evolutionary arms races

Rotifers find answer to parasites by blowing on the wind

edyong — Thu, 28 Jan 2010 13:00:01 +0000

Rotifers find answer to parasites by blowing on the wind

When our lives are in danger, some humans go on the run, seeking refuge in other countries far away from the threats of home. Animals too migrate to escape danger but one group - the pond-living bdelloid rotifers - have taken this game of hide-and-seek to an extreme.

If they are threatened by parasitic fungi, they completely remove any trace of water in their bodies, drying themselves out to a degree that their parasites can't stand. In this desiccated state, they ride the wind to safety, seeking fresh pastures where they can establish new populations free of any parasites.

This incredible strategy may be partially responsible for another equally remarkable one - the complete abandonment of sex. For over 80 million years, the bdelloids (pronounced with a silent 'b') have lived an asexual existence. Daughters are identical clones of their mothers~~, budded off from her body~~. No males have ever been discovered. For this reason, Olivia Judson once described bdelloid rotifers as an "evolutionary scandal". Their sexless lifestyles simply shouldn't work in the long run.

Ditching sex allows an animal to efficiently pass all of its genes to the next generation without having to seek out a mate. This should give asexual animals a big advantage but not so. Sex provides fuel for evolution. Every time two individuals meet in flagrante, their chromosomes are joined, shuffled and re-dealt to the next generation. In this way, sex begets diversity, remixing genes into exciting new combinations.

This diversity is a vital weapon in the never-ending war against parasites. Parasites, with their large populations and short generations, are quick to evolve new ways of exploiting their hosts. They could have their run of a genetically uniform population and soon bring it to its knees. A sexually active species is a harder target. With genes that shuffle every generation, new anti-parasite adaptations are always just one bout of mating away. And so it goes, again and again, with hosts constantly having to outrun their parasites and sex acting as the getaway vehicle.

So asexual reproduction, for all its immediate gains, should be a poor long-term strategy compared to the dynamic nature of sex. Bdelloids have clearly addressed this problem and thanks to the last few years of research, we know how. They have evolved ways of achieving every single one of the many benefits of sex, without actually doing the deed. Escape parasites? They've got that covered. Shuffle their genes? They do that too. Generate genetic diversity? Check.

The latest of these studies comes from Christopher Wilson and Paul Sherman from Cornell University. They pitted rotifers of the species Habrotrocha elusa against a lethal and exclusive adversary - a fungus called Rotiferophthora angustispora, which only infects bdelloids. Once eaten, the fungal spores produce cells that threaten to devour the rotifer from the inside out within just 2 weeks.

But the rotifers have a defence. They can completely dry out their bodies into a dormant state called a "tun" and they can survive that way for up to 9 years. All it takes is a drop of water to revive them. The fungus isn't so resilient. Its spores are fractured and shattered by the same drying process that the rotifers actively initiate.

Without this drying defence, the rotifers were completely exterminated by the fungus. If they managed to turn into tuns, they gained a temporary reprieve, but if any individuals hadn't dried out in time, fungal cells soon emerged from their corpses and infected the rehydrated survivors. Only if the rotifers stayed dormant for over 3 weeks did they fully escape the parasites.

In nature, the tuns rarely stay in the same place. They're light enough to catch the slightest wind, which helps when fleeing from a waterborne fungus. To simulate these travels, Wilson and Sherman placed a heavily infected population in a wind chamber, with air currents approximating a light breeze. Sure enough, the dormant rotifers were blown into empty dishes around 30 to 40cm away and after a week, they were rehydrated. Fresh populations started thriving in 17 of the 24 new dishes and 10 of these were completely fungus-free.

This simple experiment shows that H.elusa can escape its parasite by hitchhiking to safety aboard the wind. In the experiment, the rotifers found safety from parasites after three weeks of desiccation or 30 metres of flight. In the wild, they can almost certainly travel further and dry out for longer. In fact, they are such efficient wind-riders that some species have spread all over the globe and they regularly turn up in samples of rain and wind.

It's not just the parasites they outrun - they also escape the drawbacks of their very asexual existence. Sexual animals need to keep up their liaisons to continually outmanoeuvre their parasites and predators. But rotifers, through their extreme abilities, can leave the arms race altogether, decoupling their fates from those of enemies that co-evolve with them.

That's not the only trick they have. Sex produces greater genetic diversity, but rotifers can do this through other means. The same drying process that allows them to blow away on breezes also breaks their cell membranes and shatters their DNA. That allows them to smuggle in genes from surrounding species. The gene pools of entire kingdoms are available to them. Today, their genomes are rife with foreign DNA from bacteria, fungi and even plants, many of which are fully functional.

They can also put different copies of the same gene to different uses. All animals have two copies of their various genes, but without sex to mingle them, those of rotifers can become decoupled. They become free to evolve in new directions and acquire new roles. These animals get two genes for the price of one.

Through these adaptations, the bdelloids have avoided the seemingly inevitable extinction that awaits so many asexual animals. Sex is almost universal among complex life. Less than 1% of animal species reproduce solely through cloning and almost all of them evolved quite recently. Asexuality is a strategy found on the thinnest twigs of the tree of life, suggesting that most eventually die out. Bdelloids are exceptions. As Nick Lane describes in his wonderful book Life Ascending, they "have become biological celebrities, chaste exceptions in a world obsessed with sex, passing like monks through a red light district."

Reference: Wilson and Sherman. 2010. Anciently Asexual Bdelloid Rotifers Escape Lethal Fungal Parasites by Drying Up and Blowing Away. Science http://dx.doi.org /10.1126/science.1179252

Images: by Diego Fontaneto and Bob Blaylock.

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edyong Thu, 01/28/2010 - 08:00

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I can't get enough bdelloid info. More is better.

I didn't quite understand this sentence, though: "rotifers, through their extreme abilities, can leave the arms race altogether, decoupling their fates from those of enemies that co-evolve with them."

Was this referring to their ability travel to an uncontaminated location and found a new population as an individual, without need of a mate that could be infected?

Thanks for the post. A minor quibble, though "Daughters are identical clones of their mothers, budded off from her body": Bdelloids, like other rotifers, do not reproduce by budding, they actually produce diploid eggs by mitosis which start their development with no need of sperm. Budding is a different mechanism of cloning, involving no eggs but pieces of tissue, which is found in plants but also in some invertebrates such as anemones.

We need a new "All Bdelloids, All the Time" blog. A cable channel, too. And an institute. Maybe a religion.

Brian - in local populations, the rotifers co-evolve with their parasites in evolutionary "arms races" where adaptations are balanced by counter-adaptations. But by leaving, the rotifers break that futile cycle.

Blackbird - fair point. Fixed.

Nathan - you speak the truth.

Just to hammer home that you have the nitpickiest commenters ever, I think it was John Maynard Smith who first called rotifers a scandal in "The Problems of Biology" in 1986.

Ah but I never claimed that Judson said it first, merely that she once said it. Ka-pow!

Dang! Outpedanted.

Fantastic post.

Love the post, love the personas of the posters. Regarding Nathan's request for more bdelloid media, and Ed's graphic "Ka-pow" response, I flashed on a Speed Racer-like cartoon featuring these self-desiccating asexual superheros.

Two questions, if you have time: if the bdelloids escape the parasitic fungi by dessicating themselves so the fungal cells and spores shatter, how do the bdelloids survive when their cell membranes and DNA also rupture?

And why haven't the fungi evolved similarly to survive- 80 million years seems a long time to not develop tolerance to dessication.

Thanks Ed, returning to lurking mode now.

Rotifers are the fucking best!

Reviewer 3 - the quick answer to the first one is that they're really, really good at repairing their DNA. If I'm remembering correctly, their chromosomes are tetraploid, meaning they have four copies of each, rather than our two. This means that if anything breaks, they have multiple back-ups to copy the missing information from. Imagine that your computer had three back-up harddrives. It would be virtually impossible for you to lose your valuable data because you would always have a working spare.

The answer to the second question is less clear, but it's not trivial to tolerate dessication. "Extremophiles" of this sort aren't exactly common. You've got rotifers, tardigrades, Deinococcus, and maybe a few others.

With respect to Reviewer #3's second question, how can we be certain that the fungi have also been co-evolving with the rotifers for 80 million years? These particular fungi could well have evolved to become rotifer parasites much more recently.

It seems to me that the desiccation strategy would work against almost any parasite, and could have first evolved as a defense against different parasites, possibly now extinct. Over the bdelloids' 80 million year history, and entire sequence of different parasite species could have "had a go" at them, each failing in turn to evolve an adequate counter to the self-dessication strategy, and ultimately going extinct, eventually replaced by another new species of parasite, while the rotifers march on.

Why don't the fungus keep up? I think the global distribution of bdelloids might have something to do with it; there's always a population in some place that is uncomfortable for any particular fungus. Also, since bdelloids don't need to make contact with other bdelloids, a fungus that develops any dessication tolerance has to find a mate without help from the bdelloids. Furthermore, since each bdelloid is its own lineage, the ones we have now are all descended from billions of generations not successfully colonized by any fungus.

A more successful strategy would seem to be getting your genes adopted piecemeal directly into the bdelloid genome, retro-wise. This seems somehow less satisfying from a parasitical standpoint, but I would have a hard time expressing why. I doubt Nature would find any such argument persuasive.

How do we know that the bdelloids are all "female"? I don't what the sexual determination of rotifers is but apparently the bd's don't have that?
-Terry

I'll bet the "dry up and blow away" defense is less than perfect from the bdelloid's viewpoint, because it seems likely that quite a few of them will die somewhere instead of finding a good home. So from the fungus viewpoint, it's not a problem - why try to evolve the same capability when you should focus on reproducing in your current pond (and presumably sending spores out that will eventually found new populations elsewhere).

Tobacco plants foil very hungry caterpillars by switching pollinators to hummingbirds

edyong — Thu, 21 Jan 2010 11:00:53 +0000

Tobacco plants foil very hungry caterpillars by switching pollinators to hummingbirds

The partnerships between flowering plants and the animals that pollinate them are some of the most familiar in the natural world. The active nature of animals typically casts the plants as the passive partners in this alliance, but in reality, they're just as involved. That becomes particularly apparent when the animals renege on their partnership.

Nicotinia attenuata, a type of wild US tobacco, is usually pollinated by hawkmoths. To lure them in, it opens its flowers at night and releases alluring chemicals. But pollinating hawkmoths often lay their eggs on the plants they visit and the voracious caterpillars start eating the plants. Fortunately for the plant, it has a back-up plan. It stops producing its moth-attracting chemicals and starts opening its flowers during the day instead. This simple change of timing opens its nectar stores to a very different pollinator that has no interest in eating it - the black-chinned hummingbird.

Danny Kessler from the Max Planck Institute first noticed the tobacco plant's partner-swapping antics by watching a population of flowers that was overrun by hawkmoth caterpillars. Nearly every plant was infested. To Kessler's surprise, around one in six flowers started opening between 6 and 10am, rather than their normal business hours of 6 and 10pm. To see if the two trends were related, Kessley deliberately infested plants from another population with young hawkmoth larvae.

Eight days later, and 35% of the flowers had started opening in the morning, compared to just 11% of uninfested plants. The flowers use a cocktail of various chemicals to lures in night-flying moths, but the main ingredient is benzyl acetone (BA). A large plume gets releases when the flower opens at night. It's so essential that genetically modified plants, which can't produce BA, never manage to attract any moths. Nonetheless, the flowers that opened in the morning never produced any BA.

By artificially boosting the nectar yield of specific flowers, Kessler showed that hawkmoths are more likely to lay eggs on plants that reward them with the most nectar. So by putting off the adult hawkmoths from visiting the flowers, the plants gained a reprieve from future onslaughts by their larvae.

The larvae themselves prompt the switch. As they munch away, their saliva releases complex mixtures of fats and amino acids into the wounds they create. This cocktail trigger a genetic alarm in the plant's cells, which culminates in a burst of jasmonic acid. This all-important plant chemical coordinates a variety of defences, from producing poisons to summoning predators and parasitic wasps. In this case, it's responsible for shifting the flowers' blooming schedule.

Kessler demonstrated the role of the caterpillars' saliva and jasmonic acid through a clever series of experiments. Even if no larvae are around, just adding their saliva to artificial wounds causes some plants to switch to the morning opening hours. If the plants are genetically modified so that they can't produce jasmonic acid, the entire process grinds to a halt, rescued only by the artificial addition of jasmonic acid.

Having solved the problem of the very hungry caterpillars, the plants still need pollinators. Again, the revised opening schedule provides the solution. Through painstaking field observations, Kessler showed that hummingbirds were strongly attracted to the morning blossoms, almost always visiting these flowers first. The birds have apparently learned to associate the shape of the opened flowers with the prospect of a rich, early-morning beakful of nectar. The plant gets a new partner, while avoiding the unwanted shenanigans of its old one.

Hummingbirds, of course, never eat other parts of the plant but if they're such compliant partners, why doesn't the tobacco plant always open its flowers in the morning? We don't know, but Kessler suggests that the birds, for all their strengths, may not be quite as reliable as the moths. Hummingbirds are more likely to drink from multiple flowers on the same plant, which would lead to a lot of self-fertilisation. They're more restricted by geographical factors, such as the presence of nearby nest sites. And, unlike hawkmoths, they can't be summoned across long distances through the simple use of smell.

Picture by Stan Shebs

Reference: Kessler et al. 2010. Changing Pollinators as a Means of Escaping Herbivores. Current Biology http://dx.doi.org/10.1016/j.cub.2009.11.071

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edyong Thu, 01/21/2010 - 06:00

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Fascinating!

This is awesome - love this study!

Very cool stuff, Ed! One of these days you'll have to come to the Colonies for a visit to Arizona, and I'll be happy to show you all the hummingbirds you could possibly imagine.

The TickleMe Plant which can close its leaves and lower its branches when tickled uses this defense against insects and herbivores. Once the insect lands on the plant or the animal begins to eat it, the plant moves like and animal causing the would be predator to find a less active plant.
My students love their TickleMe Plants and can't wait to get to class to tickle theirs. We get our supplies to grow them at http://www.ticklemeplant.com

Museum butterfly collections chronicle evolutionary war against male-killers

edyong — Thu, 10 Sep 2009 10:00:34 +0000

Museum butterfly collections chronicle evolutionary war against male-killers

The drawers of the world's museums are full of pinned, preserved and catalogued insects. These collections are more than just graveyards - they are a record of evolutionary battles waged between animals and their parasites. Today, these long-dead specimens act as "silent witnesses of evolutionary change", willing to tell their story to any biologist who knows the right question to ask.

This time round, the biologist was Emily Hornett, currently at UCL, and her question was "How have the ratios of male butterflies to female ones changed over time?" You would think that the sex ratios of insects to mirror the one-to-one proportions expected of humans but not if parasites get involved.

The bacterium Wolbachia is arguably the world's most successful parasite, infecting around 20% of all insects, themselves an extraordinarily successful group. It can infect eggs but not sperm, which means that females can pass the bacteria on to their offspring, but males cannot. As a result, Wolbachia has it in for males - they are evolutionary dead-ends, and the bacterium has many strategies for getting rid of them. It can kill them outright, it can turn them into females and it can prevent them from mating with uninfected females. As a result, populations infected with Wolbachia can be virtually male-free.

To study the effect of Wolbachia on butterfly populations, Hornett (great name for an entomologist)turned to collections of the blue moon butterfly (Hypolimnas bolina). This beautiful species was heavily collected by entomologists between 1870 and 1930 and their efforts have stocked the museums of the world with specimens. While these were long dead, Hornett found that many of them contained viable DNA and she used them to develop a genetic test for Wolbachia infections.

She validated her Wolbachia test by using butterflies collected by the entomologist H.W.Simmons in Fiji over 70 years ago. Simmons carefully recorded the numbers of males and females in his butterflies and noted some very unusual all-female brood. Sure enough, Hornett confirmed that only mothers who tested positive for Wolbachia produced these skewed clutches, while those that were infection-free gave birth to the standard bisexual broods.

Satisfied that her test was accurate, Hornett cast her net further. She looked at specimens collected from five populations of blue moon butterflies collected from the Phillippines, Borneo, Tahiti, Fiji and Samoa between 73 and 123 years ago. The butterflies are well studied to this day, so Hornett could compare the proportion of Wolbachia infections then and now. In butterfly time, this represents a gap of 500 to 1000 generations separating the specimens from their modern descendants.

The results show that the butterfly and the bacterium have been engaging in a heated evolutionary battle throughout the Pacific. The male-killer's dominance has fluctuated greatly, rising in some areas and falling in others, while the butterfly has repeatedly evolved to resist its sex-skewing antics.

Among the historical populations, only the Philippine group had a high proportion of infected males - a sign that they had developed a resistance to Wolbachia, which has been maintained to this day. In Borneo, the butterflies have evolved resistance over time - collectors didn't capture a single infected male in 1878 (or indeed, very many males at all), but such individuals are commonplace now. This rise of resistance parallels the real-time events that Hornett, together with Emily Dyson and Greg Hurst documented in Samoa a couple of years ago.

Only Viti Levu in Fiji ha actually managed to avoid Wolbachia infection, rather than simply shrugging it off. In 1886, 45% of the population were infected, but this climbed to 92% by 1934, before falling back to 59% by modern times. Remarkably, this trend mirrors notes written by field biologists studying the butterflies over the years. In 1888, Mathew counted 2 females for every male. In the 1920s, at the height of the infection, Poulton estimated 12 females to every male, while modern counts have returned to a two-to-one ratio.

Elsewhere, the bacterium's fortunes have been better. On the Polynesian islands of Ua Huka and Tahiti, the male-killer was either absent or rare in historical populations (again, supported by written records) but common today. Unsurprisingly, male blue moon butterflies are a rarity in these places.

This isn't just a matter of nebbish census-taking - the dominance of male-killer infections can have drastic consequences for their hosts' biology. For a start, the absence of males would dramatically shrink the population size, reducing its genetic diversity and leaving it ill-prepared to face future environmental changes, or even others predators or parasites.

The results portray a geographic mosaic of infection and suppression, across the entire Pacific. The migration of butterflies from island to island may help to spread beneficial adaptations to populations that need it. Indeed, the suppressive ability that has protected the Philippine males for over a century may have contributed to the resistant populations of Samoa and Borneo. Perhaps in time, Tahiti and Ua Huka will benefit too.

Hornett's work also shows that the balance of power can shift very dramatically between a host and its parasite over a relatively short span of time. In fact, these cycles may happen so rapidly that many may be completely invisible to us - a pattern of sex distortion that stays invisible because we aren't looking quickly enough.

Fossil-hunters can glimpse the threads of evolution by looking at the bones of ancient animals and look at how they differed from their modern relatives. That tells us about evolution on geological time-scales, spanning millions of years. But museum specimens can act as fossils of a sort, offering a peephole on evolutionary changes over a much shorter and more contemporary scale.

Reference: Current Biology DOI 10.1016/j.cub.2009.07.071

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edyong Thu, 09/10/2009 - 06:00

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Virus and bacteria team up to save aphid from parasitic wasp

edyong — Fri, 21 Aug 2009 08:00:47 +0000

Virus and bacteria team up to save aphid from parasitic wasp

Viruses and bacteria often act as parasites, infecting a host, reproducing at its expense and causing disease and death. But not always - sometimes, their infections are positively beneficial and on rare occasions, they can actually defend their hosts from parasitism rather than playing the role themselves.

In the body of one species of aphid, a bacterium and a virus have formed a unlikely partnership to defend their host from a lethal wasp called Aphidius ervi. The wasp turns aphids into living larders for its larvae, laying eggs inside unfortunate animals that are eventually eaten from the inside out. But the pea aphid (Acyrthosiphon pisum) has a defence - some individuals are infected by guardian bacteria (Hamiltonella defensa) that save their host by somehow killing the developing wasp larvae.

H.defensa can be passed down from mother to daughter or even sexually transmitted. Infection rates go up dramatically when aphids are threatened by parasitic wasps. But not all strains are the same; some provide substantially more protection than others and Kerry Oliver from the University of Georgia has found out why.

H.defensa's is only defensive when it itself is infected by a virus - a bacteriophage called APSE (or "A.pisum secondary endosymbiont" in full). APSE produces toxins that are suspected to target the tissues of animals, such as those of invading wasp grubs. The phage infects the bacteria, which in turn infect the aphids - it's this initial step that protects against the wasps.

Oliver set up populations of pea aphids that were all genetically identical. Some of them carried H.defensa in their bodies while others did not, and of the infected individuals, some were also ridden with the APSE-3 phage. He unleashed the wasp A.ervi upon all of these individuals, and waited.

The results clearly demonstrated the importance of having both virus and bacteria - with the full alliance intact, less than 10% of the aphids fell victim to the wasp. If one or both of the partners was absent, the wasp successfully parasitized 90% of its potential hosts. Without the phage, the aphids might as well not have been infected by H.defensa at all, for all the good it did.

The phage bestows protective powers on the bacteria thanks to a single gene that encodes a toxic protein. It also makes a lot of it - the toxin-producing gene is almost three times as active as one of the most active genes in the bacterium's own genome. How exactly this poison kills the wasp is unclear and it's worth noting that H.defensa's has toxins of its own. The most intriguing possibility is that somehow the viral and bacterial poisons work together to kill the wasp, but that's a theory to be tested in the future.

In lab conditions, the alliance between bacterium and virus is a very stable one, but in the wild, H.defensa has a bad habit of spontaneously losing its all-important phage lodger. Even within a single aphid, only some bacteria have viral genes integrated into their own. No one knows why the virus should disappear so frequently. Perhaps it's not an entirely welcome tenant and its presence carries some sort of cost that occasionally outweighs its benefits. Perhaps only phage-free H.defensa can be passed on from aphid to aphid.

Regardless, case of the pea aphid is a great example of the role of phages as vehicles for genes, shuffling them between bacteria and providing the carriers with extra superpowers. In almost all cases, these gene transfers affect the ability to cause disease. It's a phage that carries the CTX toxin that the bacterium Vibrio cholerae needs to cause cholera. It's a phage that carries the Shiga toxin that turns harmless strains of Escherichia coli into a bug capable of causing severe food poisoning.

The APSE-3 phage is different - its toxin, far from turning its host into a more effective parasite, actually turns it into an anti-parasite defender. Faced with an enemy that threatens them all, the virus, the bacterium and the aphid have formed an evolutionary alliance, with infection as its foundation.

Of course, not all parasitic wasps are foiled by viruses - some have managed to turn them into biological weapons, stealing their genes and using them to subdue their caterpillar prey...

Reference: Science 10.1126/science.1174463

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edyong Fri, 08/21/2009 - 04:00

Minor correction, Ed - although Dr. Oliver might not consider it so minor:

"She unleashed the wasp A.ervi upon all of these individuals, and waited."

Actually, he unleashed the wasp. You linked to his faculty bio page, but apparently never looked at it, because Kerry Oliver is awfully rugged-looking and bearded for a 'she.'

How is the virus transmitted? Seems like there would be selection for protecting the host only if it's transmitted to host offspring, presumably inside with the bacteria. If it ever evolves the ability to infect and reproduce in the wasps, I'd expect it to suppress rather than strengthen host defenses.

Oh God - n00b error! You know, I once sent a female scientist a write-up of their work for fact-checking and referred to them as "he" throughout. She sent it back with a couple of technical corrections but didn't spot the gender-bending one.

Ford-
My impression is that the phage is usually transmitted along with the infected facultative symbiont from parent to offspring, though it can be lost (rendering the Hamiltonella worse than useless). Since this is a phage, there would have to be a susceptible microbe in the wasps for it to infect.

Ed, you use the construction "H.defensa's" in a way that seems odd to me more than once in the article. Is it just a cut'n'paste anomaly, or am I missing some meaning?

Once again I am led to doubt that the phage should really be treated as an organism independently from the bacterium. You don't always wear your hat, but it's still your hat.

One jump from chimps to humans - the origin of malaria

edyong — Mon, 03 Aug 2009 15:00:43 +0000

One jump from chimps to humans - the origin of malaria

Swine flu has made the world all too aware of the possibility of diseases making the leap from animal hosts to human ones. Now, we know that another disease made a similar transition from chimpanzees to humans, several thousand years ago. This particular infection is caused by a parasite, and a very familiar and dangerous one - Plasmodium falciparum, the agent responsible for malaria.

Transmitted by the bite of mosquitoes, P.falciparum infects over 500 million people every year. Its closest relative is a related parasite, Plasmodium reichenowi, which infects chimpanzees. Leading an international research team, Stephen Rich from the University of Massachussetts has discovered that P.reichenowi is no mere relative - it's actually P.falciparum's ancestor.

Rich compared the genes of the two species to build a Plasmodium family tree, which showed that all of the 133 known strains of P.falciparum, from all parts of the world, are united one a single branch on the P.reichenowi lineage. The stem of that branch represents a single event where P.reichenowi crossed the species barrier from chimps to humans.

The new study was possible because of eight newly collected samples of P.reichenowi from wild and captive chimps. Until now, only a single sample of this species had ever been isolated. Armed with fresh samples, the team focused their attention on three genes - cytB, clpC and 18s rRNA. They found that those of P.reichenowi are very varied, much more so than its genetically uniform cousin P.falciparum (even though we have over 16 times as many samples of the latter). Chances are that any two samples of P.reichenowi are more genetically distinct that either one is to P.falcarium.

Rich is sitting on the fence as to whether this means that the two species should actually be united as one. But he says that his results clearly rule out the idea that the two Plasmodium species have evolved separately from a common ancestor that itself infected the last common ancestor of chimps and humans. Instead, it's more likely that the genetically uniform species, P.falciparum, evolved from the diverse one, P.reichenowi.

This isn't a new idea. As far back in the 1950s, the renowned anthropologist Frank Livingstone suggested that P.falciparum was an unwitting gift from chimpanzees. He reasoned that as agriculture increased the size of human societies, our encroaching presence in the dwindling forest homes of chimps provided many opportunities for a canny parasite to find fertile new infecting grounds.

Rich suggests that the jump from chimps to humans was driven by an evolutionary battle fought by Plasmodium parasites and their hosts at a genetic level. Part of Plasmodium's complicated life cycle involves infecting red blood cells, leading to one of the classic symptoms of malaria - anaemia. To infiltrate these cells, the parasites recognise a molecule on their surface called GYPA, using a sensor protein of its own called EBA-175.

In chimps, GYPA has a chemical cap called Neu5Gc and it's this adornment that P.reichenowi recognises. Neu5Gc is one of a group of chemicals called sialic acids, and it's produced from a related member called Neu5Ac; just one oxygen atom separates the two. The conversion process is driven by a gene called CMAH - a gene that works in chimps, but that's been crippled in humans. As a result, our GYPA proteins are capped with Neu5Ac instead of Neu5Gc, which explains why P.reichenowi infects chimps but not us.

Some scientists think that a mutation that disabled the CMAH gene provided our early ancestors with temporary relief from P.reichenowi. By blinding the parasite to the GYPA protein, the mutation gave us a powerful advantage. But it wasn't a permanent one; Plasmodium upped the stakes by evolving a version of the EBA-175 that was more relaxed in its choice of targets. This new weapon stuck to human proteins capped with Neu5Ac just as well as they did to chimp proteins capped with Neu5Gc.

This counter-move allowed the parasites to re-exploit their old victims and in time, one lineage - Plasmodium falciparum - developed a preference for the Neu5Ac target. Rich thinks that this final step occurred relatively recently, between five and ten thousand years ago, and gave the parasite an extreme ability to cause disease in humans. The benefits of this mutation allowed P.falciparum to quickly expand worldwide, which probably accounts for its lack of genetic diversity.

The fact that P.falciparumhas only once found a way to infect humans suggests that it's very difficult for the parasite to evolve a protein arsenal that recognises a new type of sialic acid. The same could even be true for other unrelated infections, such as flu. Sialic acids are actually the big obstacle in the way of bird flu viruses spreading freely among humans. The cells of human and bird airways bear slightly different sialic acids on their surfaces and bird flu viruses, for the moment, have a preference for the bird version.

Reference: doi:10.1073/pnas.0907740106

More on malaria:

Genetically-modified mosquitoes fight malaria by outcompeting normal ones

Size matters for mosquitoes but medium-sized males do better

edyong Mon, 08/03/2009 - 11:00

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Thanks for posting about this. I just heard about it on the radio and hoped you might have a post to clarify things. Thanks for not letting me down! The great evolutionary biologist William Hamilton died from malaria a few years back. Definitely a huge disease.

What is P. falcarium? Is it a new name for P. falciparum?

Re: Dale Hoyt
P. falcicarium is not a new name for P. falciparum. It' an unfortunate typographical error occurrred most prominently on the top of the diagramtic illustration. I saw not too long ago a person received an appreciative service award in a rather large public organization his name misspelled on the certificate. He was astonished encountering his misspelled name, and at first he felt that he should report immediately in a righteous indignation, but he calmed himself down, thinking that the administrators worked hard for the year end annual event. Not only that, but also they might be so embarassed that they could turn even to hate him for their self-preservation, knowing a crevice of the human nature. So, he took nearly a month of his time and reported about the error to the three top authorities, and they corrected the error without a hitch. It seems that the error can and does occur at all levels as even seen recently as small as a typographical error made a plane rider take a wrong plane and realized only after he arrived at an unintended location that there was only one letter ommission on his ticket purchase. We must simply check one more time always before we make submitions. I esteem the author Ed Yong greatly, and if the typo can happen even to him, I too must forgive my own not infrequent errors and feel not so embarrassed.

AriSan

LOL!
I meant diagramatic, not "diagramtic" in my letter above.

AriSan

AriSan has just demonstrated Muphry's Law - where anyone commenting on matters of spelling or grammar will themselves make mistakes of spelling and grammar. ;-)

Snails get sexy when parasites are around

edyong — Fri, 24 Jul 2009 08:00:46 +0000

Snails get sexy when parasites are around

In Lake Alexandrina, New Zealand, a population of snails is under threat from a parasitic flatworm, a fluke aptly known as Microphallus. The fluke chemically castrates its snail host and uses its body as a living incubator for its larvae. But the snails have a weapon against these body-snatching foes - sex.

The New Zealand mud snail Potamopyrgus antipodarum is found throughout island's freshwater habitats. They breed either sexually or asexually through cloning, and the two strategies vary in prevalence throughout the lake. In the shallower waters round its margins, sex is the name of the game, but in the deeper waters towards the lake's centre, snails are more likely to opt for cloning.

Kayla King from Indiana University has shown that it's the concentration of the local parasites that drives this gradient of sex. The flukes spend their adult lives in ducks and they rely on the birds inadvertently scooping up their larvae while feeding. In Lake Alexandrina, ducks only feed in the shallow waters around the lake's margins so these areas are hotspots for parasites, and for co-evolutionary wars between them and their snail hosts. Sex provides the snails with the genetic ammunition they need to stay in the game.

The snails and their parasites beautifully support and illustrate the principles of the Red Queen hypothesis, which suggests that one of the chief benefits of sex lies in providing the genetic innovation necessary to outfox parasites in evolutionary arms races.

Asexual animals have massive advantages over sexual ones - they can produce more offspring quickly and they never have to dilute their own genetic contributions with those of a partner. All in all, they should rapidly replace sexual counterparts unless something negates their advantages - something like competition against parasites. To a parasite, a population of asexual clones with their identical genomes, are an easy target. Any adaptation that gives them the upper hand against one individual will work against the whole group.

Sexual individuals, on the other hand, have genetically diverse offspring, for every time a male and female mate, their genes mingle in new and interesting combinations. That makes them far more difficult to game when the targets are constantly shifting.

King collected snails from the shallow and deep parts of Lake Alexandrina and nearby Lake Kaniere. She also captured Microphallus flukes from these two lakes and one other, and pitted them against the various snails. The results showed that the parasites are strongly adapted to their local sub-populations of snails. For example, the Alexandria flukes are much more infectious to the shallow-water sexual snails from the same lake than they than the deep-water asexual ones. The shallows are indeed co-evolutionary hotspots.

The parasites were only good at infecting snails from the same local area - the shallow part of their own particular lake. Snails were foreign to them if they came from either different lakes, or from the deeper parts of the same one (even if those regions were only a few metres away). This rules out the possibility that the sexual behaviour of the shallow-water snails makes them more vulnerable to parasites. The opposite explanation, that more parasites lead to more sex, is the likelier one.

Reference: Current Biology 10.1016/j.cub.2009.06.062

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edyong Fri, 07/24/2009 - 04:00

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Who would have thought that sex was basically about worms?

I hope you don't mind me sharing a Finnish palindrome, just in case there are other Finnish readers:

"Toi loiskeston ne hennot seksioliot."

Roughly translated, it says, "It's them delicate sex creatures that brought parasite tolerance." (The grammar is nonstandard in the original.)

As a High School Science Teacher in a small conservative town, I am famous for showing what my students call the "Sex Video". It is the "Eyes of Nye on Genetic Diversity", and explains that we have sex to stay ahead of the parasites. Nye also uses his bad humor and the Red queen to explain this.
Thanks for a concrete example that I can use in the classroom.

Heh, Potamopyrgus antipodarum is also highly invasive in a number of US West Coast watersheds. And reproduce entirely asexually for the most part, though if I recall the readings I did for a undergrad evol.bio presentation, it is reproducing semi-sexually in one population in France.

Anyhow, if flukes are tightly bound to sexual populations, then this does make biocontrol via introducing flukes a bit more on the fun side. Depending on the source populations for the invasive groups. Though if I recall correctly, each invasive population was mostly 1 clonal line...

Sorry to leave the comment here, but the post directly following -- the one about noise and birds -- appears to be broken in some way. I canNOT get to the rest of the article. Not via the "read more" link, nor the title link.

Those flukes are a bit like Robin Hood - robbing from the snails, to give to the ducks. Certainly in terms of length, that is.

Traumatic insemination - male spider pierces female's underside with needle-sharp penis

edyong — Tue, 28 Apr 2009 17:00:27 +0000

Traumatic insemination - male spider pierces female's underside with needle-sharp penis

The courtship rituals of the spider Harpactea sadistica start innocently enough, with a dance and a hug. The male spider taps the female gently with his front legs and embraces her. But from that point onwards, things for the female go rapidly downhill. The male bites her and she becomes passive, allowing him to manoeuvre her into position. Like all spiders, his genitals are found next to his head, on a pair of appendages called the pedipalps. But unusually, his penis ends in a needle-sharp tip called an embolus.

The embolus sits at the end of a loop called the conductor. The male hooks one of these loops around the opposite embolus to steady it. Then, by rotating the anchored needle, he drives the point straight through the female's underside and ejaculates directly into her body cavity. On average, he does this six times, moving slowly downwards and alternating between his two penises. The entire cringeworthy sequence lasts about 15 minutes and throughout it, the male spider never penetrates the female's actual genital opening.

The species was discovered in Israel last year by Milan Rezac from the Crop Rsearch Institute in the Czech Republic. He named it well. H.sadistica practices a style of sex that's understandably known as "traumatic insemination". It's disturbingly common among insects and other invertebrates, and is most famously practiced by bedbugs. But this is the first time that the behaviour's been seen among the chelicerates - the group of animals that includes spiders, scorpions and mites.

You can see it happening in the videos below. In the first, the male spider bites and incapacitates the female. In the second, he hooks the conductor of one pedipalp around the embolus of another and, with rotating motions, drives it into the female. These videos aren't pretty - you've been warned.

When most spiders mate, the male ejects sperm into the female's vulva, and she stores it in a special pouch called the spermatheca. These pouches allow females to control when she fertilises her eggs, by voluntarily shunting sperm onto them before she lays. It also means that the last male to mate with her typically sires most of her offspring, because his sperm flushes out those of earlier liaisons.

This system means that there's a fever-pitched sperm competition between males. Different species have evolved bizarre adaptations to get an edge in these conflicts. Some guard their mates after sex. Others leave behind "mate plugs" - chastity belts that block the female's genitals. The males of one species take this to an extreme, by spontaneously dying during intercourse and turning their entire bodies into a mating plug.

But H.sadistica's brutal sexual practice allows it to bypass the competition altogether because the male's sperm can travel directly to the female's ovaries through her bloodstream. By wresting control of fertilisation from females, males ensure that when they mate, their sperm isn't shunted aside at the risk of later removal. The female's genitals support that idea, for her spemathecae have atrophied and are extremely small. Since the male doesn't ejaculate in the usual place, she has no need to keep around storage space for his sperm.

The wounds don't seem to harm the female, and Rezac never saw blood leaking from the punctures. If the females are put off by their repeated stabbings, they don't show it. Within days, they are usually ready for a new mate. It might even be positively beneficial for the female to mate with males who practice traumatic insemination. The sons of such a partnership would themselves be better at circumventing the female's sexual stores and having more offspring of their own.

It's entirely possible that in the future, the H.sadistica females will evolve a countermeasure to the male's stabbing genitals. Female bedbugs, for example, have developed a set of secondary sperm containers called 'spermalege' at the place where the males make their entries. They have effectively shunted the entire sexual process - from penetration to sperm storage - to a different part of their bodies. The original genital opening is only used to lay eggs. The same adaptations have cropped up in other insects too but H.sadistica doesn't have them... yet.

Reference:Rezac, M. (2009). The spider Harpactea sadistica: co-evolution of traumatic insemination and complex female genital morphology in spiders Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2009.0104

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edyong Tue, 04/28/2009 - 13:00

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Ok, that is just too gruesome....but fascinating. I must be sick...or a biologist. :-)

Seriously though, that is truly fascinating as examining adaptations and counter-adaptations (an arms-race, as it were) in organisms allows you to imagine evolution in action rather more vividly.

Wow, what a prick!

Evolution at work!

And the female spider doesn't even get to eat the male spider after that?

I also notice that the male is larger than the female, a reversal of the usual case amongst spiders.

I'm thinking of making a short (mostly v-log style) video where I talk about this. Is it alright if I use that second video inside of mine?

(I would, of course, link back here in the video description.)

Is it bad that the phrase "traumatic insemination" kinda makes me giggle?

TMI! TMI!

SciencePundit - but of course.

Christie - yes. Yes it is. You have incurable NERD... ;-)

Spiderman ain't so sexy now, huh?

GO PENIS!!!!!!!!

The rebellion of the ant slaves

edyong — Wed, 01 Apr 2009 06:30:07 +0000

The rebellion of the ant slaves

Humans aren't the only species that have had to deal with the issue of slavery. Some species of ants also abduct the young of others, forcing them into labouring for their new masters. These slave-making ants, like Protomagnathus americanus conduct violent raids on the nests of other species, killing all the adults and larva-napping the brood.

When these youngsters mature, they take on the odour of their abductors and become the servants of the enslaving queen. They take over the jobs of maintaining the colony and caring for its larvae even though they are from another species; they even take part in raids themselves. But like all slave-traders, P.americanus faces rebellions.

Some of its victims (ants from the genus Temnothorax) strike back with murderous larvae. Alexandra Achenbach and Susanne Foitzik from Ludwig Maximillians Universty in Munich found that some of the kidnapped workers don't bow to the whims of their new queen. Once they have matured, they start killing the pupae of their captors, destroying as many as two-thirds of the colony's brood.

Ants that are targeted by slave-makers take massive hits to their colonies and they are under intense pressure to resist these marauders. But all the defences discovered so far happen before the raids have been successfully completed. They involve better fighting skills, quicker reaction times when enemies are spotted, hastier escapes and so on.

Some scientists have suggested that strategies like this would be impossible to develop because the enslaved workers are caught in an evolutionary trap. Far away from their own colony, and sterile themselves, there is no way for them to increase their reproductive success. But Achenbach and Foitzik have rejected this idea - their conclusion is that by conducting assassinations within their new home, they severely reduce the slave-makers' numbers and their ability to conduct raids. That safeguards the future of their relatives.

Achenbach and Foitzik collected 88 colonies of the slave-making P.americanus ant that had abducted workers from three species of Temnothorax. They found that the workers clearly care for the larvae, and nearly all of them were raised until their pupated. But at that point, the slaves' behaviour changed dramatically, taking on a more homicidal bent.

Two-thirds of pupae died before they hatched. The mortality rate was even higher (83%) for pupae containing queens, but very low (3%) for those containing males. The duo saw that the captives were deliberately killing the healthy pupae. In about 30% of cases, as in the photo, the workers would gang up to literally pull the developing ants apart. Another 53% of the pupae were killed by neglect, by workers who moved them out of the nest chamber.

These murders were solely the acts of the slaves. No P.americanus worker ever lifted a mandible against its own pupae. Nor are the deaths a reflection of a generally poor standard of care on the part of Temnothorax. In their own colonies, the majority of pupae hatched, with just 3-10% dying before that happened.

This rebellion takes its toll on the slave-makers and may explain why the nests of P.americanus tend to be very small. The captives may never reproduce themselves, but they do their part for their relatives back home by crippling the workforce of the slave-makers. These indirect benefits are particularly pertinent to Temnothorax ants because a single colony can occupy many different nests - a family of sisters spread out over a large area. If one nest is raided, it pays to ensure that none of the related nests are targeted later.

Other species use similar strategies to reject attempts by parasites to usurp their parental efforts. For example, cuckoos all over the world lay eggs with varyingly strong resemblances to the eggs of other birds, in an attempt to turn them into unwitting foster parents. Hosts develop sharper recognitions skills and cuckoos develop more strikingly matched eggs. In Australia, this arms race has escalated to the point where the superb fairy wren has adopted a different strategy - it ignores the egg and recognises the young bird, killing it if it turns out to be a bronze cuckoo.

At the moment, P.americanus is on the losing end of a similar evolutionary arms race with its hosts. But it's an old parasite with a long history with Temnothorax. Achenbach and Foitzik believe that the rebellious slaves are a recent countermeasure against the problem of slavery and the impetus to evolve resistance is now on the slavers. They need to develop counter-adaptations to prevent most of their brood from dying at the jaws of their captives. The war is not yet over.

Reference: Achenbach, A., & Foitzik, S. (2009). FIRST EVIDENCE FOR SLAVE REBELLION: ENSLAVED ANT WORKERS SYSTEMATICALLY KILL THE BROOD OF THEIR SOCIAL PARASITE

Evolution, 63 (4), 1068-1075 DOI: 10.1111/j.1558-5646.2009.00591.x

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edyong Wed, 04/01/2009 - 02:30

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Great article! Thanks!

Wow, that is a fascinating study!

Wow--fascinating. And it's so interesting that animal behaviour that is advantageous for other members of the species, though not for the individual, persists.

I thought this may have been an April Fool's until I saw the reference checked out. Amazing things these ants are capable of!

Some scientists have suggested that strategies like this would be impossible to develop because the enslaved workers are caught in an evolutionary trap. Far away from their own colony, and sterile themselves, there is no way for them to increase their reproductive success.

But you make that objection to many altrustic acts. I thought the selfish gene paradigm would have ended that type opf thinking.

That should be "you CAN make that objection", as in anyone can. I wasn't accusing Ed of making any such objections.

I was with Max there until, like him, I clicked the reference. The truth is stranger than fiction!

This is very interesting.

Normally evolution is thought of as being competition among individuals.

The fact that we are altruistic requires us to expand the idea of evolution to groups.

Here we actually have evolution at the level of competition between species. That is, individuals are acting in a fashion to benefit their specie, rather than just themselves or their colonies (assuming that their destruction of the Protomognathus americanus benefits their entire species rather than just their own colonies).

How does this evolution that benefits the entire species work? This is not simple. If an individual (colony) develops a trait that is detrimental to Protomognathus americanus, how does that trait grow within the specie? It is after all not obvious that this trait should survive competition within the specie. A colony that has this trait, does not have an advantage against other colonies of the same species which are competitors for resources.

A specie that has this resistance trait will be successful, but explaining how the trait grew within the specie is not simple.

The long term equilibrium cannot be where Protomognathus americanus are excessive, as they depend on other ant species for survival. Their fraction of the total ant population (of all species) cannot increase beyond a point. This co-existence probably provided enough time for other species to develop the resistance.

I, too, thought it was an April Fool's joke when I saw the title. "Rebellion of the ant slaves"? Sounds almost as implausible as the comic strip I saw some time ago that showed "enlightened ants" who refused to enslave other ants. And yet, it's true! Astonishing!

It does, indeed, make evolutionary sense, kin-selection and all, yet, it's still a rather impressively complex behavior to be developed in such a way.

@Jayanta:

The direct benefits are not at the level of the species*. The ants' home colonies are still around, and benefit from their enslaved members' actions. The slaves are benefiting their sisters back home. Ant colonies are large families, all with the same parents, generally sharing Â¾ of their genes. Thus, the genes that result in this rebellion are likely to be shared by others in their home colony. The survivors in the home colony thereby benefit, and spread out.

*There is likely to be some benefit to unrelated colonies. However, this is an evolutionarily neutral situation, as the benefit to unrelated colonies is not actually depriving their home colony of benefits, particularly as the closest nests to the P. americanus colony are likely to be the slaves' home colony, and thus, they are the ones who receive the greatest benefit

Hahaha I just saw this on BB and thought "I should blog this before that Ed Yong character spits out 800 perfectly formed words on it" - foiled again!

"Some scientists have suggested that strategies like this would be impossible to develop because the enslaved workers are caught in an evolutionary trap. Far away from their own colony, and sterile themselves, there is no way for them to increase their reproductive success."

This makes more sense if you do not consider the worker ants as individual organisms. They are, after all, created by their queen to serve her purposes, and, being sterile, have no evolutionary interests of their own to look after. Saying that they are "in an evolutionary trap" and therefore have no reason to act against their captors is similar saying that the toxins of a poisonous beetle which has been ingested by a predator are "in an evolutionary trap" and have no reason to poison their captor.

For those with access to the Chronicle of Higher Education, myrmecologist Joan Herbers provides a worthy criticism of the terminology of ant slavery:

http://chronicle.com/weekly/v52/i29/29b00501.htm

Paper Hand wrote "The direct benefits are not at the level of the species*. The ants' home colonies are still around, and benefit from their enslaved members' actions."

Thanks, there has to be something like this going on for this evolution to succeed. Maybe groups like this which had this, went on to expand till they became the entire specie.

Ben Morris wrote "This makes more sense if you do not consider the worker ants as individual organisms. They are, after all, created by their queen to serve her purposes, and, being sterile, have no evolutionary interests of their own to look after."

I agree. As I wrote in this post a couple of days back: A better comparison would be comparing (worker and other non-reproducing) ants to the white blood cells in your veins, which will attack bacteria without hesitation, though it may lead to their own destruction.

Alex wrote: For those with access to the Chronicle of Higher Education, myrmecologist Joan Herbers provides a worthy criticism of the terminology of ant slavery:

I remember when this article came out, and there's a nice response by James Trager available on line as well:

http://www.notesfromunderground.org/archive/april%202007/opinion/trager…

I learned the coinage "dulosis" early in my entomological education, and (unlike Herbers) I prefer that term, but I think Trager also makes a good case for "cleptergy", which means "theft of work."

"Normally evolution is thought of as being competition among individuals."

I remember on the first day of Genetics my professor really, really drove home that evolution is about populations and not individuals. The individuals may be the source of the mutation, but populations evolve as that spreads.

Cool stuff, good writing too, mr. Yong!

regards,

Turtuga Blanku

*Solar Power Music at: www.TurtugaBlanku.com*

Parasites can change the balance of entire communities

edyong — Sat, 28 Feb 2009 11:00:42 +0000

Parasites can change the balance of entire communities

Conspiracy theories, TV thrillers and airport novels are full of the idea that the world is secretly run by a hidden society. We have come up with many names for this shadowy cabal of puppet-masters - the Illuminati, the Freemasons, and more. But a better name would be 'parasites'.

Every animal and plant is afflicted by parasites. The vast majority are simple, degenerate creatures, small in size and limited in intelligence. They affect our health and development, and even our behaviour and culture. And by pulling the strings of key species, parasites can change the face of entire habitats.In a typical school textbook, an ecosystem consists of plants that feed plant-eaters, who in turn, line the bowels of predators. But parasites influence all of these levels, and as such, they can change the structures of entire communities.

The idea that nature is secretly manipulated by these tiny, brainless creatures is unsettling but manipulate us, they do. And by changing the behaviour of their hosts, parasites can change the face of entire habitats. Chelsea Wood and colleagues from Dartmouth College have found compelling evidence for this, by showing that a tiny flatworm can alter the structure of a tidal habitat by infecting small marine snails.

These tidal boundaries of the coastal Atlantic are home to the common periwinkle (Littorina littorea), a type of marine snail. It invaded America's shores about two centuries ago and has spread successfully along the eastern seaboard. It acts as this habitat's equivalent of cattle, grazing the ephemeral algae that grows in rocky tidal pools.

But they are not alone. The periwinkles are unwitting passengers for the parasitic fluke Cryptocotyle lingua, a kind of flatworm. In most parts of the coast, the flukes are relatively rare, but in some parts of New England where seabirds are plentiful as many as 90% of periwinkles can be infected.

Like most parasites, the fluke has an amazingly complicated life cycle. Snails become infected by accidentally eating eggs spread in the droppings of seabirds. The larvae develop in their bodies by eating the snails inside out. They eventually give rise to a free-swimming creature that finds and infects fish, which are then eaten by seabirds, completing the cycle.

By devouring the snails' internal organs, the flukes wreck their hosts' digestive and reproductive systems. Wood reasoned that the neutered and malnourished snails must be have altered feeding habits and tested this idea in the lab and the field. She found that the flukes put infected snails on an involuntary diet, and they ate far less algae than uninfected ones.

While many parasites can directly alter their hosts' behaviour, Wood believes that nixing the periwinkles' appetities is not part of Cryptocotyle's plan. It's just an incidental side effect of infection and happens because the snails' crippled digestive glands could not cope with the normal amount of food. And because the parasites trashed their reproductive systems and several other organs, they needed less energy to begin with.

Even so, by reducing the snails' feeding, the parasites could potentially affect the entire tidal community. Wood demonstrated this by setting up cages in the tidal zone containing uninfected snails or infected ones. Sure enough, after a month or so, cages that housed infected snails had about 60% more algae than those with uninfected ones.

In the long-term, these effects are likely to be even larger as the fluke castrates its unfortunate hosts and lowers their life expectancy. Over time, this would reduce the size of the whole snail population and give the algae an even greater chance to grow.

These snails' lost appetites ripple out through the entire habitat. Infected snails mean more algae, which provide more food for other invertebrates. The algae also crowd out the rocky real estate that barnacles attach themselves too, and the loss of barnacles reduces the numbers of blue mussels that coexist with them.

Even though it never comes into contact with these other tidal players, the fluke indirectly influences all of them. Nestled within the body of a snail, it pulls the strings of the entire ecosystem. This is one of the few instances where the effects of parasites on ecosystems have been carefully documented. Millions of similar dramas must play out all over the world, for half of the planet's species are parasitic. It's not our world, it's theirs.

Reference: Wood, Byers, Cottingham, Altman, Donahue & Blakeslee. 2007. Parasites alter community structure. PNAS 104: 9335-9339.

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edyong Sat, 02/28/2009 - 06:00

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Fascinating interconnections.

I'm telling you - parasites rule the world. We just don't know it yet.

So cool! It's particularly interesting because Littorina littorea is (probably) a non-native species introduced from Europe in the 19th century. I wonder how this affected the fluke population?

It's like the line in Bladerunner: "You know the score, pal. You're not a parasite, you're little people".

So cool and creepy! I am a web content taxonomist with NIAID, and as it happens I've been working my way through parasites recently.
They fascinate and repulse me -- which I'm sure is a rather typical reaction :)

When I tried looking up more on this particular parasite, Google rather helpfully suggested that Cryptocytole lingua should be spelled Cryptocotyle lingua. The results for your version only came up with this page and the original post at the old wordpress site, and another page that no longer exists

I was trying to check whether this was the same snail parasite that was discussed in Carl Zimmer's Parasite Rex, which when it infected fish after being in a snail, appeared to make them turn on their sides to reflect sunlight off their bodies more often than usual to attract birds.. been meaning to dig that book out again anyway!

Zombie periwinkles creeping over the ocean floor howling "Brine!"

Wow. And people say that the small wont rise and take the day. Now wouldn't it just be too bad if we discovered that these wonderful little worms were affecting humans in some obscure but significant way?

I don't think the periwinkles would mind.

Mud time capsules show evolutionary arms race between host and parasite

edyong — Wed, 11 Feb 2009 07:30:36 +0000

Mud time capsules show evolutionary arms race between host and parasite

This is the fifth of eight posts on evolutionary research to celebrate Darwin's bicentennial.

Life can sometimes be a futile contest. Throughout the natural world, pairs of species are locked in an evolutionary arms race where both competitors must continuously evolve new adaptations just to avoid ceding ground. Any advantage is temporary as every adaptive move from a predator or parasite is quickly neutralised by a counter-move from its prey or host. Coerced onward by the indifferent force of natural selection, neither side can withdraw from the stalemate.

These patterns of evolution are known as Red Queen dynamics, after the character in Lewis Carroll's Through the Looking Glass who said to Alice, "It takes all the running you can do, to keep in the same place." These arms races are predicted by evolutionary theory, not least as an explanation for sex. By shuffling genes from a mother and father, sex acts as a crucible for genetic diversity, providing a species with the raw material for adapting to its parasites and keep up with the arms race.

We can see the results of Red Queen dynamics in the bodies, genes and behaviours of the species around us but actually watching them at work is another matter altogether. You'd need to study interacting species over several generations and most biologists have neither the patience nor lifespan to do so. But sometimes, players from generations past leave behind records of the moves they made. Ellen Decaestecker and colleagues from Leuven University found just such an archive in the mud of a Belgian lake.

The lake is home to a small crustacean called a water flea (Daphnia magra) and a parasitic bacteria Pasteuria ramosa that lives inside it. Both species can undergo dormant states, and Decaestecker found that the lake's sediment preserves members of this sleeping fauna from up to 39 years ago. Every layer of sediment acts as a time capsule, preserving members from previous generations

Decaestecker sampled cylinders of sediment form the lake and revived dormant Daphnia eggs and parasite spores from different levels, representing intervals of 2-4 years. With these, she managed to hatch living Daphnia and pit them against parasites from their past, present and future.

On average, she found that the bacteria infected the water fleas more successfully if they came from the same time period than if they hailed from the past. As time went on, the bacteria picked up new adaptations that made them more effective parasites.

But Decaestecker also found that bacteria from a flea's future were also less infectious than its contemporaries. It seems that the parasites' upper hand is short-lived for the fleas evolve their own counter-adaptations. As the bacteria continue to adapt to the changing defences of their hosts, they trade-off the ability to infect the current generation with their ability to infect the previous ones.

This particular race isn't quite as one-sided as it might appear for slowly. Over time, the bacteria didn't become any better at infecting the water fleas, but those that did caused more virulent disease. They produced more reproductive spores and millions of these take up the fleas' bodies and effectively castrate them. Over time, the reproductive success of the infected fleas fell.

Reference: Decaestecker, E., Gaba, S., Raeymaekers, J.A., Stoks, R., Van Kerckhoven, L., Ebert, D., De Meester, L. (2007). Host-parasite 'Red Queen' dynamics archived in pond sediment. Nature, 450(7171), 870-873. DOI: 10.1038/nature06291

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edyong Wed, 02/11/2009 - 02:30

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Parasites

The arms race analogy isn't entirely apt because the parasite needs the host to reproduce and the predator needs prey to reproduce. Otherwise we get what humans have done--overfishing, for example, to the point of extinction. If the predator developed an unbeatable mechanism for attacking prey, then subsequent generations would starve, no?

This seems to be in exact agreement with Hamilton's theory of the role of parasites in the evolutionary maintenance of sex. See "Narrow Roads of Gene Land", his amazing collected works. It is not really true that sex increases diversity: rather, as Hamilton's simulations showed, it allows the preservation of once and-future anti-parasite strategies that are temporarily useless in the cycle. Gradual improvement of strategies (plural) happens at a much slower scale than the fast evolutionary scale of the parasite. Very exciting confirmation of the theory, in fact! (Current views are that this is not the whole role of sex; still, it seems to be a major factor.)

Intrigued by Lilian's comment. Is that a prediction that evolutionary theory can make. That we should find examples of predators/parasites wiping out their prey/host populations because a mutated mechanism makes them unbeatable?

I would be careful about saying that any mutation is "unbeatable". Not that you did, but others have in the past and it's often a failure of imagination. As Lesley Orgel put it, "Evolution is cleverer than you.".

That being said, here's an example of a predator/prey relationship, where some populations of predators appear to have temporarily won the arms race. Note that I say temporarily. Also note that in this case, prey defence might not limit individual predators but prey population size will limit predator population size. The seemingly invincible snakes are still restrained by the fact that if they eat all their prey, they won't have enough food, their numbers will fall, giving the newts a chance to rebound. And so on.

It seems a little Lamarckian to say that parasites and hosts evolve new adaptations in an arms race, is it not? A parasite invades a host and reproduces. Some members of that parasite species have the adaptations necessary to survive the onslaught of the immune system or other defense mechanism of the host, so they survive and reproduce, making it more likely that their offspring will live--as long as they don't kill the host species. The host species' members with the adaptations necessary to survive the onslaught of the parasitic invation will survive to reproduce, making their offspring more likely to survive, as well. What happens over time is that the two species will become more specialized and dependent on each other, and any adaptation that makes it more likely for some of one or the other species to die makes it certain that only particular members of each species will survive to reproduce. That's why past and future parasites cannot be successful in present hosts; they are specialized to survive in a host with particular adaptations because of the adaptations they have themselves. Parasites tend to survive because of rapid reproduction, which tends to make their genetic variation more rapid, as well, since some genes are added with each generation. It's not likely that parasites will go away in a given host species, regardless of the rapidity of host variation.

Species go extinct all the damn time. Can't parasite pressure be a factor, even a major factor, or even the factor, in an extinction? Is there any inherent natural limit on how damaging a parasite's innovation may be? That we don't notice this much must be selection bias: we mostly see processes that last, and most of the sudden extinctions (that we didn't arrange ourselves!) happened before we came along.

Furthermore, parasites/predators can often switch to another, possibly less hospitable, host if they eliminate the present favorite, so their numbers need not decline as fast as their prey's. Finally, a very few remaining exclusive predators may suffice to finish off the prey population if it reproduces slowly enough.

For copious examples of extinction by predation, examine any Pacific island. For near-historical drama, consider humans vs. the passenger pigeon.

Sometimes arms races are won, and sometimes the victory is Pyrrhic.