Numbers arent enough

Okay, so there are like 20,000 polar bears left.

4,000 tigers.

1,600 Pandas.

Meh, who cares, right? I mean, there are still some. 1,600 plus the ones in zoos. 'Endangered' animals are fine! Yeah... No. Minor problem with decreasing population numbers: Its more than just the numbers. Its genetic diversity within those numbers. If those 1,600 pandas are all we have left, and those 1,600 pandas are genetically similar, they are in big trouble.

Easy example? Tasmanian Devils. While there are still 20,000-50,000 Tasmanian Devils left, they are being slaughtered by an infectious tumor. An infectious tumor that is taking advantage of the fact there is very little genetic diversity in the remaining Devil population.

Ive talked about MHC molecules a couple times on this blag, but let me just post up a quick reminder-- There are two kinds of MHCs. MHC I is on the surface of allll of your cells. It contains little bits of the proteins your cells are making, just as an 'everything is normal' flag. If the cell becomes infected with a virus, it will start throwing up viral 'flags' so cytotoxic T cells know to kill it. MHC II molecules are only on the surface of certain immune cells-- ones that eat bacteria/pathogens. They put little bits of nommed up pathogen in MHC II to get other immune cells all riled up so they can clear the infection.

MHC I molecules are really, really important. Lots of viruses (including HIV-1) will down-regulate MHC I in the cells the infect in the hopes of 'hiding', but there are immune cells (natural killer cells) that recognize the lack of MHC I, and will kill a cell thats not sending up the 'everything is fine' flags.

MHC I are also important when it comes to transplants-- there are >800 different MHC I alleles in humans. If the MHC I molecules of the donor and the recipient are different, the transplant will be rejected. Thats why its so hard to find the perfect donor for all the people in need of transplants, and why a close relative is usually your best bet.

And, MHC I are important when it comes to cancer. Cancer cells do not present 'normal' proteins in their MHC Is. CTLs recognize when cancerous cells are presenting mutated proteins, waaaaay too much of a 'normal' protein, or proteins that are only supposed to be expressed in embryos (embryonic cells are supposed to multiply really fast, normal cells arent).

So to get back to the Tasmanian Devils, they are currently dealing with an epidemic of a tumor. No, not a virus that causes a tumor, an actual tumor cell is passed from Devil to Devil via bites and fights (thus its usually found on the Devils faces, and is called 'Devil Face Tumor Disease'). The tumor gets so large that the little guys eventually starve to death... Im not linking to pictures. Too sad...

But what is really odd about this tumor is that the Devils immune systems ignore it. Now, from what Ive just typed about MHC I, we know of a few ways the Devils immune systems should be knocking this tumor down.

1-- Tumors try to down-regulate MHC I. If this Devil tumor does the same thing, then it should be killed by natural killer cells.
2-- The tumor is essentially a 'transplant'. It is not self. The tumor should present non-self in MHC I and be killed by CTLs, just like if we gave someone who needed a heart transplant a baboon heart.
3-- The tumor is a tumor. Its presenting all kinds of crazy proteins that the Devils, with their healthy, normal immune systems, should be clearing just fine!

But they arent. Infected Devils die in about a year.


No genetic diversity. Remember how I said humans have over 800 different MHC I alleles? Tasmanian Devils... their MHC I? Um, its all the same.

Transmission of a fatal clonal tumor by biting occurs due to depleted MHC diversity in a threatened carnivorous marsupial (free online!!)

They took white blood cells from Devils from all over Australia and mixed them all up. Normally when I do that with human white blood cells, they get all pissed off. Turns into an immune cell version of 'Beat It' (which makes them easier to infect with HIV-1, for my experiments). Immunologists call it a 'mixed lymphocyte response'. Devil white blood cells? They all had a tea party. They loved each other. No mixed lymphocyte response. Functionally, the MHC I molecules from the entire Devil population are identical.

What does this mean? From the authors discussion:

DFTD is a transmissible tumor that spreads through a population due to a lack of histocompatibility barriers. The disease has progressed rapidly due to low diversity at MHC loci and the propensity of devils to bite each other around the face and mouth during mating and fights for food.
These findings reinforce the need for conservation biologists to focus on genetic diversity at functionally import ant loci that play a role in population fitness when designing conservation strategies. For devils, ensuring maximum MHC diversity in insurance populations is paramount. In light of the devils inability to recognize the clonal facial tumors as foreign and mount an immune response, the only course of action is the isolation of unaffected animals and the ongoing removal of affected animals from the population.

They also mention the lack of genetic diversity in cheetahs, seals, and a couple species of whales. There are, relatively, 'lots' of Devils left (maybe about 50,000). 'Lots' might not be enough to save them from extinction.

And there are 1,600 pandas.



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On the bright side, "4,000 Tigers" would make a good band name.

... I'm sorry. I got nothing. I really just get a sick feeling in the pit of my stomach when I think of the species that could go extinct in my lifetime. Or the ones that already have, like the Yangtze Dolphin.

By minimalist (not verified) on 22 Sep 2008 #permalink

Yoo, hypothetically, if humans had no histocompatibility barriers, tumors could spread from person to person the same way, but we could also interchange pretty much any vital organ so long as blood types were the same.

Oh, and I forgot to add, Panda conservation is taking millions upon millions of conservation dollars that could be better spent on other animals which can currently be saved from extinction and have the genetic diversity necessary to survive. I think if Chris Packham ever gets his giant panda dinner, I'd share...

Oh Jared! NOooooo....:-(
" I think if Chris Packham ever gets his giant panda dinner, I'd share..."

On a different, brighter note ...
One of my kittehs had a problem w/ her nose that truly resembled the tas devil affliction. After a couple grand was spent on her ...(steroids, anti-biotics every 2 months)- we started feeding her wet food. It went away. Apparently she just needed more fat in her diet. No more vet bills! Yay!! :-)

Sod it, I did not know that it was this bad; I knew that Pandas were in trouble, but I didn't know that tigers were so close to extinction, and the polar bears aren't in such good shape either.

Ah, this explains all the strange blisters I saw in Coyote Ugly's in Nashville, TN.

By Jimmy Page (not verified) on 22 Sep 2008 #permalink

So why are those prairie dogs they bred back from only seven individuals doing alright? (-ish)

The black-footed ferrets seem to be doing okay so far, but the reason the population dropped down to less than 20 (from just over 100) was precisely because of disease outbreaks. Not sure what's being done to prevent susceptibility to further outbreaks, if indeed anything can be done other than crossing one's fingers.

By minimalist (not verified) on 22 Sep 2008 #permalink

Minimalist,yea, the population reached 18, but those still had considerable diversity due to frozen sperm being used to fertilize eggs effectively adding some genetic diversity back into the population... Also, since they do live in fairly low human population density area, some small colonies may still exist in the wild. Remember Wyoming only has a human population density of 2/km^2 in average over the whole state, with most actually being uninhabited...

Abbie, I've got a question for you, I've been trying to find out of some of these endangered species have gene banks set up yet, but so far, I've only found gene banks for the cute and cuddly animals and for the plants. I was wondering if you knew of any others for the not-so-cute animals out there....

Thanks for that post Abbie,absolutely intriguing !
I was not aware that this facial tumour spreads that way,helped by a loss of genetic diversity.

I wonder what would happen if I bit into my identical twin's melanoma(hypothetically),or my brother's.Fascinating !

I saw a show about the cancerous devils a few years ago. It was fascinating but so very sad.

Some species may bounce back from near extinction and seem to do well, like the ferrets, but if they come across a disease they have little immunity to they'll be wiped out in a flash. Those who survived whatever disease(s) almost wiped out the species may have a natural immunity to those diseases, but there may be others they are highly vulnerable to. Even if we think we brought a species back from the edge, all it takes is one germ they no immunity to and they are gone. That's why diversity is so critical.

Wow, fascinating. Presumably the lack of any different MHC I's means that they can also share organs with relatively few problems.

It seems amazing that they have that little variation, shows how bad it's got i guess. I'll definately take a look at the linked paper, it sounds interesting...

Ah, ferrets! Sorry. And thank you. It musta been a decade since I saw that programme.

Poor buggers. Cuter than any panda.

Stacy, they have an odor, yes, but if they are neutered or spayed, it shouldn't really be strong unless you don't bathe them... I have one, he doesn't really have any smell since I had him neutered...

I find that some ferret owners smell worse than their pets. :)

By minimalist (not verified) on 23 Sep 2008 #permalink

The MHC protein issue also came up yesterday on

Debunking Creationism, a new blog spinoff from Debunking Christianity. There's a good video clip over there specifically about cheetahs.

(The founders of Debunking Christianity have also just started a blog called Debunking the Christian Right. Not much going on there yet, but worth a look in a few days - if Debunking Christianity is any indication, the new blogs are going to be good.)

By themadlolscien… (not verified) on 23 Sep 2008 #permalink

I've met my fair share of ferret owners, and yes, minimalist, you are correct in that assertion, many ferret owners do, in fact, mark their pets...

I wonder how much genetic diversity the devils had in the first place. Was there some sort of genetic bottleneck event in their recent evolutionary past? 50,000 individuals is a lot for them to all have the same MHC-Is!

@synapse: methinks you are correct
"Tasmanian devils are likely to have gone through a population bottleneck on the island 10,000 years ago during the last Ice Age, said McCallum, explaining their low diversity. Numbers may have plummeted to less than 500 animals at that time, as cool conditions pushed them into a few remaining suitable patches of habitat.

McCallum likened their diversity to that of cheetahs in Africa, which also suffered a bottleneck at some time in the past. One study in the 1980s showed that diversity was so low in the big cats something akin to that between close relatives in humans that skin grafts between unrelated animals would not be rejected. "

10,000 years ago? I don't think a bottleneck that long ago fully explains the low MHC diversity, when we compare with cases like this:

"The San Nicolas Island fox (Urocyon littoralis dickeyi) is genetically the most monomorphic sexually reproducing animal population yet reported and has no variation in hypervariable genetic markers. [...] We examine variation of five loci within the MHC of San Nicolas Island foxes and find remarkably high levels of variation. Further, we show by simulation that genetic monomorphism at neutral loci and high MHC variation could arise only through an extreme population bottleneck of <10 individuals, â10-20 generations ago, accompanied by unprecedented selection coefficients of >0.5 on MHC loci."

windy @ 29, it depends on the mutation rates and other factors which may be keeping that MHC the same, like, for example, other aspects of the immune system and other possible causes. Secondly, they do vary, just not enough and not where it counts.

There was a population decline in Devils as well as Thylacines in the early C20, attributed by some sources to a 'distemper-like disease' (e.g. mentioned in the Wikipedia thylacine article); I think the Quolls (Dasyurus spp.) also went through a rough patch at the same time. Tasmania is small enough and marginal enough that, over the course of the last 10,000 years, relatively large carnivores like these may have repeatedly been reduced to small numbers (as, possibly, were indigenous humans). If windy's example of the San Nicolas Island fox is comparable, MHC monomorphism in extant Devils may imply they almost completely died off in the relatively recent past (early C20 or earlier, certainly not later).

By John Scanlon FCD (not verified) on 23 Sep 2008 #permalink

John, #31

Tazzies were once hunted as pests up until relatively recently, and so their numbers may have dropped very low. At the same time I wonder about the 50,000 number. Google "tasmanian devil" and check the sites you find.

Follow Up

The figure seems to be around 50,000 tazzies, but the animal is now listed as endangered thanks to the DFTD parasite. That and competition from red foxes and domestic dogs.

Whereto tigers; is this the number in the wild, or does it include those held in captivity?

How close is state of the art germline engineering to producing devils with novel MHC sequences?

By Robert S. (not verified) on 01 Sep 2010 #permalink

Wasn't there a surgeon who cut his hand while removing a malignant tumour, and subsequently transplanted part of it onto that cut, where it then thrived and had to be removed? *goes to NEMJ*