evolgen

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Not all animals must have sex with another individual to produce perfectly viable offspring. And neither do humans, thanks to technological breakthroughs in artificial insemination. But what about those critters that do not require masturbation and meat basters to produce babies sans contact with another individual? Remarkably, this is quite common in the animal kingdom, although different animals go about doing it in different ways.

Caenorhabditis elegans, the roundworm that has become a popular model in developmental biology, lives in populations made up almost entirely of hermaphrodites. There are no true females, and only 1% of all individuals are male. A hermaphrodite will usually mate with itself (i.e., they self-fertilize), but the occasional male does get lucky and inseminates a hermaphrodite. Why males even exist is up for speculation, but this is not parthenogenesis.

For a real example of parthenogenesis, we should take a look at the hymenopterans, an order of insects that includes bees, wasps, and ants. Many of these guys are notable for the fact that unfertilized eggs give rise to males, while fertilized eggs gives rise to females. Because the males are born from unfertilized eggs, this is parthenogenesis. The females, on the other hand, are produced the same way most sexually reproducing animals make females — via the combination of a sperm (the male gamete) with an ovum (the female gamete). Therefore, the males are haploid (they have one copy of the genome, inherited from their mother), and the females are diploid (they have two copies of the genome, one from each parent).

Males born via parthenogenesis in hymenopterans can be thought of as half-clones of their mothers, and fathers share their entire genome with their daughters. This leads to interesting relationships between relatives because the genetic relatedness is not reciprocal. For example, while males have 100% of their genome in common with their mothers and daughters (as opposed to 50% in other sexually reproducing organisms), the mothers and daughters are only 50% related to their sons and fathers. Additionally, females are more closely related to their sisters than to their daughters, which may explain why worker females are so cooperative in bee hives.

And there are the exceptional cases, like that of a Komodo dragon last year (see here for a summary of the gory details and speculation on how it happened, and here for a post from Sandy). Unlike the hymenopterans, parthenogenesis in reptiles is not a default state, but results from accidents in the creation of the ovum. In bees, wasps, and ants, the males develop from haploid gametes that have not been inseminated by a male. In reptiles, however, the virgin births result from a misstep in the production of the ovum causing it to be diploid. These reptiles wouldn’t survive as haploids, but, because the ovum has a complete complement of genes and chromosomes, it can develop into a healthy animal.

So, what one means by “virgin birth” determines how someone can answer the question of what’s the deal with it. There are multiple ways to give rise to an animal without mating, only one of which is parthenogenesis. Parthenogenesis can be the default state (as in hymenopterans) or it can be a quirky event that is touted as exceptional (as in reptiles).

Comments

  1. #1 Jim Thomerson
    December 3, 2007

    Parthenogenisis is fairly common in Poecillid fishes and in lizards. Also occurs occasionally in turkeys (read that someplace.) Poecilia formosa is a sexual parasite, requiring copulation with another poecillid male to activate, but not fertilize, the egg. Parthenogenic species of fish and lizards are all, or mostly, of hybrid origin. There is one rivulid which has populations with varying percentages of self-fertilizing hermaphrodites.

  2. #2 ihateaphids
    December 3, 2007

    Don’t forget about the aphids!! Aphids are some of the most awesome parthenogens out there. They have what is known as ‘telescoping generations’, which essentially means that the parthenogenetic offspring developing within an individual is already developing her own parthenogenetic offspring! It’s like a homunculus.

  3. #3 Eric Pedersen
    December 3, 2007

    Unfortunately, the whole “more related to sisters than the mothers” thing doesn’t really work well in honeybees. It’s pretty much dependant on queens mating only once, and honeybees are ridiculously polyandrous: each queen mates with an average of 10-12 males, and have been known to mate with up to 24 males. (see http://tiny.cc/Gpsqm for instance). It may be a nice explanation for where altruism got its start in the hymenopterans, but it doesn’t really explain why they’ve stayed altruistic.

  4. #4 Jim Thomerson
    December 3, 2007

    Worker bees are sterile females and don’t have daughters. The only way a worker bee can hope to pass her genes on is by helping her mother, the Queen. So the worker bee has only inclusive fittness and no simple Darwinian fitness. Queens are solitary and may well have killed sisters, daughters, or mother in the process.

  5. #5 Eric Pedersen
    December 3, 2007

    There is actually one way workers can pass on their genes independent of the queen: laying workers. Worker bees have the ability to lay unfertilized drone eggs. You see this in hives where the queen has died, and the workers haven’t managed to raise a new queen. You can get laying workers in queen-right hives, but the other workers will generally chew the eggs up.

    Basically, this means that your average hive is chock-full of workers that would happily try and increase their own fitness, but aren’t willing to let any other worker increase its own fitness at their expense.

  6. #6 Jim Thomerson
    December 4, 2007

    A side comment: there was an apiary at the Abby of St. Thomas. Gregor Mendel had plans to extend his genetic work to animals by working on bees. Fortunately he got diverted and did not do so. No doubt he would have been completely confused. If you get a chance to visit the Abby, there is a very interesting Mendel Museum there.

  7. #7 Ian
    December 6, 2007

    Since you’ve begun to discuss issues with determining what’s a species, would you consider writing a blog post on just how species are differentiated in asexual organisms, especially with regard to viral and bacterial organisms? I’d sure appreciate it and I think many others would too.

    Or should this be another “Ask a science blogger” question?!

  8. #8 RPM
    December 6, 2007

    Ian, check out John Wilkin’s blog, Evolving Thoughts, for discussions of species concepts. And go ahead and submit that as an Ask a Science Blogger question. There are multiple bloggers here that are (somewhat) qualified to answer it.

  9. #9 Ian
    December 10, 2007

    Thanks for the pointer. Wilkins has material there that I’ve obviously missed, and can more than likely sate my curiosity. Thanks!

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