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Counterintuition: Bdelloid Rotifers

A while back, it was noticed that there was an “ancient asexual scandal” (Judson 1996): Bdelloid Rotifers. These are cool little animals that live in ponds and streams, but which go against the received wisdom that sex is a hedge against environmental challenges that asexual organisms cannot have without some genetic sharing. So far as we know, these are entirely female (Birkey 2004, Welch 2004). And they have now gene sharing. For 80 million years or so. So they ought to be extinct. They aren’t.

There are several hypotheses on the board: one is that they are able to share genes occasionally when rehydrating (they can become entirely dried out during hot periods, only to rehydrate when the rains come (Arkhipova 2008). Another is that they undergo ionising radiation mutation that introduces novelties at a higher rate than usual (Gladyshev 2008). A third: the “spare” gene (since genes are paired in the double helix) is free to evolve independently (like a duplicated gene in sexual organisms, Pouchkina-Stantcheva 2007). It’s Ed’s post that inspires me to write this post.

Esoteric organisms challenge our conceptions in biology. Any general concept ought to include all the cases, including the weird ones. And here’s a hitch: according to the most widely used conceptions of species, bdelloids are not species. This is counterintuitive in two ways: it is opposed to the sexual reproductive conception of species, and it is counter to the notion that a concept like species must be general. It also challenges our presumption for the explanation of sex.

We ought not to think that a conception or definition or hypothesis that works in one part of biology must work in all others, and yet biologists themselves often behave as if this were true. That is another challenge: why is this? The answer, I believe, is that biology is both highly diverse, and also massive. Nobody can be more than basically informed about all of it. So, the temptation to overgeneralise from one’s experience of one’s own special group, whether that is birds or bees or botany, is overwhelming. After all, biologists are deeply evidence based: they study hard the organisms they specialise in. Ideas that are formed by evidence are hard to dislodge and easy to generalise (that makes sense, after all; you want evidence based ideas to be robust). Hence, in cases where the field (biology overall) is very particularistic, generalisations are likely to be less than useful.

Looking at asexuals, or lateral genetic transfer, or superorganisms, and so on, is a good antidote to the poison of overgeneralisation. Counterituition is needed in biology, as in any special science like history, economics, or anthropology. And, most of all, in philosophy.

Arkhipova, I., M. Meselson, and E. Gladyshev. 2008. Massive Horizontal Gene Transfer in Bdelloid Rotifers. Science 320:1210-1213.

Birky, C. W. 2004. Bdelloid rotifers revisited. Proceedings of the National Academy of Sciences USA 101 (9):2651-2652.

Gladyshev, E., and M. Meselson. 2008. Extreme resistance of bdelloid rotifers to ionizing radiation. Proceedings of the National Academy of Sciences 105 (13):5139.

Judson, Olivia P., and Benjamin B. Normark. 1996. Ancient asexual scandals. Trends in Ecology and Evolution 11 (2):41-46.

Pouchkina-Stantcheva, Natalia N., Brian M. McGee, Chiara Boschetti, Dimitri Tolleter, Sohini Chakrabortee, Antoaneta V. Popova, Filip Meersman, David Macherel, Dirk K. Hincha, and Alan Tunnacliffe. 2007. Functional Divergence of Former Alleles in an Ancient Asexual Invertebrate. Science 318 (5848):268-271.

Welch, J. L. M., D. B. M. Welch, and M. Meselson. 2004. Cytogenetic evidence for asexual evolution of bdelloid rotifers. Proceedings of the National Academy of Sciences USA 101 (6):1618-1621.


  1. Ingrid Jakobsen Ingrid Jakobsen

    Nitpick: the double helix doesn’t have anything to do with why there are two copies of the genes. Bdelloid rotifers, like many animals, are diploid: they have two copies of all their genes, where each copy includes both strands of the double helix.

  2. Jay Jay Jay Jay

    I think the problem lies in the definition of species. Many biologists have never heard of bdelloids or their lack of sex, so it’s possible when those biologists who defined a species a two groups incapable of interbreeding, they may not have known about these cute guys (or girls, as it may be).

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