Field of Science

Bdelloid Rotifers-Ancient Asexuals?

It appears that I started this blog just in time: in a paper just-out today in PLoS Biology, entitled Independently Evolving Species in Asexual Bdelloid Rotifers Fontaneto et al. describe their recent results on the "scandalous" bdelloid rotifers:
"The evolution of distinct species has often been considered a property solely of sexually reproducing organisms. In fact, however, there is little evidence as to whether asexual groups do or do not diversify into species. We show that a famous group of asexual animals, the bdelloid rotifers, has diversified into distinct species broadly equivalent to those found in sexual groups. We surveyed diversity within a single clade, the genus Rotaria, from a range of habitats worldwide, using DNA sequences and measurements of jaw morphology from scanning electron microscopy. New statistical methods for the combined analysis of morphology and DNA sequence data confirmed two fundamental properties of species, namely, independent evolution and ecological divergence by natural selection. The two properties did not always coincide to define unambiguous species groups, but this finding is common in sexual groups as well. The results show that sex is not a necessary condition for speciation. The methods offer the potential for increasing our understanding of the nature of species boundaries across a wide range of organisms."
In other words, these data suggest that bdelloids seem to evolve just like "normal" sexual eukaryotic species. Hmm...if it quacks like a duck...
The article is accompanied by a commentary entitled Who Needs Sex (or Males) Anyway? by Liza Gross and the study has already been picked up by the media, including this story on Science Daily.

Thanks to Coturnix for the lead. Thanks also to David Mark Welch & Matthew Meselson for the image: see Meselson's website for the legend.


  1. I'm confused by:
    "The evolution of distinct species has often been considered a property solely of sexually reproducing organisms. In fact, however, there is little evidence as to whether asexual groups do or do not diversify into species."

    doesn't this imply that there would only be one asexual species?

  2. The main issue is how to distinguish such "species" one from the other if they are not reproducing sexually. The root of this "problem" is in applying the biological species concept. This is a serious issue in defining "species" in prokaryotes (well, at least some people think it's serious). In sum, it gets down to "what is a species?". In this case, these asexual organisms appear to be separate species in the same way that sexuals differ from one another (both morphologically and molecularly).

  3. Are you arguing for cryptic sexual reproduction in bdelloid rotifers? Are people jumping to conclusions without a good understanding of life history?

  4. rpm, you got my drift. maybe the ? in the post title tipped you off...

    Yes, I am skeptical of the asexual status of bdelloid rotifers. It's not for lack of looking: people have been doing so for more than a century and still there is no evidence for sex or males. More recently, molecular data have been brought to bear on the question. This is mostly the work or David & Jessica Mark Welch and Matthew Meselson. These data are all consistent with asexuality.

    But as I am very fond of saying: "Absence of evidence is not evidence of absence". Some other putative ancient asexuals have been recently de-bunked (e.g., darwinulid ostracods). So, even though the bdelloid life histories are reasonably well-understood, we may still have something to learn.

    As for the paper at hand, I'm not criticizing it per se. I think that the data and analyses are quite well-done. The problem I have centers on what the initial assumption is and how that affects the conclusion. If bdelloids are indeed asexual, then this is a surprising and novel answer. If they are sexual, then the answer they obtained is the expected one.

  5. Can't you come to the same conclusion by (e.g.) noting that Gram positive and Gram negative bacteria exist?

    They are morphologically and molecularly distinct. They are not sexual. However they are sufficiently similar to be sure beyond reasonable doubt that they shared a common ancestor at some point.

    Therefore asexual organisms can speciate.

    How can anyone *not* believe that asexual organisms can speciate/diversify? There exists a diversity of asexual organisms, and yet we believe them to share common ancestry!

    I must be missing something.

  6. I also suspect that bdelloids may have very cryptic males. The darwinuloid ostracods were another maleless group, but not too long ago lost that distinction. I had a relevant discussion of this topic in a recent post.

  7. Mr. Logsdon, could you please explain why it was/is believed that new species cannot evolve from asexual organisms? Is it because asexual reproduction does not allow for enough genetic variation? And if so, does that mean that asexual organisms diversifying into new species indicates that it is not natural selection?

  8. I'm sorry for the delayed reply here, but sometimes procrastination pays. It turns out that John Wilkins has posted recently on the topic of microbial species concepts on his blog Evolving Thoughts.

  9. If rotifers did have sex, would they tag you .. I don't know but I hereby tag you.


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