Ancient asexuals: darwinulids not exposed

Sir

The paradox of sex, using bdelloid rotifers as an example of ancient asexuality, is highlighted in your News Feature 'Sex-starved and still surviving' (Nature 452, 678–680; 2008). But bdelloids are not the only certain ancient asexuals — there are other contenders for the title.

The box on darwinulid ostracods does no justice to research findings on other putative ancient asexuals. Sex in darwinulids has not been conclusively demonstrated. The three males in a single species of the darwinulid genus Vestalenula, found among thousands of females, did not have identifiable sperm (nor did any of the investigated females), suggesting that these specimens are non-functional atavisms.

Perhaps the most important result to emerge from the discovery of these males is that a ghost male of the model darwinulid species Darwinula stevensoni, supposedly described in 1870, can now be dismissed, because the morphology of the copulatory appendages of the Japanese males indicates that the presumed penes of the D. stevensoni male were different parts of its anatomy.

Neither does D. stevensoni bear any genetic signature of recombination. You correctly note that it has very low nuclear genetic variability, but fail to mention that this is expected of any organism, asexual or sexual, that has active gene-homogenizing mechanisms, such as gene conversion or efficient DNA repair.

On the basis of the available evidence, bdelloid rotifers are ancient asexuals. Likewise, the species D. stevensoni has now been asexual for about 25 million years. To dismiss all darwinulids as putative asexuals is as wrong as claiming ancient asexual status for the group as a whole.

See also If you don't need change, maybe you don't need sex

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Martens, K., Schon, I. Ancient asexuals: darwinulids not exposed. Nature 453, 587 (2008) doi:10.1038/453587b

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