Better than sex

Asexual organisms are of fascination to biologists because most multicellular organisms have chosen sex in some form or other for reproduction. The interest stems from the clear theoretical advantage of asexual reproduction which results in the transfer of a full complement of genes to an offspring rather than the 50 per cent that results from sexual reproduction. Researchers are keen to determine why sexual reproduction persists. A major support for sex is that it offers a mechanism for purging the genome of debilitating mutations through recombination. But there are now more than 20 hypotheses concerning the advantages of sex, considered the 'queen of evolutionary problems'. But is it possible that some organisms have developed mechanisms to preserve their genetic integrity while reproducing asexually? New studies reported in the Proceedings of the Royal Society, London, (published on-line) suggest that one tiny crustacean may have achieved just this. Isobelle Schoen and Koen Martins at the Royal Belgian Institute of Natural Sciences in Brussels, have been studying the 1mm-long ostracod, Darwinula stevensoni, that is thought to have reproduced asexually for 20-25 million years. They have compared genetic sequences at three regions in the genome from individuals taken across a wide geographic range including South Africa, France, Italy, Belgium and Ireland. They compared the variation between individuals and also at one site with a related sexual species. Their results revealed significantly higher variation amongst the sexual species and remarkable similarity between D. stevensoni populations sampled at all of the regions they examined. But these results present a conundrum: studies on the well-known and long-asexual group of minuscule animals, the bdelloid rotifers thought to have been asexual for 30 million years, has found that there is genetic divergence between different populations which is thought to result from the lack of genetic interchange. Known as the 'Meselson effect' the variation is predicted as a marker of high allelic divergence. The authors have studied a number of possible reasons for the observed homogeneity in D. stevensoni and conclude that although gene conversion may be responsible for similarities at one of the sites they studied, mechanisms for enhanced DNA repair may be responsible for the homogeneity at the other sites. If this turns out to be the case then these small crustaceans may have beaten sex at its own ubiquitous and influential game. Sex secrets: The minute ostracod crustacean group contains species with a long history of asexual …