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Genomics: an animal reproducing asexually tells its secrets

​The genome of a bdelloid rotifer – a microscopic animal whose surprising capacity for survival and reproduction is the subject of much debate – has been sequenced and analyzed by an international consortium jointly directed by the CEA-Genoscope and the University of Namur. The results of this research – to which CNRS and INRA also contributed – have just confirmed biological and paleontological data suggesting that bdelloid rotifers have adopted asexual reproduction exclusively for tens of millions of years. This “evolutionary scandal”, hitherto viewed with skepticism by some members of the scientific community, has now been clarified. Analysis has shown that the bdelloid genome is not only incompatible with sexual reproduction, but that it also displays mechanisms allowing the animal to avoid the harmful genetic consequences of asexual reproduction.

Published on 22 July 2013

Asexual reproduction is often regarded as an evolutionary dead end in that it is thought to induce deleterious mutations (i.e. detrimental to the organisms affected) from one generation to the next, inevitably causing the species to die out. This explains why researchers have long been interested in bdelloid rotifers, microscopic animals whose method of reproduction appears to be exclusively asexual.

Analysis of the Adineta vaga bdelloid rotifer’s genome has proved that the animal is incapable of sexual reproduction, which requires homologous chromosomes from both parents carrying genes in the same order. The researchers discovered, however, that while Adineta vaga has two copies of genes, these are not in the same order and are sometimes even located on the same single chromosome. This means that, unlike other animal species sequenced until now, Adineta vaga has no homologous chromosomes. This organization is not compatible with the formation of gametes (reproductive cells), which are essential for sexual reproduction.The analysis also revealed abundant traces of gene conversions, which can be likened to a genetic “cut and paste” operation in which a copy of one or more genes is copied to and replaces another gene elsewhere in the genome. The authors suggest that this mechanism could gradually attenuate, if not totally eliminate, the accumulation of deleterious mutations.This would seem to put an end to the debate as to whether or not bdelloid rotifers are asexual. More importantly, the study suggests that scientists can now analyze the genome structure of a species to determine whether it reproduces sexually or asexually. If bdelloid rotifer species have managed to survive without sexual reproduction for millions of years, it is quite likely that other animals can do the same.This research thus challenges the commonly held belief that sexual reproduction is vital to the survival of animal species. Furthermore, it shows that asexual reproduction is also a viable long-term evolutionary strategy for certain animal species.

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