On April 24, 2006, PED Seminar Series Presents

Why does sexual reproduction exist?

by Professor Matthew Meselson

Sexual lineages almost certainly extend without interruption back to the common ancestor of sexually-reproducing species. In contrast, asexual lines occasionally arise from sexual populations and may initially thrive but almost invariably are short-lived on an evolutionary time scale. Although there are numerous hypotheses, what drives the early extinction of asexual lines and why sex can prevent it are fundamental unsolved problems of biology.

Apparently also tracing continuously back to the origin of sexually-reproducing organisms are parasitic retrotransposons – mobile genetic elements that insert themselves into chromosomes and are present in virtually all eukaryotes. Although such elements reproduce autonomously, their increase is constrained by processes dependent on sexual reproduction. We have therefore proposed that upon the abandonment of sexual reproduction the number of such elements will increase indefinitely, eventually driving the asexual lineage to extinction and therefore contributing importantly to the advantage of sex.

This speculation is supported by the lack of retrotransposons in bdelloid rotifers – a class of invertebrate animals that has persisted and evolved for tens of millions of years apparently without sexual reproduction. Including 370 identified species, bdelloid rotifers are characteristically found in the water films of mosses and lichens, in rain gutters and in other ephemerally aquatic habitats. Unlike nearly all other metazoans, bdelloids are able to thrive in such habitats because they can survive desiccation at any stage of their life cycle by entering a metabolically quiescent state of anhydrobiosis from which they are able rapidly to recover upon rehydration. We have recently found that, like the desiccation and radiation-resistant bacteria of genus Deinococcus and unlike nearly all other organisms, bdelloids can repair and recover from very high levels of radiation-induced DNA breakage. This suggests that it is the adaptation of bdelloids to desiccating habitats and its associated DNA damage and repair that has depleted their genomes of retrotransposons and made it possible for them to avoid the early extinction suffered by other asexuals.

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