Sexual reproduction in bdelloid rotifers

Nearly all eukaryotes reproduce sexually, either constitutively or facultatively, and nearly all that are thought to be entirely asexual diverged recently from sexuals, implying that loss of sex leads to early extinction and therefore that sexual reproduction is essential for evolutionary success. Nevertheless, there are several groups that have been thought to have evolved asexually for millions of years. Of these, the most extensively studied are the rotifers of Class Bdelloidea. Yet the evidence for their asexuality is entirely negative -- the failure to establish the existence of males or hermaphrodites.

Opposed to this is a growing body of evidence that bdelloids do reproduce sexually, albeit rarely, retaining meiosis-associated genes and, in a limited study of allele sharing among individuals of the bdelloid Macrotrachela quadricornifera, displaying a pattern of genetic exchange indicating recent sexual reproduction. Here we present a much larger study of allele sharing among these individuals clearly showing the occurrence of sexual reproduction, thereby removing what had appeared to be a serious challenge to hypotheses for the evolutionary advantage of sex and confirming that sexual reproduction is essential for long term evolutionary success in eukaryotes.

We also discuss the relation between bdelloid life history and population structure and the possible benefit of outcrossing in restoring advantageous genome-wide epistatic interactions disrupted by conversion and deletion.