Recombination in bdelloid rotifer genomes: asexuality, transfer and stress.

Bdelloid rotifer genomes show signatures of asyngamous recombination leading to loss of heterozygosity. This has been hypothesised to occur via chromosome pairing during egg formation through a meiosis-like but non-reductional process. Evidence of genetic exchange in bdelloid populations requires further validation in our view. Around 10% of bdelloid genes have been acquired horizontally from other kingdoms of life; the mechanisms and consequences of high horizontal gene transfer (HGT) prevalence remain to be confirmed by functional and comparative studies. Bdelloids tolerate desiccation and radiation stress, and appear to repair the resulting DNA double-strand breaks by different mechanisms in the soma versus germline; other aspects of stress tolerance remain to be investigated. Links between parthenogenesis, desiccation tolerance and horizontal gene transfer have been hypothesised, but several key steps remain to be tested. Bdelloid rotifers constitute a class of microscopic animals living in freshwater habitats worldwide. Several strange features of bdelloids have drawn attention: their ability to tolerate desiccation and other stresses, a lack of reported males across the clade despite centuries of study, and unusually high numbers of horizontally acquired, non-metazoan genes. Genome sequencing is transforming our understanding of their lifestyle and its consequences, while in turn providing wider insights about recombination and genome organisation in animals. Many questions remain, not least how to reconcile apparent genomic signatures of sex with the continued absence of reported males, why bdelloids have so many horizontally acquired genes, and how their remarkable ability to survive stress interacts with recombination and other genomic processes. [ABSTRACT FROM AUTHOR]