Genome structure of bdelloid rotifers : shaped by asexuality or desiccation?

Abstract

Bdelloid rotifers are microscopic invertebrate animals best known for their ancient asexuality and the ability to survive desiccation at any life stage. Both factors are expected to have a profound influence on their genome structure. Recent molecular studies demonstrated that, while the gene-rich regions of bdelloid genomes are organized as co-linear pairs of closely related sequences and depleted in repetitive DNA, subtelomeric regions harbor diverse transposable elements and horizontally acquired genes of foreign origin. While asexuality is expected to result in depletion of deleterious transposons, only desiccation appears to have the power to produce all of the uncovered genomic peculiarities. Repair of desiccation-induced DNA damage would require the presence of a homologous template, maintaining co-linear pairs in gene-rich regions, and selecting against insertion of repetitive DNA which might cause chromosomal rearrangements. Desiccation may also induce a transient state of competence in recovering animals, allowing them to acquire environmental DNA. Even if bdelloids engage in rare or obscure forms of sexual reproduction, all these features could still be present. The relative contribution of asexuality and desiccation to genome organization may be clarified by analyzing whole-genome sequences and comparing foreign gene and transposon content in species which lost the ability to survive desiccation.