Evolution and speciation without sex in bdelloid rotifers

Summary

The possibility for independently evolving entities to form and persist in the absence of sexual recombination in eukaryotes has been questioned; nevertheless, there are organisms that are notorious to be asexual and to have apparently diversified into multiple species as identified by taxonomists. These organisms have therefore been named as an evolutionary paradox. We explore three alternative hypotheses attempting to solve the apparent paradox, focusing on bdelloid rotifers, the most studied group of organisms in which all species are considered asexual: (1) they may have some hidden form of sex; (2) species do not represent biological entities but simply convenient names; (3) sex may not be a necessary requirement for speciation. We provide ample evidence against the first two hypotheses; then, we explore the role of sex in speciation comparing bdelloid and monogonont rotifers, and conclude with some caveats (e.g. the recently discovered role of horizontal gene transfer and the interpretation of genomic evidence) that could still change our understanding of speciation in asexual animals.