The consequences of life without sex: an examination into taxonomy and evolution of the anciently asexual bdelloid rotifers

Abstract

The anciently asexual bdelloid rotifers are a ubiquitous and ecologically important group coined “evolutionary scandals” owing to their diversification and persistence over evolutionary time despite the pressures of obligate parthenogenesis. Understanding the biodiversity of rotifers, in the context of reproductive mode, will aid in the understanding of how differences in sex and ecology drive biodiversity patterns. Given that resolved taxonomy is a prerequisite to investigating larger scale macroevolutionary patterns, and that rotifer taxonomy is confounded by morphostasis, cryptic diversity, and a dearth of expert taxonomists, this thesis first deals with the use of DNA taxonomy as a tool to alleviate the taxonomic crisis so prevalent in the group. Results from multiple meta-analyses exploring the effects of choice in gene, phylogenetic reconstruction methods, and species delimitation metrics, provide better guidelines for DNA taxonomy. The analyses suggest that coalescent-based DNA taxonomy using the Generalised Mixed Yule Coalescent model in conjunction with the cytochrome oxidase 1 subunit c gene analysed in a Bayesian inference phylogenetic framework using BEAST, provides realistic and tangible species clusters (evolutionarily significant units; ESU) in both asexual and sexual organisms. Using these ESUs, the efficacy of DNA barcoding for identifying rotifer species is examined. These analyses suggest that the sexual monogonont rotifers are more readily identifiable than the asexual bdelloid rotifers. Explicit comparison of genetic discreteness of asexual and sexual rotifers indicates that sexual rotifers are separated by larger genetic and phylogenetic gaps than asexual rotifers and thus are more discrete. Combined, these analyses indicate that sex, specifically reproductive isolation, is the predominant factor in explaining why species form discrete clusters rather than a continuum of forms. Finally, a dated multilocus molecular phylogeny of Bdelloidea is reconstructed. The phylogeny suggests that bdelloid rotifers have persisted for at least 40 million years, but that bdelloid higher taxa are typically polyphyletic.