No sex, No problem? Mutation accumulation in asexual animals

Abstract

According to classical predictions, sex is beneficial because it allows for purging of slightly deleterious mutations via effective purifying selection. In asexual organisms mutational load increases over time, eventually leading to their genomic decay. Further, in the absence of sex, haplotypes are expected to accumulate mutations independently and diverge in parallel. This is commonly known as ‘the Meselson effect’ and regarded as strong support for truly asexual evolution. Several single gene studies tested accumulation of slightly deleterious mutations and the Meselson effect. However, genome-based studies of independent evolutionary replicates are essential to account for among-gene variance and lineage-specific effects. Such studies are missing for some important, i.e. well suited and long-term, asexual animals. In this thesis, I tested if both predictions, mutational load and haplotype divergence, are commonly met in asexual animals. For this, I generated and investigated genome data of different, asexual and closely related sexual arthropod species. While the effectiveness of purifying selection was decreased in asexual Timema stick insects, there was no overall difference between asexual and sexual representatives of eight major hexapod groups. Surprisingly, the mutational load in oribatid mites was even lower compared to their sexual relatives. Possibly, effective purging of slightly deleterious mutations is enabled by their very large population sizes. In the asexual oribatid mite species Oppiella nova , population genomic patterns including parallel haplotype divergence were consistent with the Meselson effect. This strongly supports its evolution in the absence of sex. In the discussion, I present a comparative overview of existing studies on mutation accumulation and haplotype divergence in animals. I show that accumulation of slightly deleterious mutations is not commonly found in asexual animals indicating that sex per se is not a prerequisite for their long term persistence. Some “scandalous” asexual animals may well have survived in the absence of sex for millions of years.