Genome Evolution of Asexual Organisms and the Paradox of Sex in Eukaryotes

The predominance of sex in eukaryotes is still enigmatic. Sex, a composed process of meiosis and mixis cycles, confers high costs but the selective advantages remain unclear. In this review, we focus on potentially detrimental effects of asexuality on genome evolution. Theory predicts that asexual lineages should suffer from lack of meiotic DNA repair, accumulation of deleterious mutations, proliferation of transposable elements, among others. Here, we compare the different genomic features, life cycles, developmental pathways, and cytological mechanisms in the major eukaryotic groups, i.e., in protists, animals, fungi, and plants. In general, it is difficult to disentangle lineage-specific features from general features of asexuality. In all groups, forms of asexuality are predominantly facultative or cyclical. A variety of mixed or partial sexual developmental pathways exists, maintaining some components of sexuality, while obligate asexuality appears to be rare in eukaryotes. The strongest theoretical prediction for negative consequences of asexuality is decreased effectiveness of selection compared to sexuality. While some studies have shown increased rates of mutation accumulation in asexuals, others using whole-genome comparisons did not find this pattern. Various mechanisms exist that can alleviate the negative consequences of accumulation of negative mutations. More empirical data are needed to understand comprehensively the role of genome evolution for the maintenance of sex.