What can asexual lineage age tell us about the maintenance of sex?

Sexual reproduction is both extremely costly and extremely common relative to asexuality, indicating that it must confer profound benefits. This in turn points to major disadvantages of asexual reproduction, which is usually given as an explanation for why almost all asexual lineages are apparently quite short-lived. However, a growing body of evidence suggests that some asexual lineages are actually quite old. Insight into why sex is so common may come from understanding why asexual lineages persist in some places or taxa but not others. Here, we review the distribution of asexual lineage ages estimated from a diverse array of taxa, and we discuss our results in light of the main mutational and environmental hypotheses for sex. Along with strengthening the case for wide variation in asexual lineage age and the existence of many old asexual taxa, we also found that the distribution of asexual lineage age estimates follows a surprisingly regular distribution, to the extent that asexual taxa viewed as "scandalously" ancient merely fall on the high end of this distribution. We interpret this result to mean that similar mechanisms may determine asexual lineage age across eukaryotic taxa. We also derive some qualitative predictions for asexual lineage age under different theories for sex and discuss empirical evidence for these predictions. Ultimately, we were limited in the extent to which we could use these data to make inferences about the maintenance of sex by the absence of both clear theoretical expectations and estimates of key parameters.