What Can Asexual Lineage Age Tell Us about the Maintenance of Sex?

Abstract

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.

Spatial and temporal escape from fungal parasitism in natural communities of anciently asexual bdelloid rotifers

“…Lineages that abandon sex are relieved of the costs of making males, the genetic load of recombination, and the energy and health costs of finding and competing for mates [3]. Nevertheless, obligately asexual plant and animal lineages are very rare, and nearly always short-lived compared with the sexual clades in which they arise [4]. Various explanations have been suggested for the scarcity of asexuals, but current theoretical and empirical evidence supports a key role for antagonistic coevolutionary interactions [5 -7].…”

General environmental heterogeneity as the explanation of sexuality? Comparative study shows that ancient asexual taxa are associated with both biotically and abiotically homogeneous environments

“…(2003), Neiman et al. (2009), Schurko et al. (2009), Schwander and Crespi (2009), and Speijer et al.…”

“…Some researchers consider a lineage to be AA if it reproduces obligately asexually for at least 50,000 generations or 0.5 million years (Law & Crespi, 2002a); some prefer one million generations (Schwander, Henry, & Crespi, 2011), yet others just speak about “millions of years” (Judson & Normark, 1996; Normark et al., 2003). It was even suggested that AAs are not substantially different from other asexuals and their delimitation is more or less arbitrary (Neiman et al., 2009). It is not the aim of this study to argue for the substantial difference of AAs from other asexuals or against it.…”

“…As we outlined in the previous section, the phenomenon of asexual “terminal twigs contra ancient asexuals” is still somewhat controversial, and its real existence is being discussed (see, e.g., Janko, Drozd, & Eisner, 2011; Neiman, Meirmans, Meirmans, Schlichting, & Mousseau, 2009; Schön, Martens, & Rossi, 1996; Schwander & Crespi, 2009). Regardless of these discussions, it is obvious that out of all the secondary asexual clades only the AAs have been able to survive or even diversify in an asexual state for millions of years (Judson & Normark, 1996; Neiman et al., 2009; Normark, Judson, & Moran, 2003; Schurko, Neiman, & Logsdon, 2009; Schwander & Crespi, 2009).…”

“…Regardless of these discussions, it is obvious that out of all the secondary asexual clades only the AAs have been able to survive or even diversify in an asexual state for millions of years (Judson & Normark, 1996; Neiman et al., 2009; Normark, Judson, & Moran, 2003; Schurko, Neiman, & Logsdon, 2009; Schwander & Crespi, 2009). This is the main reason that our study is based exclusively on AAs as they already proven to be evolutionarily viable in the long term.…”