Do males pay for sex? Sex-specific selection coefficients suggest not

Abstract

Selection acting on males can reduce mutation load of sexual relative to asexual populations, thus mitigating the twofold cost of sex, provided that it seeks and destroys the same mutations as selection acting on females, but with higher efficiency. This could happen due to sexual selection-a potent evolutionary force that in most systems predominantly affects males. We used replicate populations of red flour beetles (Tribolium castaneum) to study sex-specific selection against deleterious mutations introduced with ionizing radiation. We found no evidence for selection being stronger in males than in females; in fact, we observed a nonsignificant trend in the opposite direction. This suggests that selection on males does not reduce mutation load below the level expected under the (hypothetical) scenario of asexual reproduction. Additionally, we employed a novel approach, based on a simple model, to quantify the relative contributions of sexual and offspring viability selection to the overall selection observed in males. We found them to be similar in magnitude; however, only the offspring viability component was statistically significant. In summary, we found no support for the hypothesis that selection on males in general, and sexual selection in particular, contributes to the evolutionary maintenance of sex.

Sexual selection on spontaneous mutations strengthens the between‐sex genetic correlation for fitness

“…This result has been confirmed in a series of D. melanogaster studies that test for sex-specific fitness effects of mutation accumulation Sharp and Agrawal 2013), inheritance of classical mutations (Sharp and Agrawal 2008;Hollis et al 2009), and inbreeding depression (Morrow et al 2008;. However, observations in two different species of seed beetles based on radioactively induced mutations have provided mixed results (Grieshop et al 2016;Prokop et al 2017). In the context of our experiment on Drosophila serrata, we therefore expected to see a greater fitness decline in males than in females in our limited selection (LS) lines when compared against the ancestor.…”

The contribution of the mitochondrial genome to sex‐specific fitness variance

“…; Prokop et al. ), but if so, males should exhibit higher fitness variance than females, regardless of the mode of inheritance. The prevalence of male‐biased fitness effects among random mutations poses a challenge for empirically testing for elevated male fitness variance due to maternal inheritance.…”

“…Second, Drosophila mutation-accumulation experiments (see above) indicate that the fitness effects of nuclear-encoded mutations may typically be larger for males than for females (see Mallet et al 2011;Sharp and Agrawal 2013). It is unclear whether this specific pattern is widespread in other populations or species (e.g., Power and Holman 2015;Grieshop et al 2016;Prokop et al 2017), but if so, males should exhibit higher fitness variance than females, regardless of the mode of inheritance. The prevalence of male-biased fitness effects among random mutations poses a challenge for empirically testing for elevated male fitness variance due to maternal inheritance.…”

Perception costs of reproduction can magnify sexual selection

“…At a more conceptual level, understanding what factors modulate sexual selection is equally important. For example, sexual selection in males has been suggested to ameliorate or even balance the twofold costs of sex to females via the purging of deleterious mutations 9 , but whether selection on males is strong enough for the maintenance of sex remains an open question 10 . Given that the underlying mechanisms described so far appear to be highly conserved 4,5 , we contend that perception costs of reproduction could play an important yet previously unrecognized role in the evolution of male and female adaptations and life histories.…”