Sexual selection increased offspring production via evolution of male and female traits

Female-limited responses in remating rate and mating duration in the experimental evolution of a beetle Callosobruchus chinensis

Mating rate optima often differ between the sexes: males may increase their fitness by multiple mating, but for females multiple mating confers little benefit and can often be costly (especially in taxa without nuptial gifts or mala parental care). Sexually antagonistic evolution is thus expected in traits related to mating rates under sexual selection. This prediction has been tested by multiple studies that applied experimental evolution technique, which is a powerful tool to directly examine the evolutionary consequences of selection. Yet, the results so far only partly support the prediction. Here, we provide another example of experimental evolution of sexual selection, by applying it for the first time to the mating behaviour of a seed beetle Callsorobruchus chinensis. We found a lower remating rate in polygamy-line females than in monogamy-line (i.e. no sexual selection) females after 21 generations of selection. Polygamy-line females also showed a longer duration of first mating than monogamy-line females. We found no effect of male evolutionary lines on the remating rate or first mating duration. Though not consistent with the original prediction, the current and previous studies collectively suggest that the observed female-limited responses may be a norm, which is also consistent with the conceptual advances in the last two decades of the advantages and limitations of experimental evolution technique.

Selection in males purges the mutation load on female fitness

Theory predicts that the ability of selection and recombination to purge mutation load is enhanced if selection against deleterious genetic variants operates more strongly in males than females. However, direct empirical support for this tenet is limited, in part because traditional quantitative genetic approaches allow dominance and intermediate-frequency polymorphisms to obscure the effects of the many rare and partially recessive deleterious alleles that make up the main part of a population's mutation load. Here, we exposed the partially recessive genetic load of a population of Callosobruchus maculatus seed beetles via successive generations of inbreeding, and quantified its effects by measuring heterosis-the increase in fitness experienced when masking the effects of deleterious alleles by heterozygosity-in a fully factorial sex-specific diallel cross among 16 inbred strains. Competitive lifetime reproductive success (i.e., fitness) was measured in male and female outcrossed F1s as well as inbred parental "selfs," and we estimated the 4 × 4 male-female inbred-outbred genetic covariance matrix for fitness using Bayesian Markov chain Monte Carlo simulations of a custom-made general linear mixed effects model. We found that heterosis estimated independently in males and females was highly genetically correlated among strains, and that heterosis was strongly negatively genetically correlated to outbred male, but not female, fitness. This suggests that genetic variation for fitness in males, but not in females, reflects the amount of (partially) recessive deleterious alleles segregating at mutation-selection balance in this population. The population's mutation load therefore has greater potential to be purged via selection in males. These findings contribute to our understanding of the prevalence of sexual reproduction in nature and the maintenance of genetic variation in fitness-related traits.

Environmental drivers of sexual dimorphism in a lizard with alternative mating strategies

Understanding the relative importance of sexual and natural selection in shaping morphological traits is a long-standing goal of evolutionary ecology. Male-biased sexual size dimorphism (SSD) is typically associated with male-male competition. Similarly, male polymorphisms are considered a consequence of competitive social interactions. This classic paradigm overlooks the fact that environmental factors mediate social interactions and can lead to ecological adaptations. Common side-blotched lizards, Uta stansburiana, are a model system for this paradigm due to well-known rock-paper-scissors social dynamics between male morphs. SSD in this species has been considered primarily a consequence of social interactions, with male size resulting from the number of morphs in each population and female size being constrained through fecundity benefits. We test if the environment explains intraspecific variation in SSD and number of male morphs in U. stansburiana. By compiling data from 49 populations, we show that environmental variables are stronger predictors of SSD than the number of male morphs. Similarly, we show that the environment mediates SSD and potentially contributes to morph loss in colder environments. We propose that the environment favours smaller males in areas of high seasonality. Our results demonstrate the importance of the environment as a mediator of SSD.