Sex‐specific plasticity and genotype × sex interactions for age and size of maturity in the sheepshead swordtail, Xiphophorus birchmanni

Abstract

Responses to sexually antagonistic selection are thought to be constrained by the shared genetic architecture of homologous male and female traits. Accordingly, adaptive sexual dimorphism depends on mechanisms such as genotype‐by‐sex interaction (G×S) and sex‐specific plasticity to alleviate this constraint. We tested these mechanisms in a population of Xiphophorus birchmanni (sheepshead swordtail), where the intensity of male competition is expected to mediate intersexual conflict over age and size at maturity. Combining quantitative genetics with density manipulations and analysis of sex ratio variation, we confirm that maturation traits are dimorphic and heritable, but also subject to large G×S. Although cross‐sex genetic correlations are close to zero, suggesting sex‐linked genes with important effects on growth and maturation are likely segregating in this population, we found less evidence of sex‐specific adaptive plasticity. At high density, there was a weak trend towards later and smaller maturation in both sexes. Effects of sex ratio were stronger and putatively adaptive in males but not in females. Males delay maturation in the presence of mature rivals, resulting in larger adult size with subsequent benefit to competitive ability. However, females also delay maturation in male‐biased groups, incurring a loss of reproductive lifespan without apparent benefit. Thus, in highly competitive environments, female fitness may be limited by the lack of sex‐specific plasticity. More generally, assuming that selection does act antagonistically on male and female maturation traits in the wild, our results demonstrate that genetic architecture of homologous traits can ease a major constraint on the evolution of adaptive dimorphism.

Phenotypic and genetic integration of personality and growth under competition in the sheepshead swordtail, Xiphophorus birchmanni

“…Thus the reduced growth and survival at higher density are consistent with our density treatment having manipulated the level of competition as intended. We previously reported a weak trend toward later and smaller maturation at high early life density in these fish, highlighting the fact that other aspects of life history are also impacted (Boulton et al 2016). In contrast however, we found no evidence of density treatment effects on individual behavior.…”

“…We previously reported a weak trend toward later and smaller maturation at high early life density in these fish, highlighting the fact that other aspects of life history are also impacted (Boulton et al. ). In contrast however, we found no evidence of density treatment effects on individual behavior.…”

“…Data analyzed here were from a broader study (Boulton et al 2014;Boulton et al 2016). Previously described methods are thus kept accordingly brief.…”

“…GS was defined as the geometric mean number of fish in i ’s group, averaged across months up to and including the observation, and included to control for effects of mortality (reducing group size from the starting n = 8). SR was similarly defined as the geometric mean (across previous months) of the proportion of i's tank mates that are mature males (see Boulton et al ). Both variables were (arithmetic) mean‐centered across all individuals to aid model interpretation.…”

Sex differences in age-to-maturation relate to sexual selection and adult sex ratios in birds

“…Sex differences in maturation are attributed to selective factors operating differently on males and females, which may lead to divergent optima for the sexes (Fairbairn ), and may tend to facilitate sexual conflict (Boulton et al. ). Nevertheless, the nature of these selective forces and thus the ultimate causes of maturation bias, are controversial and not fully understood (Wiley ; Stamps and Krishnan ).…”