Project

Quantitative genetics of the extreme sexual size dimorphism

Goal: Females and males commonly differ in the expression of traits. The evolution of sexual dimorphism requires sex-specific selection and at least partly independent genetic variation between the sexes. However, females and males share an almost identical genome that constrains the sexes to respond independently to the selection and may result in a stage when one or both sexes express traits outside their optima. Quantitative genetics provides tools to predict the extent to which the evolution of sexual dimorphism is genetically constrained between sexes by assessing the cross-sex genetic correlation. The cross‐sex genetic correlation can be estimated as rmf =COVAmf∕sqrt(VAf ∗VAm), where COVAmf is the additive genetic covariance between the sexes, and VAm and VAf are additive genetic variances of males and females, respectively. When is close to unity, the sexes are assumed to have a nearly identical genetic architecture for the trait and evolution of sexual dimorphism should be constrained; close to zero values of rmf indicate complete independence in the genetic architecture of the trait between males and females and thus sex independent evolution. A cross‐sex genetic correlation between zero and one suggests that some of the genes acting on the shared trait already differ between males and females and indicates a further possibility for the evolution of sexual dimorphism in the trait. In this project, we aim to assess genetic variances and cross‐sex genetic correlations of size in an extremely sexually-size dimorphic spider, Nephilinis cruentata. In these spiders, females are considerably larger than males, they weigh more than 70X more than males. Our preliminary analyses found rmf close to zero suggesting that females and males do not share genetic architecture for size, indicates a resolved intra-locus sexual conflict and potential for further sex independent evolution of size. The result reflects differences in the effects of sexual and natural selection on body size between the sexes. The amount of genetic variation is significantly lower in females compared to males implying that females have been under the stronger directional selection (for fecundity) compared to males that are more plastic. 

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Simona Kralj-Fišer
added a research item
Adult body size, development time, and growth rates are components of organismal life histories, which crucially influence fitness and are subject to trade-offs. If selection is sex-specific, male and female developments can eventually lead to different optimal sizes. This can be achieved through developmental plasticity and sex-specific developmental trajectories. Spiders present suitable animals to study differences in developmental plasticity and life history trade-offs between the sexes, because of their pronounced sexual dimorphism. Here, we examine variation in life histories in the extremely sexually size dimorphic African hermit spider (Nephilingis cruentata) reared under standardized laboratory conditions. Females average 70 times greater body mass (and greater body size) at maturity than males, which they achieve by developing longer and growing faster. We find a small to moderate amount of variability in life history traits to be caused by family effects, comprising genetic, maternal, and early common environmental effects, suggesting considerable plasticity in life histories. Remarkably, family effects explain a higher variance in male compared to female life histories, implying that female developmental trajectories may be more responsive to environment. We also find sex differences in life history trade-offs and show that males with longer development times grow larger but exhibit shorter adult longevity. Female developmental time also correlates positively with adult body mass, but the trade-offs between female adult mass, reproduction, and longevity are less clear. We discuss the implications of these findings in the light of evolutionary trade-offs between life history traits.
Simona Kralj-Fišer
added a research item
Males and females are often subjected to different selection pressures for homologous traits, resulting in sex-specific optima. Because organismal attributes usually share their genetic architectures, sex-specific selection may lead to intralocus sexual conflict. Evolution of sexual dimorphism may resolve this conflict, depending on the degree of cross-sex genetic correlation (rMF) and the strength of sex-specific selection. In theory, high rMF implies that sexes largely share the genetic base for a given trait and are consequently sexually monomorphic, while low rMF indicates a sex-specific genetic base and sexual dimorphism. Here, we broadly test this hypothesis on three spider species with varying degrees of female-biased sexual size dimorphism, Larinioides sclopetarius (sexual dimorphism index, SDI = 0.85), Nuctenea umbratica (SDI = 0.60), and Zygiella x-notata (SDI = 0.46). We assess rMF via same-sex and opposite-sex heritability estimates. We find moderate body mass heritability but no obvious patterns in sex-specific heritability. Against the prediction, the degree of sexual size dimorphism is unrelated to the relative strength of same-sex versus opposite-sex heritability. Our results do not support the hypothesis that sexual size dimorphism is negatively associated with rMF. We conclude that sex-specific genetic architecture may not be necessary for the evolution of a sexually dimorphic trait.
Simona Kralj-Fišer
added a project goal
Females and males commonly differ in the expression of traits. The evolution of sexual dimorphism requires sex-specific selection and at least partly independent genetic variation between the sexes. However, females and males share an almost identical genome that constrains the sexes to respond independently to the selection and may result in a stage when one or both sexes express traits outside their optima. Quantitative genetics provides tools to predict the extent to which the evolution of sexual dimorphism is genetically constrained between sexes by assessing the cross-sex genetic correlation. The cross‐sex genetic correlation can be estimated as rmf =COVAmf∕sqrt(VAf ∗VAm), where COVAmf is the additive genetic covariance between the sexes, and VAm and VAf are additive genetic variances of males and females, respectively. When is close to unity, the sexes are assumed to have a nearly identical genetic architecture for the trait and evolution of sexual dimorphism should be constrained; close to zero values of rmf indicate complete independence in the genetic architecture of the trait between males and females and thus sex independent evolution. A cross‐sex genetic correlation between zero and one suggests that some of the genes acting on the shared trait already differ between males and females and indicates a further possibility for the evolution of sexual dimorphism in the trait. In this project, we aim to assess genetic variances and cross‐sex genetic correlations of size in an extremely sexually-size dimorphic spider, Nephilinis cruentata. In these spiders, females are considerably larger than males, they weigh more than 70X more than males. Our preliminary analyses found rmf close to zero suggesting that females and males do not share genetic architecture for size, indicates a resolved intra-locus sexual conflict and potential for further sex independent evolution of size. The result reflects differences in the effects of sexual and natural selection on body size between the sexes. The amount of genetic variation is significantly lower in females compared to males implying that females have been under the stronger directional selection (for fecundity) compared to males that are more plastic.