Sex-Specific Genetic Variance and the Evolution of Sexual Dimorphism: A Systematic Review of Cross-Sex Genetic Correlations

Abstract

The independent evolution of the sexes may often be constrained if male and female homologous traits share a similar genetic architecture. Thus, cross-sex genetic covariance is assumed to play a key role in the evolution of sexual dimorphism (SD) with consequent impacts on sexual selection, population dynamics, and speciation processes. We compiled cross-sex genetic correlations (r MF ) estimates from 114 sources to assess the extent to which the evolution of SD is typically constrained and test several specific hypotheses. First, we tested if r MF differed among trait types and especially between fitness components and other traits. We also tested the theoretical prediction of a negative relationship between r MF and SD based on the expectation that increases in SD should be facilitated by sex-specific genetic variance. We show that r MF is usually large and positive but that it is typically smaller for fitness components. This demonstrates that the evolution of SD is typically genetically constrained and that sex-specific selection coefficients may often be opposite in sign due to sub-optimal levels of SD. Most importantly, we confirm that sex-specific genetic variance is an important contributor to the evolution of SD by validating the prediction of a negative correlation between r MF and SD.

Heritability and cross-sex genetic correlations of early-life circulating testosterone levels in a wild mammal

“…We also do not know how neonatal testosterone levels in any taxa relate to their hormone levels later in life, which may be under stronger selection. While little is known about the magnitude of between-sex genetic correlations in hormone traits overall, the correlation found in this study is unexpectedly strong given that physiological traits typically have the lowest cross-sex genetic correlation for any trait type [25]. Thus, to date, the strength of selection and the degree of sexual antagonism for this trait remain unresolved, and predicting its likely evolutionary dynamics requires further research.…”

The baubellum is more developmentally and evolutionarily labile than the baculum

“…A major unsolved question is to what extent sexually dimorphic characters are constrained by the shared genome of males and females (Poissant et al., 2010). Sexual dimorphism is expected to be greatest in species where different optima can be reached via sex‐specific expression of the genome and response to selection.…”

“…At one extreme, a particular state may be beneficial in one sex, but harmful in the other. In the absence of sex‐specific modification of expression, the species will evolve to a phenotypic compromise, where neither sex can reach its optima because of counterselection in the other sex (Poissant et al., 2010). One evolutionary solution to such sexual conflict is sex‐specific expression of the genome, freeing each sex to evolve its own trait value, or even for one sex to lose the trait if it is nonfunctional or deleterious.…”

“…Sexual dimorphism, where the same trait takes on different states in the two sexes, is a nearly ubiquitous phenomenon in nature, and understanding the evolutionary forces that lead to sexual dimorphism is an important goal for evolutionary biology (Clutton‐Brock, 2007; Poissant et al, 2010). …”

QTL mapping for sexually dimorphic fitness-related traits in wild bighorn sheep

“…In theory, the evolution of sexual dimorphism depends on the presence of sex-specific genetic variance (Lande, 1980). Although such variance has been documented in a large number of organisms Poissant et al, 2010a), including bighorn sheep (Poissant et al, 2008), little is known about its molecular underpinning and micro-evolutionary dynamics. Dissecting the genetic architecture of sexually dimorphic traits in bighorn sheep would thus also provide insights into the molecular mechanisms facilitating the independent evolution of males and females.…”