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The Quantitative Genetics of Sexual Differences: New Methodologies and an Empirical Investigation of Sex-Linked, Sex-Specific, Non-Additive, and Epigenetic Effects

Abstract

Phenotypic differences between females and males are widely observed in organisms with separate sexes. How these sexual dimorphisms evolve despite the sexes sharing a majority of their genome remains an unresolved issue in evolutionary biology as theoretical models often disagree over the genetic mechanisms that are predicted to facilitate the evolution of sexual dimorphism. In this dissertation, I develop quantitative genetic methods for estimating sex-specific non-additive genetic and sex-linked additive genetic (co)variances. I then empirically quantify the genetic effects underlying population differences in sexually dimorphic traits using the water strider Aquarius remigis.

I show that differences between the sexes in genetic architectures bias estimates of additive genetic variance if these differences are improperly incorporated into quantitative genetic analyses. I then develop the nadiv software package for the R statistical program to facilitate estimation of non-additive genetic (co)variances using the "animal model". Next, I use simulations to demonstrate that estimates of sex-specific additive genetic variances and between-sex additive genetic correlations are biased when sex-linked additive genetic variance is ignored.

I create a genetic model of a sex-linked locus to derive general expressions for the covariance between relatives due to sex-linked genes applicable under any form of global sex chromosome dosage compensation. These expressions lead to the development of formulae and algorithms (incorporated into nadiv) to create sex-linked relatedness matrices for use in animal model analyses. I further show that the way by which sex-linked relatedness matrices under the various forms of dosage compensation differ from one another implies that unbiased estimates of sex-linked additive genetic variance can still be obtained even when the particular form of dosage compensation is unknown.

Using population crosses of water striders, I show the net genetic effects contributing to sexually dimorphism differ between female and male water striders. I demonstrate that the magnitude of this difference in epistatic and dominance maternal genetic effects positively covaries with the magnitude of sexual dimorphism across a range of morphological traits. This is the first study to show that sex-specific non-additive genetic effects correlate with the degree of sexual dimorphism across traits.

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