Sex-specific selection and the evolution of sexual dimorphism

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Sexual dimorphism is arguably the most pervasive form of intra-specific diversity in the animal kingdom, affecting many phenotypes, including morphology, physiology, behaviour and life history. I propose six different objectives, organized around three related themes: I. Sexual dimorphism and sex-biased gene expression. For many dimorphisms, gene expression differences between males and females (sex-biased expression) for genes present in both sexes are key. Many studies have tried to connect sex-biased genes to sexual selection, with profoundly discordant results. This is likely a product of the way we currently measure gene expression from complex aggregations of many cell types, which cannot determine whether sex-biased expression is the result of regulatory differences within similar cells (rewiring within cell types), or is due to differences in the abundance of cell types resulting from sex-specific developmental trajactories. We have recently installed a state-of-the-art single-cell RNA-Seq (ScRNA-Seq) system in the lab to differentiate gene expression differences due to regulatory reprogramming of cell types from those due to developmental differences in cell type abundance. We will use this study the cause and molecular signatures of sex-biased gene expression, and determine whether we can indeed study sexual selection in the molecular signature of sex-biased genes. II. How does sex-specific selection shape genomic diversity? We have recently implemented bioinformatic and population genomic approaches to detect the subtle but clear signal of sex-specific selection and sexual conflict from whole-genome DNA sequencing data within generations. This is a major advance on previous work measuring molecular evolutionary properties of sex-biased genes, as it allows us to work within ecologically relevant time scales and compare populations with different ecologies. Moreover, this approach makes it possible to determine the causes of sexual conflict. We will implement this approach to study the footprint of sexual conflict over the life cycle and across different natural populations. III. Why do sex chromosomes diverge at radically different rates? Sex chromosomes are the only region of the genome that differ between the sexes. A key question in sex chromosome research is why some sex chromosomes remain homomorphic, while others show profound differences between the X and Y. We recently characterized the sex chromosomes across several Poeciliid species, revealing that the degree of sex chromosomes differentiation is highly variable both within and across species for a single sex chromosome system. This clade therefore is a powerful natural replicate of sex chromosome evolution that can be used to understand differences in divergence and degradation of the Y chromosome. We will leverage this diversity to ask what prevents some Y chromosomes from decaying while others diverge so quickly.