Sexual Dimorphism, Throat Coloration, and the Evolution of Gene Expression in Threespine Stickleback Brain Tissue

Abstract

Elucidating the causes and mechanisms underlying variation in sexual dimorphism is a critical issue in evolution, medicine, and ecology. In the threespine stickleback (Gasterosteus aculeatus), the leading vertebrate model for evolutionary research, sexual dimorphism in color and morphology is known to vary dramatically among populations. Marine (anadromous) populations typically show greater morphological dimorphism and more consistent color dimorphism. Here I examine how sexual dimorphism of gene expression in the brains of stickleback fish varies with life history across populations and with color pattern. I studied one marine population that exhibits sexual dimorphism in throat coloration; two stream-resident populations that express intense red coloration in both sexes; one that exhibits much less coloration in females; and one stream-resident population that lacks red coloration in either sex. This gene expression data set was collected previously in my laboratory but had not been mapped to the most recent, substantially improved stickleback genome. Data was trimmed and remapped to the newest threespine stickleback genome (UGA_version5) using RNA sequence alignment with fastp and STAR. To measure the expression level, the number of reads associated with each gene from the reference genome was considered using featureCounts. Remapping of the reads to the newest stickleback genome and a principal component analysis (PCA) of the data shows a strong divergence in sex for all five populations on the first principal component and some differentiation by population on the second principal component. DESeq2 differential expression analysis of population interaction indicates a higher level of upregulation in males for significant genes in the marine populations as compared to the freshwater populations in both magnitude and frequency.