PATTERNS OF SEX-BIASED GENE EXPRESSION AND GENE PATHWAY EVOLUTION IN DROSOPHILA

Abstract

Sexual dimorphism, or the phenotypic differences that exist between males and females of the same species, is widespread throughout nature. Sexually dimorphic traits are primarily generated by differences in gene expression between the sexes, commonly known as sex-biased gene expression. In this dissertation, I explore the evolutionary patterns and consequences of sex-biased gene expression across Drosophila species. The most obvious sexually dimorphic characteristics exist in adult stages and consequently patterns of sex-bias in early Drosophila development have not been well-studied. In chapter 1, I examine patterns of sex-biased gene expression during ontogeny in two closely related Drosophila species belonging to the D. pseudoobscura group (D. pseudoobscura and D. persimilis). This study provides insight into global patterns of sex-bias gene expression throughout development between species. The visual pathway in Drosophila shows abundant evidence for sex-biased and species-specific differential gene expression. In chapter 2, across 12 different Drosophila species, I examine rates of protein sequence evolution of genes in this pathway to determine if observed differences in gene expression correlate with rates of evolutionary change at the level of protein sequence. As a whole the visual pathway in Drosophila exhibits strong conservation at the level of protein sequence over 65 million years of evolutionary time suggesting that observed differences in levels of transcription are the result of differences in the underlying regulatory mechanisms. The comparative molecular evolutionary analysis of the visual pathway revealed a novel isoform-specific lineage-specific duplication event of the key signal transduction activator gene G-alpha-q. In D. melanogaster, G-alpha-q is present as a single-copy and alternatively spliced in a tissue- and isoform-specific manner. The same gene is duplicated in an isoform-specific manner in the species belonging to the subgenus Drosophila such that each duplicate appears to retain the exon complement of only one of the splice-variants. In chapter 3, using experimental and computational approaches, I examine the evolution of the gene structure and expression of these novel isoform-specific duplicates. This analysis revealed a mechanism by which duplicate genes can evolve novel functions and expression patterns (including sex-biased expression patterns) while retaining their ancestral functions.