Molecular evolution of sex determination genes in closely related species of the Drosophila melanogaster subgroup

Abstract

<p>Recent studies have highlighted the importance of sex and reproductive related traits in speciation. Hence, an evolutionary examination of the primary regulators of sexual differentiation is an important step in understanding the origins of phenotypic diversity. In this thesis, a molecular analysis of sequence variation is performed on the three interacting sex determining genes, transformer (tra ), transformer-2 (tra-2 ) and doublesex ( dsx ), among closely related species of the Drosophila melanogaster subgroup. At the dsx locus, both female- and male-specific DSX isoforms are found to be conserved relative to other loci sequenced from species of the D. melanogaster subgroup. However, levels of selective constraints on the male-specific portion of the dsx protein has varied considerably across Dipteran lineages and indicate the apparent heterogeneity in selection pressures across taxa. Meanwhile, tra and tra-2 are demonstrated to be among the most rapidly evolving loci found in the D. melanogaster subgroup. The presence of large arginine-serine rich insertions in sibling species indicate that TRA is tolerant to substantial amino acid change. Generally, within and between species patterns of variation in the sibling species do not deviate from a neutral model of evolutionary change. At the tra-2 locus, the ratios of replacement to synonymous substitutions were significantly different within vs. between species indicating the presence of diversifying selection at this locus. Polymorphism data from D. simulans corroborates this result and thus, provides evidence that positive selection may be acting directly on a gene involved in the primary sex determination hierarchy. The high levels of variability found among sex determining genes indicate the evolutionary potential of this important sexual system. The amenability of this sexual system to tolerate large genetic perturbations may expedite the generation of evolutionary novelties. Since sex determination genes control various components of the mating system, this high degree of genetic variability may be an important source of heritable genetic material for sexual selection--an important driver of speciation--to act upon.</p>