The genomic basis of the response to female-limited X-chromosome evolution

In this thesis, I attempted to achieve a better understanding of the nature of X-linked polymorphic loci using a female-limited X chromosome (FLX) experimental evolution in Drosophila melanogaster. I expected that expressing the evolved X chromosome will result in an increase in female fitness and a decrease in male fitness. I first investigated the effect of an experimentally evolved female-limited X chromosome on male reproductive traits (Paper I). Secondly, I examined how the genome-wide expression pattern responds to the presence of the evolved X chromosome (Paper II), and then I analysed the changes in allele frequencies across the genome (Paper III). Finally, I attempted to study the changes in genetic variances and covariances in sexually homologous traits in response to FLX evolution, as well as change in the cross-sex genetic correlations for these sexually homologous traits (Paper IV).
Contrary to the initial expectation, I found evidence of trade-offs between various components of male reproductive success rather than an overall decline (Paper I). However, I identified a more ‘feminized’ gene expression profile as the result of FLX evolution (Paper II) and found evidence of adaptation in the methodological control treatment which was a necessary part of the experimental design. The analysis of differences in allele frequencies between selection regimes showed no evidence of overrepresentation of SA loci on the X chromosome, but these results suggest an interesting avenue for future study of sexual conflict over sensory ability (Paper III). Finally, I found evidence of a breakdown in the intersexual genetic correlation for locomotory activity in FLX populations compared to control populations (Paper IV).
These results indicate that the X chromosome may not possess as many SA mutations as previously thought, and they are by nature difficult to study in a species with old, already highly degenerated sex chromosomes. However, the results presented here highlight the importance of sex-specific selection pressures in shaping the genetic architecture of many traits.