Evolution of sex chromosome and sex specific expression

Abstract Many species have genetic sex determination and sex chromosomes that differ in size between males and females. This heteromorphy in sex chromosomes has evolved due to the suppression of recombination between Y and X (or Z and W in birds) following the accumulation of sex-specific alleles near sex-determining loci. The degree of gene expression imbalance between the sexes and also relative to autosomes is controlled by mechanisms of dosage compensation. Previous microarray studies on two bird species, the zebra finch (Taeniopygia guttata) and the common whitethroat (Sylvia communis), have shown a male-biased expression pattern in the Z sex chromosome, which was explained by incomplete dosage compensation mechanism in female birds. Sex chromosome evolution can be studied and better understood in systems with neo-sex chromosomes (new sex chromosomes are parts of autosomes that is translocated and then fused to a sex chromosome) where the degeneration of the sex-limited chromosome is still in its infancy. A neo-sex chromosome recently detected in Sylvioidea passerines might help unraveling the sex chromosome evolution riddle in birds. In this thesis, microarray data for zebra finch and common whitethroat are re-assessed for sex-biased genes and their function, with an emphasis on the neo-sex chromosome part in the common whitethroat. Differential expression in common whitethroat detected by RNA-sequencing is also assessed in the same way and compared to the microarray results. The male-biased gene expression pattern on the Z chromosome was confirmed by both techniques, and some interesting genes related to sexual reproduction were identified on the Z chromosome but also on the neo-sex chromosome part. Popular science summary: Evolution of sex chromosome and sex specific expression In many species the sex chromosome combination differs between males and females. In mammals the males bear a XY and females a XX combination while in birds the males are the homogametic (ZZ) and females the heterogametic (ZW) sex. An important factor in the evolution of sex chromosomes is dosage compensation which is a mechanism balancing the X (or Z) gene expression with the autosomal gene expression. The evolution of sex chromosomes can be better understand in systems like neo-sex chromosomes (newer parts of sex chromosomes that come from the fusion or translocation of an autosome and a sex chromosome) and that is because they offer a unique possibility for studying the evolution of sex-linked genes due to the still limited degeneration of the sex-limited chromosome. Previous microarray studies on two bird species, the zebra finch (Taeniopygia guttata) and the common whitethroat (Sylvia communis), have shown a male-biased expression pattern in the Z sex chromosome. This was explained by a partial dosage compensation mechanism in female birds. A neo-sex chromosome was recently found in the great reed warbler (Acrocephalus arundinaceus), the common whitethroat and the skylark (Alauda arvensis) but not in non-Sylvioidea passerines which means that the neo-sex chromosome appeared at the base of the Sylvioidea branch of the avian phylogeny (47.4-37.6 million years ago). In the present project the microarray results on male and female zebra finches and common whitethroats of Naurin et al. (2011) were reassessed. Special attention was paid in the neo-sex chromosome since it might contain genes that offer selective advantages in one of the sexes. A visualization of the correlation between gene expression and linked loci in each chromosome was attempted too. Furthermore, RNA sequencing was performed for the same common whitethroat individuals and the results were analysed and compared to the microarrays’ results. Conclusion The analysis confirmed the male-biased gene expression pattern on the Z chromosome in both zebra finch and common whitethroat. Sex-biased genes that were related to sexual reproduction were detected at the telomeric region of the neo-sex chromosome as well as at the fusion point with the ancestral chromosome. Since recombination cessation in the Sylvioidea neo-sex chromosome has been confirmed to start at these regions, this finding supports theories about recombination cessation at neo-sex chromosomes starting either at the fusion point or at sexually antagonistic loci. Finally, based on the results, RNA sequencing and microarrays were suggested to complement each other. Advisor: Bengt Hansson Master´s Degree Project 45 credits in Molecular Ecology, 2012-2013 Department of Biology, Lund University