Avian Genome Evolution - Gene Expression, Gene Divergence and Sexual Dimorphism

This study has focused on: (i) differences in how males and females use the genes in the genome, (ii) how sex chromosomes evolve and (iii) how the evolutionary rate of genes are affected by the chromosome on which they are located on and the gene expression level they experience. A large-scale microarray was developed for the zebra finch, Taeniopygia guttata and techniques were developed so that this microarray could also be used for wild passerine birds. Results from this thesis show that sexual dimorphism in gene expression is extensive in the brain of birds and that most genes involved in this sexual dimorphism is expressed at higher levels in males than in females and are located on the sex chromosome called the Z chromosome. Results further indicate that female birds dosage compensate to quite a high extent, a fact that has been questioned in recent studies. Dosage compensation is the process by which the sex with only one Z chromosome achieves gene expression on Z-linked genes that is balanced with gene expression from autosomal chromosomes, which are all present in the genome in two copies. Moreover, results from this study show that dosage compensation is more likely to be achieved for genes with low expression levels than for highly expressed genes on the Z chromosome. Furthermore, we have demonstrated that genes on the Z chromosome have very high evolutionary rates between species and that genes on large autosomal chromosomes evolve at a higher pace than on small chromosomes. We also find that genes with low gene expression levels evolve faster than other genes in the genome and results indicate that genes with medium high gene expression are the most conserved in birds. Moreover, genes on the Z chromosome which have higher expression in males than in females, so called male-biased genes, evolve faster than unbiased genes on the Z chromosome. Male-biased Z-linked genes were also shown to evolve at the same rates regardless of their gene expression level, a pattern which was not true for unbiased Z-linked genes or autosomal genes. Lastly, we have also identified a previously un-know sex-chromosome in birds. Gene expression analyses and linkage mapping shows that chromosome 4a, a regular autosome in the zebra finch, has become a neo-sex chromosome in warblers (Sylviidae). Our results indicate that the avian Z chromosome is highly likely to have played a large role in speciation and adaptive evolution in birds, and opens up for interesting comparisons with the well-studied sex chromosomes in for example mammals