Evolution of sex chromosomes and sex-biased gene expression

Sex chromosomes are chromosomes that carry vital sex-determining genes. Different sex determination systems and sex chromosomes have evolved from autosomes independently in different taxa; nevertheless, they share strikingly similar characteristics. Sex chromosomes undergo somewhat unique evolutionary processes due to paucity in recombination and transmission through only one of the sexes. As a result, sex chromosomes play an important role in genomic conflicts, adaptation and even speciation. In general, most sex determining chromosomes, due to their lack of recombination and low population size, experience accelerated evolution and the accumulation of repetitive sequences which render them hard to study. Neo-sex chromosomes (sex chromosome–autosome fusions) have been identified in different species and provide great opportunities for further understanding of sex chromosome evolution. In this study, we have confirmed a previously reported neo-sex chromosome in the common whitethroat by applying differential allele expression analysis. Our results also indicate that the neo-sex chromosome has undergone several evolutionary processes manifested in elevated expression and higher single nucleotide polymorphisms (SNPs) content. Additionally, some SNPs suggest Z, W polymorphisms which may be confirmed by further functional studies. (Less)