Abstracts (first author)
Short- and long-term evolution of gene expression and sequence divergence on the avian Z chromosome
Sex chromosomes provide unique opportunities to study the fundamental evolutionary forces that act on the entirety of the genome. Because of their unusual pattern of transmission, biological differences between males and females cause sex chromosomes to experience distinct evolutionary environments, which in turn influence coding sequence, gene expression, and even splice forms. The avian Z chromosome is separated into multiple distinct strata, formed by numerous recombination suppression events that have evolved independently across different avian lineages. This provides a unique opportunity to examine the effects of sex-specific selection on the same chromosome across evolutionary time scales.
We first mapped out the fine-scale evolutionary history of the Z chromosome across a clade of birds, encompassing roughly 80 million years. Our results show that replicate strata have formed independently across this clade. We then used multiple integrated next-generation datasets to measure gene expression and gene sequence divergence across four species within this clade that experience different sex-specific selection regimes. Our data indicate that male-biased gene expression accumulates over time across Z chromosome strata, with older strata showing greater levels of male-bias. This is consistent with the fact that Z-linked genes are more often selected for male-specific effects. We also show that sex chromosome divergence influences sex-specific alternative splicing, leading to the loss of female-specific exons from the Z chromosome.