A bird’s white-eye view on the early steps of ZW neo-sex chromosome evolution

This collection include datasets, genome sequences, figures and scripts used for the manuscript Leroy et al. "A bird's white-eye shot: looking down on a new avian sex chromosome evolution".

Abstract : Chromosomal organization is relatively stable among avian species, especially for sex chromosomes. Sylvioidea species however, harbor a unique pair of neo-sex chromosomes, originating from a parallel translocation of a region of the ancestral 4A chromosome on both W and Z chromosomes. In this study, we took advantage of this unusual event to study the early stages of sex chromosome evolution. To do so, we sequenced, assembled, organized the scaffolds along chromosomes and annotated genes of a Zosterops borbonicus female (ZW), a species of Sylvioidea. Molecular phylogenetic dating under various methods and calibration sets confidently confirmed the recent diversification of the genus Zosterops (1-3.5 million years ago), thus representing one of the most exceptional rates of diversification among vertebrates. We then combined genomic coverage comparisons of five males and seven females, and homology with the zebra finch genome to identify sex chromosome scaffolds, as well as the candidate chromosome breakpoints for the two translocation events. We observed reduced levels of within-species diversity in both translocated regions and, as expected, even more so on the neoW chromosome. In order to compare the rates of molecular evolution in genomic regions of the autosomal-to-sex transitions, we then estimated the ratios of non-synonymous to synonymous polymorphisms (ⲡN/ⲡS) and substitutions (dN/dS). Based on both ratios, no or little contrast between autosomal and Z genes was observed, thus representing a very different outcome than the higher ratios observed at the neoW genes. In addition, we report significant changes in base composition content for translocated regions on the W and Z chromosomes and a large accumulation of transposable elements (TE) on the new W region. Our results revealed contrasted signals of molecular evolution changes associated to these autosome-to-sex transitions, with congruent signals of a W chromosome degeneration yet a surprisingly weak support for a fast-Z effect.