RT Journal Article
SR Electronic
T1 Evolutionary dynamics of sex chromosomes of palaeognathous birds
JF bioRxiv
FD Cold Spring Harbor Laboratory
SP 295089
DO 10.1101/295089
A1 Luohao Xu
A1 Simon Yung Wa Sin
A1 Phil Grayson
A1 Daniel E. Janes
A1 Scott V. Edwards
A1 Timothy B. Sackton
YR 2018
UL http://biorxiv.org/content/early/2018/04/05/295089.1.abstract
AB Standard models of sex chromosome evolution propose that recombination suppression leads to the degeneration of the heterogametic chromosome, as is seen for the Y chromosome in mammals and the W chromosome in most birds. Unlike other birds, palaeognaths (ratites and tinamous) possess large non-degenerate regions on their sex chromosomes (PARs or pseudoautosomal regions), despite sharing the same sex determination region as neognaths (all other birds). It remains unclear why the large PARs of palaeognaths are retained over more than 100 MY of evolution, and the impact of these large PARs on sex chromosome evolution. To address this puzzle, we analysed Z chromosome evolution and gene expression across 12 palaeognaths, several of whose genomes have recently been sequenced. We confirm at the genomic levels that most palaeognaths (excepting some species of tinamous) retain large PARs. As in neognaths, we find that all palaeognaths have incomplete dosage compensation on the regions of the Z chromosome homologous to degenerated portions of the W (differentiated regions or DRs), but we find no evidence for enrichments of male-biased genes in PARs. We find limited evidence for increased evolutionary rates (faster-Z) either across the chromosome or in DRs for most palaeognaths with large PARs, but do recover signals of faster-Z evolution similar to neognaths in tinamou species with mostly degenerated W chromosomes (small PARs). Unexpectedly, in some species PAR-linked genes evolve faster on average than genes on autosomes. Increased TE density and longer introns in PARs of most palaeognaths compared to autosomes suggest that the efficacy of selection may be reduced in palaeognath PARs, contributing to the faster-Z evolution we observe. Our analysis shows that palaeognath Z chromosomes are atypical at the genomic level, but the evolutionary forces maintaining largely homomorphic sex chromosomes in these species remain elusive.