Repeated sex chromosome evolution in vertebrates supported by expanded avian sex chromosomes

Sex chromosomes have evolved from the same autosomes multiple times across
vertebrates, suggesting that selection for recombination suppression has
acted repeatedly and independently on certain genetic backgrounds. Here,
we perform comparative genomics of a bird clade (larks and their sister
lineage; Alaudidae and Panuridae) where multiple sex chromosome–autosome
fusions appear to have formed expanded sex chromosomes. We detected the
largest known avian sex chromosome (195.3 Mbp) and show that it originates
from fusions between parts of four avian chromosomes: Z, 3, 4A and 5.
Within these four chromosomes, we found evidence by using phylogenetic
inference of five evolutionary strata where recombination had been
suppressed at different time points, and showed that stratum age explained
the divergence rate of Z–W gametologs. Next, we analysed chromosome
content and found that chromosome 3 was significantly enriched for genes
with predicted sex-related functions. Finally, we demonstrate extensive
homology to sex chromosomes in other vertebrate lineages: chromosomes Z,
3, 4A and 5 have independently evolved into sex chromosomes in fish (Z),
turtles (Z, 5), lizards (Z, 4A), mammals (Z, 4A) and frogs (Z, 3, 4A, 5).
Our results provide insights in and support for repeated evolution of sex
chromosomes in vertebrates.