Evidence of a neo-sex chromosome in birds

Neo-sex chromosomes often originate from sex chromosome–autosome fusions and constitute an important basis for the study of gene degeneration and expression in a sex chromosomal context. Neo-sex chromosomes are known from many animal and plant lineages, but have not been reported in birds, a group in which genome organization seems particularly stable. Following indications of sex linkage and unexpected sex-biased gene expression in warblers (Sylvioidea; Passeriformes), we have conducted an extensive marker analysis targeting 31 orthologues of loci on zebra finch chromosome 4a in five species, representative of independent branches of Passerida. We identified a region of sex linkage covering approximately the first half (10 Mb) of chromosome 4a, and associated to both Z and W chromosomes, in three Sylvioidea passerine species. Linkage analysis in an extended pedigree of one species additionally confirmed the association between this part of chromosome 4a and the Z chromosome. Markers located between 10 and 21 Mb of chromosome 4a showed no signs of sex linkage, suggesting that only half of the chromosome was involved in this transition. No sex linkage was observed in non-Sylvioidea passerines, indicating that the neo-sex chromosome arose at the base of the Sylvioidea branch of the avian phylogeny, at 47.4–37.6 millions years ago (MYA), substantially later than the ancestral sex chromosomes (150 MYA). We hypothesize that the gene content of chromosome 4a might be relevant in its transition to a sex chromosome, based on the presence of genes (for example, the androgen receptor) that could offer a selective advantage when associated to Z-linked sex determination loci.