title: Evolutionary strata on the chicken Z chromosome: implications for sex chromosome evolution.
abstract: The human X chromosome exhibits four "evolutionary strata," interpreted to represent distinct steps in the process whereby recombination became arrested between the proto X and proto Y. To test if this is a general feature of sex chromosome evolution, we studied the Z-W sex chromosomes of birds, which have female rather than male heterogamety and evolved from a different autosome pair than the mammalian X and Y. Here we analyze all five known gametologous Z-W gene pairs to investigate the "strata" hypothesis in birds. Comparisons of the rates of synonymous substitution and intronic divergence between Z and W gametologs reveal the presence of at least two evolutionary strata spread over the p and q arms of the chicken Z chromosome. A phylogenetic analysis of intronic sequence data from different avian lineages indicates that Z-W recombination ceased in the oldest stratum (on Zq; CHD1Z, HINTZ, and SPINZ) 102-170 million years ago (MYA), before the split of the Neoaves and Eoaves. However, recombination continued in the second stratum (on Zp; UBAP2Z and ATP5A1Z) until after the divergence of extant avian orders, with Z and W diverging 58-85 MYA. Our data suggest that progressive and stepwise cessation of recombination is a general feature behind sex chromosome evolution.
authors.affiliation: Department of Evolutionary Biology, Evolutionary Biology Centre, Uppsala University, SE-752 36 Uppsala, Sweden.
authors.forename: Lori-Jayne Lawson
authors.initials: LJ
authors.lastname: Handley
authors.forename: Helene
authors.initials: H
authors.lastname: Ceplitis
authors.forename: Hans
authors.initials: H
authors.lastname: Ellegren
mesh_terms: D000818:Animals; D005075:Biological Evolution; D002645:Chickens; D003001:Cloning, Molecular; D004247:DNA / Q000378:metabolism; D019143:Evolution, Molecular; D005091:Exons; D005260:Female; D006801:Humans; D007438:Introns; D008297:Male; D008957:Models, Genetic; D008969:Molecular Sequence Data; D010802:Phylogeny; D020161:Physical Chromosome Mapping; D011995:Recombination, Genetic; D017422:Sequence Analysis, DNA; D012730:Sex Chromosomes; D013997:Time Factors
chemical_list: D004247:DNA