Figure 2.

Independent evolutionary strata of sex chromosomes in Palaeognathae. (a) The differentiated region of sex chromosomes was divided into evolutionary strata according to the divergence level between the Z and W chromosomes. The colored dots along the branches of the phylogeny indicate the origins of evolutionary strata. The gametolog tree of KCMF1 in (b) supports a single origin of S0 in birds, and the gametolog tree of ABHD17B in (c) supports multiple independent origins of S1 in Palaeognathae. The numbers at the phylogenetic nodes indicate the bootstrapping values.