Ancestral Z–W gene pairs from three caenophidian species. (A) Phylogenetic tree of selected snake species included in this study, with branches colored to highlight relationships among species. Humans diverged from snakes 312 Myr ago. Chicken and green anole lizard diverged from snakes 280 and 167 Myr ago, respectively, and were used to resolve gene gains and losses between snakes and mammals. Snakes diverged from each other starting about 90.8 Myr ago (green). Boas (red) and pythons (blue) have independently evolved homomorphic XY sex chromosomes. Caenophidian snakes (purple) share a common ZW sex chromosome system, orthologous to the python XY. (B) Euler diagram showing overlapping sets of ancestral Z–W gene pairs identified in five-pacer viper (light purple), pygmy rattlesnake (medium purple), and mountain garter snake (dark purple) as subsets of all 1300 ancestral Z genes (green). See also Supplemental Table S3.
