Cover image

Different ancestral autosomes independently evolved into sex chromosomes in snakes, birds, and mammals. In snakes and birds, females are ZW and males are ZZ; in mammals, females are XX and males are XY. Although X and Z Chromosomes retain nearly all ancestral genes, sex-specific W and Y Chromosomes suffered extensive genetic decay. In this issue, a meta-analysis of gene loss and survival on sex-specific chromosomes shows that dosage-sensitive functions in embryonic development drove gene survival across snakes, birds, and mammals. The cover art celebrates Lunar New Year with red lanterns displaying animals from the Chinese zodiac—ox, rooster, and snake—to represent each of the three lineages. (Cover illustration by Caitlin Rausch, https://www.caitsart.com. [For details, see Bellott and Page, pp. 198–210.])