Shared mechanisms of sexual development. Typical male and female traits (represented by the different colour patterns of Catonephele butterfl ies on the cover; males have the orange colour) were thought to develop via different mechanisms in vertebrates and insects. Vertebrates were believed to primarily use hormones produced in the gonads and insects to read their sex chromosomes in each cell, in order to develop sex-specifi c traits. However, as Bear and Monteiro discuss on pages 725–732 of this issue, new research indicates that birds, mammals, and insects use both hormonal and cell-autonomous mechanisms to develop their sex-specifi c traits. Cover by Antónia Monteiro.