Sex determination by multiple sex chromosomes in Xenopus tropicalis.

Sex determination shows a great diversity of mechanisms, ranging from temperature and other environmental cues to strict chromosomal control over this process. In the case of genetic sex determination, this variability is linked to a similarly high variation of sex chromosome differentiation. In vertebrates the most widespread situation is that a pair of sex chromosomes differs in males and females. Either females can be XX and produce only one type of gamete, being the “homogametic” sex, and then males are XY and thus heterogametic, like most mammals; or if females are heterogametic (in analogy: ZW), then males have two copies of the same sex chromosome and are ZZ, like in birds. Curiously, outside of mammals and birds, the phylogenetic patterns of these two obviously contrary modes of sex determination indicate many transitions (1, 2). Many cases have been documented where closely related lineages switch between XY or ZW sex chromosomes, demonstrating that transitions between male and female heterogamety have occurred quite frequently in the evolutionary past of vertebrates (1–4). A large body of literature explains theoretically the “how” and “why” of such changes (e.g., refs. 5–7), but systems to study sex chromosome transitions experimentally or observe sex chromosome evolution in action are extremely rare. Filling up this gap, Roco et al. report in PNAS (8) the intriguing situation of the simultaneous presence of three different sex chromosomes, W, Z, and Y, in the Western clawed frog, Xenopus tropicalis.