Sex chromosome transformation and the origin of a male-specific X chromosome in the creeping vole

Mystery solved? Chromosomal sex determination arises when an autosomal locus acquires a sex-determining function. In some taxa, this process occurs often. The XY system in mammals, however, has been evolutionarily stable across a wide array of species. Fifty years ago, a variation on this norm was described in the creeping vole ( Microtus oregoni ), but the details have remained mostly unknown. Couger et al. sequenced the sex chromosomes in this species and found that the Y chromosome has been lost, the male-determining chromosome is a second X that is largely homologous to the female X, and both the maternally inherited and male-specific sex chromosomes carry vestiges of the ancestral Y. Science , this issue p. 592