Evolution: Shape-shifting vole sex determination and sex chromosomes

It has long been known that some mouse and vole species have unusual sex chromosomes, and this new genome sequencing study advances understanding of a particularly puzzling vole system.


Most mammals studied, across a wide diversity of taxa, share an XX female, XY male sex chromosome system (Figure 1A). Their X chromosomes carry largely similar sets of genes, although only part of the X in Eutherians is homologous to the Marsupial X, (part was a Marsupial autosome that was added later). The Y chromosomes of both Marsupials and Eutherians are non-recombining, and have lost almost all genes ancestrally carried on this chromosome. Only ten X-linked genes have homologues on the Ys of many Eutherians, indicating that these “ancestrally Y-linked genes” have long been evolving independently of their X counterparts. Their sequences are so diverged from their X-linked alleles that these allele pairs have been termed “gametologs”. The sex-determining system is also similar across most Eutherian lineages [1]. The so-called “mammalian master sex-determining gene” or male determiner, SRY (the gametolog of the X-linked SOX3 gene) includes a region encoding an “HMG box” that activates a regulatory network leading the gonad to develop as a testis. However, very strange things have happened in some mouse and vole species. In Microtus oregoni, the species in this new study [2], genome sequence data are providing new details that may help understand its differences from other mammals.