Research Article

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

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Science  07 May 2021:
Vol. 372, Issue 6542, pp. 592-600
DOI: 10.1126/science.abg7019

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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


The mammalian sex chromosome system (XX female/XY male) is ancient and highly conserved. The sex chromosome karyotype of the creeping vole (Microtus oregoni) represents a long-standing anomaly, with an X chromosome that is unpaired in females (X0) and exclusively maternally transmitted. We produced a highly contiguous male genome assembly, together with short-read genomes and transcriptomes for both sexes. We show that M. oregoni has lost an independently segregating Y chromosome and that the male-specific sex chromosome is a second X chromosome that is largely homologous to the maternally transmitted X. Both maternally inherited and male-specific sex chromosomes carry fragments of the ancestral Y chromosome. Consequences of this recently transformed sex chromosome system include Y-like degeneration and gene amplification on the male-specific X, expression of ancestral Y-linked genes in females, and X inactivation of the male-specific chromosome in male somatic cells. The genome of M. oregoni elucidates the processes that shape the gene content and dosage of mammalian sex chromosomes and exemplifies a rare case of plasticity in an ancient sex chromosome system.

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