A novel sex determination system in a close relative of the house mouse

Abstract

Therian mammals have an extremely conserved XX/XY sex determination system. A limited number of mammal species have, however, evolved to escape convention and present aberrant sex chromosome complements. In this study, we identified a new case of atypical sex determination in the African pygmy mouse Mus minutoides, a close evolutionary relative of the house mouse. The pygmy mouse is characterized by a very high proportion of XY females (74%, n ¼ 27) from geographically widespread Southern and Eastern African populations. Sequencing of the high mobility group domain of the mammalian sex determining gene Sry, and karyological analyses using fluorescence in situ hybridization and G-banding data, suggest that the sex reversal is most probably not owing to a mutation of Sry, but rather to a chromosomal rearrangement on the X chromosome. In effect, two morphologically different X chromosomes were identified, one of which, designated X*, is invariably associated with sex-reversed females. The asterisk designates the still unknown mutation converting X*Y individuals into females. Although relatively still unexplored, such an atypical sex chromosome system offers a unique opportunity to unravel new genetic interactions involved in the initiation of sex determination in mammals.

Sex reversal induces size and performance differences among females of the African pygmy mouse, Mus minutoides

“…Overall, 54 mice were used in the experiments with 19 males and 35 females, all adults. Seventeen females were found to carry a Y chromosome using PCR amplification of the Sry gene (following Veyrunes et al, 2010). The 18 others did not have the Sry gene, and thus were either XX or XX*.…”

“…Here, we investigated bite force and skull morphology in the African pygmy mouse, Mus (Nannomys) minutoides Smith 1834. This species is known to have a unusual sex determination system, with three types of sex chromosomes: X, Y and a feminizing X*, producing XY males and XX, XX* females and a high proportion of sex-reversed, X*Y females (Veyrunes et al, 2010). The three types of females present no differences in external morphology or gonad anatomy , but are known to vary in other traits: X*Y females have a higher reproductive success , and have a much more aggressive behaviour (latency to attack, number of attacks; Saunders et al, 2016) than the XX and XX* females.…”

Did sex chromosome turnover promote divergence of the major mammal groups?

“…Pygmy mice are famous for rearrangements of sex chromosomes and autosomes 81. There are X‐ and Y‐autosome fusions that differentiate closely related species, and some species have both, requiring segregation from a chain of four or even six chromosomes at male meiosis.…”

A study of goat SRY protein expression suggests putative new roles for this gene in the developing testis of a species with long‐lasting SRY expression

“…This newly formed sex chromosome then undergoes specialization and degeneration mechanisms (Graves, 2006). In terms of the degenerative mechanism, it is not surprising to find mammals without SRY as a testis determinant, as is the case in some rodents, which are the most rapidly evolving species (Just et al, 1995, Soullier et al, 1998Sutou et al, 2001, Veyrunes et al, 2010. In line with this idea, mice could be considered as the most evolved mammalian species in terms of Sry function; this species is in the closest position from the end-point of Sry action, corresponding to loss of the Y chromosome.…”