A new cytotype of the African pygmy mouse Mus minutoides in Eastern Africa. Implications for the evolution of sex-autosome translocations.

The African pygmy mice (genus Mus, subgenus Nannomys) are recognized for their highly conserved morphology but extensive chromosomal diversity, particularly involving sex-autosome translocations, one of the rarest chromosomal rearrangements among mammals. It has been shown that in the absence of unambiguous diagnostic morphological traits, sex-autosome translocations offer accurate taxonomic markers. For example, in Mus minutoides, irrespective of the diploid number (which ranges from 2n = 18 to 34), all specimens possess the sex-autosome translocations (X.1) and (Y.1) that are unique to this species. In this study, we describe a new cytotype that challenges this view. Males are characterized by the translocation (Y.1) only, while females carry no sex-autosome translocation, the X chromosome being acrocentric. Hence, although sex-autosome translocations (X.1) and (Y.1) are still diagnostic when one or both are present, their absence does not rule out M. minutoides. This cytotype has a large distribution, with specimens found in Tanzania and in the eastern part of South Africa. The nonpervasive distribution of Rb(X.1) provides an opportunity to investigate different evolutionary scenarios of sex-autosome translocations using a phylogenetic framework and the distribution of telomeric repeats. The results tend to support a scenario involving a reversal event, i.e., fusion then fission of Rb(X.1), and highlighted the existence of a new X1X1X2X2/X1X2Y sex chromosome system, confirming the remarkable diversity of neo-sex chromosomes and sex determination systems in the African pygmy mice.

Insights into the evolutionary history of the X-linked sex reversal mutation in mus minutoides: clues from sequence analyses of the Y-linked Sry gene.

The African pygmy mouse, Mus minutoides, is one of the very few mammal species that deviates from the classical mammalian XX/XY sex chromosome system by presenting a high proportion of fully fertile sex-reversed females. Since the still unknown sex reversal mutation is X-linked (X*), they are designated as X*Y females. Until now, X*Y females had only been identified in Southern Africa, but data were lacking for the rest of the vast sub-Saharan distribution range of this species. In this study, the PCR genotyping of the Y-linked Sry gene on 72 females from Western Africa (Guinea, Ivory Coast and Ghana) uncovered 10 sex-reversed females distributed in the 3 countries. This expands our understanding of the geographical distribution and temporal origin (dated at 0.9 mya) of the sex reversal mutation. In addition, we sequenced and analyzed a fragment of the Sry gene (including the complete high-mobility group, i.e. HMG box, and the partial C-terminal region). The results demonstrate the presence of multiple polymorphic copies of the gene as reported in other rodent species and reveal, more unexpectedly, an extremely high proportion of amino acid replacement within the HMG box. In effect, the predicted HMG box protein sequence similarity between some populations of M. minutoides is as low as 94.9%, and at the interspecific level (within genus), it drops to only 91.1% between M. minutoides and M. musculus.