The Fertile Xy Females Of Akodon (Rodentia, Sigmodontinae). The Case Of A. Azarae And A. Boliviensis

In several rodent species, as Myopus schisticolor and diverse species of Akodon, fertile XY females are present. Strikingly, at least in M. schisticolor and A. azarae, the reproductive efficiency of these XY females is higher than that of XX ones. The first hypothesis to explain Akodon XY females considered an Sry mutation. However, partial sequences of A. azarae and A. boliviensis Sry HMG-box did not reveal differences between males and XY females, a fact now confirmed by the sequencing of the complete Sty ORF of both species. The acrocentric X chromosome of A. azarae represents 8.2% of the haploid set. The material exceeding the typical mammalian length corresponds to several heterochromatic bands disposed in three different patterns. Males carry invariably one of the types (X1), and XY females one of the other two (X2 or X3); XX females present always an X1 chromosome, belonging the other to any of the types. The Y chromosome is very small, with an heterochromatic long arm. As regards A. boliviensis, its acrocentric X chromosome shows an heterochromatic short arm and a length of 7.16 to 8.9% in different populations, due to the size of the heterochromatic short arm. The long arm of its Y chromosome is also heterochromatic. Both species heterochromatic regions are positive with DAPI, pointing to a relative AT-richness. Moreover, in-situ hybridization experiments show that the gonosomal heterochromatic regions of each of these species hybridize with marked DNA from the other. In addition, FISH with a telomeric probe showed a subcentromeric mark in A. azarae X chromosome, not present in A. boliviensis. In A. azarae, the analysis of different crosses confirmed the described correlation between X type and the XY female condition, showing that the three X types are transmitted to the progeny. XY females occassionally appear in the offspring of XX ones (a result also found in A. boliviensis), and the male progeny of XY females received the X chromosome from their fathers and the Y from their mothers. These results strongly point to an unidentified X linked mutation as the cause of sex reversal, and emphasize the importance of these species as attractive models for the study of mammalian sex determination.