The distribution of the vertebrate telomeric sequence T₂AG₃ in three species of the rodent genus Akodon was examined by FISH with a peptide nucleic acid probe. In addition to the expected telomeric hybridization, non-telomeric signals were observed in the three species. In A. dolores, centromeric signals were visible in two of the four biarmed autosome pairs featuring Robertsonian polymorphism, indicating the retention of at least part of the telomeric sequences during the fusion process, and an interstitial signal of lower intensity was observed in the short arm of another. In A. boliviensis, a strong signal was observed near the centromeric end of the first chromosome pair. The first pair of A. azarae (homologous to the first pair of A. boliviensis) showed a similar but markedly amplified signal, and a subcentromeric signal in the X chromosome corresponding to a heterochromatic region; additionally, interstitial signals of lower intensity were present in one to four chromosomes in the majority of cells examined.

The populations of several species of Akodon present, besides XX females, a variable proportion of fertile XY females. In Akodon azarae, a correspondence exists between the X-chromosome C-banding pattern and the sexual phenotype of XY individuals: males carry a determinate X-chromosome type, defined by its C-banding pattern, and XY females, any of two others. To confirm the relation between X-chromosome type and the XY female condition and to investigate the hereditary transmission of these different X-chromosomes, we analyzed 50 animals captured in the field and 95 individuals corresponding to the F 1 and F 2 offspring of 16 crosses.It was seen that the correlation between X type and the sexual phenotype of XY animals is retained, and that the three X types are transmitted to the progeny. It was also observed that the male offspring of XY females receive the X-chromosome from their male parents and the Y from their mothers. These results strongly support the causal role of an X-borne mutation in A. azarae XY sex reversal, and discard a mutation of the Y-chromosome as the sole basis of this phenomenon.

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Several species of the South American genus Akodon present fully fertile XY females besides XX ones. To analyze the possibility of a Sry mutation as the cause of sex reversal in A. azarae and A. boliviensis, we determined the sequence of the Sry gene in 2 males and 3 XY females from each of these species. The Sry gene sequence was also studied in A. dolores, a species that does not have XY females. In inter-specific comparisons, the percentage identities with respect to the region analyzed varied between 96.8% and 97.9%. An ORF of 543 nucleotides was identified, and the predicted Sry proteins comprised 180 amino acids, with an HMG domain of 83 amino acids. Our results indicate that female sex reversal in A. azarae and A. boliviensis cannot be explained by sequence differences in the Sry region analyzed here, which includes the complete ORF and, together with previous results concerning the inheritance of the XY condition, show that Sry mutation is not the basis of sex reversal.

Summary The genus Akodon, comprising more than 60% of the species of the Akodontini, widely distributed in South America, presents many interesting cytogenetic atributes, such as an ample karyotypic variation (2n=10-52), intraspecific and intrapopulational polymorphisms of the autosomes and of the gonosomes, and, as in both species here studied, XY fertile females. We report a cytogenetic comparative study of A. azarae (2n=38) and of A. boliviensis (2n=40) specimens. In these species, only the sex chromosomes, the first autosomal pair and the small bibrachial autosome characteristic of the genus, can be identified in conventional preparations. Chromosome relative lengths of each species were determined from G-banded karyotypes, and a schematic representation of the G-banding patterns is presented. The comparison of these patterns shows that 16 autosomal pairs are "shared" chromosomes, corresponding to 88.3% of the autosomal complement of A. azarae and to 87.5% of that of A. boliviensis. Furthermore, A. azarae pair 2 (8.23% of its autosomal complement) has a banding pattern homologue to pair 15 and most of pair 12 of A. boliviensis, which indicates that this chromosome of A. azarae would basically result from a tandem translocation. Apparently, no correspondence exists between the remaining pair 16 of A. azarae and pair 11 of A. boliviensis. Key words Akodontini, Akodon boliviensis, Akodon azarae, Chromosomes, G-bands.The genus Akodon (Reig 1986(Reig , 1987 is the most polytypic of the akodontine tribe of the South American Sigmodontinae, comprising more than 60% of the species of the tribe (Apfelbaum and Reig 1989, Reig 1987). Its species are extensively distributed in South America, and can be found in different habitats through most part of the subcontinent. This genus is cytogenetically very interesting, not only because of its systematic complexity, but also due to the considerable karyotypic variation existent (2n= 10-52), the intraspecific variation concerning the autosomes and the sex chromosomes, and the presence of species with fertile XY females