Extraordinary chromosomal polymorphism with 28 different karyotypes in theneotropical species Akodon cursor (Muridae, Sigmodontinae), one of the smallest diploid number in rodents (2n = 16, 15 and 14)

All available published cytogenetic data show the presence of 28 different karyotypes in 311 specimens of ii. cursor as an exceptional example of high karyotype variability in a single species. Our present sample of 116 anima ls collected in the rain forest of the Atlantic coast of the states of Sao Paulo and Bahia, Brazil, show 25 karyotype constitutions. The diploid numbe r (2n) ranged from 16 to 14, and the number of autosomal arms (NF) from 26 to is, because of centric fusion and pericentric inversions involving two a utosome pairs, pericentric inversions in three other chromosome pairs, tris omy in the pair 7 and the presence of two XO females. Synaptonemal complex analysis, associated with data from experimental cross-breeding, suggested that heterozygous individuals for pericentric inversions have normal fertil ity. In this paper, we have reviewed the chromosomal data of this species, and have thus standardized the karyotype description and chromosome numberi ng. We discuss about karyotype evolution of Akodon cursor based on the freq uency and constitution of karyotypes of all different geographical samples described so far in the literature.