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Interspecies repetitive DNA homology was studied in akodont rodents related at generic and suprageneric levels. The homology was determined by taking the species Akodon molinae as the reference species. The 3H-DNA/DNA hybridization on filters showed a closer relationship between A. molinae and A. azarae, A. dolores and A. mollis than between A. molinae and Bolomys obscurus. These data agree with the taxonomical ranking of the species. The quantity and quality of the hybrid DNAs were measured by investigating their thermal stabilities and subsequent comparison to the results obtained on the reference species. These data indicate high similitude between the repetitive DNA of A. dolores and A. molinae. Increasing differences were shown to occur in the repetitive DNA of A. mollis, B. obscurus and A. azarae, respectively. Since these results coincide with the G-banding homologies and differ slightly from the taxonomical rank, it is speculated that the divergency between the DNA of A. molinae and A. azarae is the result of a differential process of DNA amplification which is not related to the phylogenetical distance separating the two species.

The cytogenetic study of a population of A. molinae showed individuals with 2n = 42 chromosomes including a large metacentric pair (chromosome 1); individuals with 43 chromosomes having one chromosome 1 and two large sub-terminal elements (chromosomes 1a and 1b), homologues of the long and short arms of the chromosome 1, respectively; and animals with 44 chromosomes having a pair of 1a and a pair of 1b chromosomes. The analysis of chromosome length combined with g- and c-bands indicate that there are only two probable mechanisms of chromosome rearrangement explaining the pair 1 polymorphism. In the first process it is assumed that chromosome 1 arose by centric fusion of two acrocentric chromosomes following pericentric inversions of the short arms of chromosomes 1a and 1b. The data given by c-bands indicate that this hypothesis should be complemented by deletions of the centromeric heterochromatin of 1a and 1b chromosomes. The second mechanism suggests that chromosomes 1a and 1b appear via a centromeric breakage of chromosome 1 followed by pericentric inversions. The blocks of c-heterochromatin in chromosomes 1a and 1b are explained by a process of saltatory replication.