Abstract
Akodon is the richest Sigmodontine genus in terms of number of species. This article analyzes the chromosomal banding produced in A. azarae, A. molinae and A. dolores by the fluorochromes 4, 6-diamidino-2-phenylindole (DAPI) and chromomycin A3 (CMA), counterstained in some cases with distamycin A (DA). A. azarae populations have fertile XY females, and A. molinae and A. dolores, a pair of closely related species, show Robertsonian polymorphisms. In the 3 species, as generally observed in mammals, DAPI-treated chromosomes presented a G/Q-like fluorescent pattern, and DA-CMA treatment gave R-type banding. The constitutive heterochromatin of A. azarae, comprising the centric regions of the autosomes (excepting the small metacentric), the Ychromosome, and several bands of the X amounting to almost 38% of its length, fluoresced with DAPI and appeared negative with DA-CMA, indicating the relative AT-richness of their repetitive components. In A. molinae and A. dolores, no characteristic fluorescence was noticed in the small heterochromatic regions of the autosomes. The heterochromatic Y of these species could be differentiated into a centric portion (approximately one third of the chromosome) that fluoresced with DACMA, and was therefore relatively GC-rich, and a distal portion that fluoresced brightly with DAPI, which was thus relatively AT-rich; the small centric band of the X fluoresced with CMA. The response of the gonosomal centric regions indicates that these chromosomes associate by their centromeric ends in diakinesis-metaphase I. These observations confirm the utility of fluorochromes that are highly selective for DNA base pair composition in cytogenetic studies, and further our understanding of the chromosomal evolution in this genus.
