Akodon azarae is one of several rodent species in whose populations XY fully fertile females are found. The chromosomes of 83 specimens (40 females, 43 males) from three localities were studied. A high incidence of XY females was observed: 21 of the females presented an XY karyotype. The X chromosome relative length corresponded to 8.2% of the genome, but a significative portion of it consisted of C-positive material, giving to the euchromatic part a relative length of 5.07%, in agreement with the estimated size of the eutherian X. Four C-banding patterns (I-IV) of the X chromosomes were observed, one of them found in only one specimen. The X chromosome of males was type I, and XY females showed an X of type II or III. XX females presented one X of type I and the other of type I, II, III or (in only one animal) of type IV. The significance of this polymorphism with respect to the XY female phenotype is briefly discussed.
SUMMARY -The chromosomes of the South American rodent Calomys musculinus have been analyzed by standard Giemsa C-banding, chromomycin A3, distamycin A and DAPI fluorochrome staining and "in situ" digestion with the restriction endonucleases Alul, Haeiii and Hindiii. Constitutive heterochromatin is restricted to the centric regions, and fluoresces after either chromomycin A3 or DAPI treatments, indicating the co-existence of AT and GC-rich sequences in these regions. The restriction enzymes assayed induce diffuse, overlapping patterns of centric and interstitial bands in the chromosomes of this species. Our results agree with other authors' biochemical analyses that indicate the existence of moderately to highly repetitive DNA sequences in C. musculinus that do not appear as a satellite in neutral CsCI DNA gradients, as well as a great abundance of the cleavage sites of these restriction enzymes in the genome of this rodent.
Summary 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 DA-CMA, 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.
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