Chromosomal and Synaptonemal Complex Analysis of Robertsonian Polymorphisms in Akodon dolores and Akodon molinae (Rodentia, Cricetidae) and their Hybrids

A cytogenetical study was performed on several populations assigned to Akodon dolores by external morphology. The specimens from Yacanto (type locality of A. dolores) and the near Villa Dolores, however, showed the karyotype of Akodon molinae, presenting the Robertsonian polymorphism of pair 1 described in this species. In the populations with “dolores” karyotype, most specimens were heterozygous for at least one Robertsonian polymorphism in pairs 1 to 5. A discriminant analysis of craniometric values failed to differentiate between groups with “dolores” or “molinae” karyotype. In the F1 and F2 individuals obtained by crossing specimens from Yacanto or V. Dolores with specimens from Chucul (“dolores” karyotype) all the possible chromosomal constitutions in pairs 1 to 5 were seen. These results favor the possibility that A. molinae represents a chromosomal race of A. dolores. In male Robertsonian heterozygotes for one or more pairs, the corresponding number of trivalent complexes was observed in micro-dispersed pachytene meiocytes; these were characterized by the presence of a short perpendicular segment formed by the pairing of the centromeric ends of the subterminal or terminal chromosomes.