Chromosomes of Octomys mimax and Octodontomys gliroides and Relationships of Octodontid Rodents

Octomys mimax possesses a chromosomal complement of 2n = 56, FN = 108, with heterochromatin restricted to the pericentric regions in all chromosomes except one autosomal pair and the X chromosome. One chromosome also has heterochromatin in a secondary constriction. C-bands of Octodontomys gliroides reveal the clear presence of pericentric heterochromatin on the X chromosome and six autosomal pairs. Lightly stained, pericentric C-bands also were present in five other autosomes. A satellite-bearing chromosome did not band. In both species, two active nucleolar organizing regions (AgNOR) were located at secondary constrictions. Based on chromosomal differences found among O. mimax and other octodontids, we hypothesize that the identical or similar 2n and FN observed for O. mimax, Aconaemys fuscus, and some other octodontid species is a result of parallel evolution.