A NEW KARYOTYPE OF CALOMYS (RODENTIA, SIGMODONTINAE)

The genus Calomys Waterhouse, 1837 is widely distributed within South America, being found in Venezuela, Colombia, Peru, Bolivia, Brazil, Paraguay, Uruguay and Argentina. Specimens of Calomys were collected in Formoso do Araguaia, Tocantins, Brazil. For chromosome characterization standard staining techniques and as Gbanding and nucleolar organizer region were used. The karyotype was 2n=46 and AN=66. The X chromosome is a medium metacentric and the Y chromosome a small acrocentric chromosome. Chromosome homologies with other species were observed. Probably, karyotype differences were basically due to Robertsonian rearrangements.


INTRODUCTION
The genus Calomys Waterhouse, 1837 is widely distributed within South America, being found in Venezuela, Colombia, Peru, Bolivia, Brazil, Paraguay, Uruguay and Argentina (MUSSER & CARLETON, 1993).It is more frequently found in open areas, particularly valleys and humid fields and along riverside forests (REIG, 1984;EMMONS & FEER, 1997).
Few chromosome studies have been conducted on the fauna of the State of Tocantins, especially studies on small mammals.The aims is to describe a new karyotype found in specimens of Calomys captured in the State of Tocantins.

RESULTS
The analysis of 147 cells revealed a karyotype of 2n=46 and AN=66 for all specimens (fig.1).The autosome pairs 1 to 11 are metacentrics or submetacentrics, gradually ranging in size from large to small, except for pair 11, which is very small; pairs 12 to 22 are acrocentrics, gradually ranging in size from medium to small, except pair 22 which was very small.The X chromosome is a medium metacentric chromosome, which could not be distinguished from the autosomes of the same morphology and size.The Y chromosome is a small acrocentric and the smallest chromosome of the complement.
G-banding permitted pairing of all autosomes of the complement, as well as the unequivocal identification of the X chromosome (fig.2).The Y chromosome showed homogeneous staining.
Twenty metaphase cells from 3 specimens were used for NOR counting.The number of labelings per metaphase ranged from 4 to 11, with 8 being the modal number (fig.3).Labeling was always observed in the short arms of the medium and small acrocentric chromosomes (fig.4).

DISCUSSION
Three Brazilian Calomys species present the same AN as the Calomys sample studied here: Calomys sp.(YONENAGA, 1975) and C. laucha tener, (SVARTMAN & ALMEIDA, 1992), both caryotype with 2n=66 and AN=66, are probably the same species.The third species, C. expulsus with 2n=36 and AN=66 (GEISE et al., 1996) is very different from C. callosus expulsus with 2n=66 and AN=68 (SVARTMAN & ALMEIDA, 1992), however this karyotype, 2n=36 and AN=66, is very similar the C. lepidus (PEARSON & PATTON, 1976), with 2n=36 and AN=68 from Peru.Calomys sp., C. laucha tener, C. expulsus and the Calomys specimens found in Tocantins present the same AN, while the corresponding 2n is quite different.Therefore, it may be supposed that the differences in these karyotypes are basically due to Robertsonian rearrangements.In this respect, 10 rearrangements may have occurred between Calomys from Tocantins with 2n=46 and AN=66 and C. laucha tener with 2n=66 and AN=66, while in the case of C. expulsus with 2n=36 and AN=66, the difference would be attributable to plus 5 rearrangements.
Calomys specimens from Tocantins also show a high karyotype similarity with C. lepidus, which presents two karyotype variants: one variant from Peru with 2n=36 and AN=68, with all autosomes consisting of two-arms (PEARSON & PATTON, 1976), and the other from Argentina with 2n=44 and AN=68, with the autosome complement consisting of 13 two-arm and 8 telocentrics pairs (ESPINOSA et al., 1997) in AN may be explained by a mechanism of the fusion or fission type.
To answer these questions, G-banding patterns were compared between Calomys from Tocantins and the C. callosus expulsus with 2n=66 and AN=68 (SVARTMAN & ALMEIDA, 1992), which present G-banding results.Although the quality of the bands and the different degree of chromatin condensation were different, homology was observed, mainly, among the first pairs these species formerly cited.The following homologies were observed, respectively, between Calomys from Tocantins and C. callosus expulsus: large arms of pair 1 with pair 1, short arms of pair 1 with pair 2, large arms of pair 2 with pair 3, and acrocentrics pairs 12 with 15 and 13 with 17.We believe that the use of more adequate banding patterns with equivalent chromatin condensation levels will permit the determination of the sequence of events that occurred.
In Calomys from Tocantins, NOR labeling was restricted to the telomeric regions of the short arms of the medium and small acrocentric chromosomes.The number of NORs ranged from 4 to 11 and the modal number 8 was similar to that observed in C. callosus expulsus and C. tener (SVARTMAN & ALMEIDA, 1992).