Echimys semivillosus, a rodent species with a very high chromosomal number

We analyzed karyologically four specimens of Echimys semivillosus (Rodentia , Echimydae) from two localities in Northern Venezuela after in vivo colchi cine treatment. The bone marrow metaphases were stained using Giemsa and C- and G-banding. The diploid number (2n=94) turned out to be among the highe st recorded for a mammalian species. The autosomal set of the karyotype of E. semivillosus consists of 11 pairs of metacentric or submetacentric chrom osomes and 35 pairs of acrocentric ones. This karyotype is different from t hat of Echimys sp. from Central Brazil. The high 2n of E. semivillosus was interpreted as an ancestral character for the Echymys genus and for caviomo rph rodents in general. Heterochromatin, revealed as C-bands, was localized at the pericentromeric regions of every chromosome and in large terminal b locks in the majority of them. The pattern of distribution of the C-positiv e regions was compared with the general patterns described for eukaryotes a nd with that occurring in species of the sister genus Proechimys.