B chromosomes in four different populations of Rhamdia quelen (Siluriformes, Heptapteridae): a comparative study of frequency and distribution.

In the present study 25 specimens of Rhamdia quelen from four different localities were analyzed cytogenetically. All showed a diploid number of 58 chromosomes, with a karyotypic formula of 36m + 16sm + 6st and FN = 116. Metacentric B chromosomes showing inter- and intraindividual variation were observed in all populations. C-banding revealed differences in the heterochromatin distribution pattern, with evidence of completely heterochromatic B chromosomes in three populations: Agua das Pedras, Agua dos Patos and Taquari rivers, and partially heterochromatic B chromosomes in the population at the fish farm station in Timbó/SC. The occurrence of B chromosomes in such distinct populations suggests that they could have arisen from the same ancestral state, before the geographic dispersal of this species.