High intraspecific karyological conservation in four species of Gymnotus (Pisces: Gymnotiformes) from Southeastern Brazilian basins

SUMMARYFour species of the genus Gymnotus from Southeastern Brazilian basins were extensively studied in relation to their karyology. A high intraspecific conservatism was detected and the cytotypes were species-specific, in relation to diploid number, NOR-bearing chromosomes and C-band pattern. No geographic chromosome races were found. The differences detected in the distribution of NOR associated heterochromatin were considered as a good evidence of the heterochromatin loss that seems to have occurred in the evolutionary story of this group of fishes. Gymnotus is a complex of species which the variation is still poor understood. The cytogenetic data here described allowed the reinterpretation of previous karyological data already available in literature and is an attempt to contribute to a better understanding of this group of Neotropical freshwater fishes.