Basic and molecular cytogenetics in freshwater Cichlidae (Osteichthyes, Perciformes). Karyotypic conservationism and divergence

The chromosomes of one Cichlasoma facetum and three Geophagus brasiliensis populations from the headwaters of adjacent river basins (Paraná State, southern Brazil), were investigated using differential staining techniques (C-banding, Ag-NORs, DAPI and CMA3) and fluorescent in situ hybridization (FISH) with 18S rDNA and 5S rDNA probes. The diploid chromosome number (2n) was invariably 48 in the four populations, with karyotypes composed of 3 submetacentric and 21 subtelo/acrocentric chromosome pairs in G. brasiliensis and 5 submetacentric and 19 subtelo/acrocentric chromosome pairs in C. facetum, with no heteromorphic sex chromosomes. The differences detected in the FN numbers between G. brasiliensis (FN=54) and C. facetum (FN=58) indicate that pericentric inversion is the probable rearrangement that led to the cytogenetic divergence between these species. The overall karyotype similarity strongly suggests a close kinship among the three G. brasiliensis populations, despite a distinct C-banding pattern showed by the Jaguariaı́va river population. The data suggest that dispersion events were responsible for the present fish distribution in the river basins analyzed. The divergence in the C-banding pattern is probably due to gene flow restriction between populations during their geological/evolutionary history.