Abstract
Two Siluriformes species endemic to the Sao Francisco River basin were characterized by conventional and differential cytogenetic analyses involving C-banding, Ag-nucleolar organizer region (NOR) and chromomycin A3 (CMA3) staining, and FISH (fluorescent in situ hybridization) with 18S and 5S rDNA probes. Conorhynchus conirostris presents a higher diploid number (2n = 60) than those detected in Pimelodidae representatives, whereas Lophiosilurus alexandri, with a karyotype of 2n = 54 chromosomes, presents a chromosomal constitution similar to that found in the family Pseudopimelodidae. Plesiomorphic characteristics such as single NORs at terminal positions are found in both species, as revealed by CMA3 and silver nitrate staining, and FISH with a 18S rDNA probe. C-banding evidenced centromeric and telomeric heterochromatic blocks distributed over most of the chromosomes with a conspicuous heterochromatin segment in a pair of submetacentric chromosomes in L. alexandri. Such karyotype data, if compared to the cytogenetic pattern of other Siluriformes species, can be partially related to their degree of endemism, favorable to the occurrence and fixation of chromosomal rearrangements. The present study in representatives from these two Siluriformes families from the Sao Francisco River contributes to a better understanding of the karyotype evolution in species of this important order of Neotropical fishes.
Highlights
Endemism seems to be a common phenomenon along Brazilian hydrographic basins and appears widespread throughout several fish groups
Fluorescent in situ hybridization (FISH) using 18S and 5S rDNA probes was carried out according to Pinkel et al (1986), with modifications
The single Ag-nucleolar organizer region (NOR) were terminally located on the long arms of a subtelocentric chromosome pair (Figure 2a), and constitutive heterochromatin was detected on the centromeric and telomeric regions of most of the chromosomes (Figure 2c)
Summary
Endemism seems to be a common phenomenon along Brazilian hydrographic basins and appears widespread throughout several fish groups. Current taxa are endemic for two reasons: vicariance or dispersal. Two major factors influence the degree of endemism of a given area, its isolation and climatic stability. A richness of endemic species can be found in areas that were isolated or stable over long periods of time (Cox and Moore, 2000). Based on the distribution of Curimatidae fish species (Characiformes), Vari (1988) proposed a division of South American rivers at the Atlantic scope into eight main endemic areas, including the São Francisco River basin. The São Francisco River basin comprises the Brazilian States of
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