Cytogenetic markers to understand chromosome diversification and conflicting taxonomic issues in Rineloricaria (Loricariidae: Loricariinae) from Rio Grande do Sul coastal drainages

South Atlantic hydrographic basin comprises several small coastal drainages isolated from each other and the basins of the Brazilian crystalline shield. Tramandai River and Patos Lagoon sub-basins occur at the southern end of this system characterized by a high level of ichthyofaunistic endemism. In the present study, we investigated the karyotype of six species of whiptails armoured catfishes from these two hydrographic basins. The results revealed interspecific diploid number variations: 2n = 64 in Rineloricaria malabarbai and R. cadeae, 2n = 68 in R. microlepdogaster, and R. aequalicuspis and 2n = 70 in R. quadrensis and R. longicauda. Species with te same diploid number are easily discriminated by karyotype formulas and the number of 5S rDNA sites. Our study provides a set of efficient chromosome markers to confirm the taxonomic status of six Rineloricaria species from Patos Lagoon and Tramandai River basins. We believe that successive vicariant process promote geographical isolation enhancing the fixation of chromosomal rearrangements, leading to the observed karyotypic variabilities. This is a plausible hypothesis if we consider that the current configuration of Rio Grande do Sul coastal drainages was shaped by a complex interation of geomofological processes and Pleistocenic/Holocenic sea-level fluctuations.

Automated summary

The study investigated the karyotypes of six species of whiptails armoured catfishes from the South Atlantic hydrographic basin, revealing interspecific diploid number variations. Species with the same diploid number can be easily discriminated by karyotype formulas and the number of 5S rDNA sites. The research suggests that successive vicariant processes promote geographical isolation, enhancing the fixation of chromosomal rearrangements leading to observed karyotypic variabilities.