South Atlantic hydrographic basin comprises several small coastal drainages isolated from each other and the basins of the Brazilian crystalline shield. Tramandaí 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.

南大西洋水文盆地包括几个相互隔离的小型沿海排水系统和巴西结晶地盾的盆地。Tramandaí 河和 Patos Lagoon 子流域出现在该系统的南端，其特征是高度的鱼类动物群落特有现象。在本研究中，我们调查了来自这两个水文盆地的六种鞭尾鲇鱼的核型。结果揭示了种间二倍体数目的变化：2n = 64 在Rineloricaria malabarbai和R. cadeae 中，2n = 68 在R. microlepdogaster和R. aequalicuspis 中，2n = 70 在R. quadrensis和R. longicauda. 具有相同二倍体数的物种很容易通过核型公式和 5S rDNA 位点的数量来区分。我们的研究提供了一组有效的染色体标记，以确认来自帕托斯泻湖和特拉曼代河流域的六种Rineloricaria物种的分类地位。我们认为连续的替代过程促进了地理隔离，增强了染色体重排的固定，导致观察到的核型变异。如果我们考虑到南里奥格兰德沿海排水系统的当前配置是由地质学过程和更新世/全新世海平面波动的复杂相互作用形成的，那么这是一个合理的假设。