Identification of cryptic species in allopatric populations of Hypostomus tietensis (Siluriformes: Loricariidae) through cytogenetics analyses

Abstract Hypostomus is the most specious genus of Hypostominae, composed of several species with high intraspecific morphological and color pattern variation, making their identification a complex issue. One of the species with problematic identification is Hypostomus tietensis that was described from a single specimen, resulting in uncertainties about its color pattern and correct identification. To assist in this context, cytogenetic analyzes were carried out in three putative populations of H. tietensis from the Upper Paraná River basin, one of them from the type locality. The three populations showed considerable cytogenetic differences, with 2n = 72 chromosomes for the population from the type locality and 2n = 76 chromosomes for the others. Terminal NORs were detected (Ag- and 18S rDNA-FISH), being simple for the type locality population (acrocentric pair 23, long arm) and the Pirapó River (subtelocentric pair 11, short arm), and multiple for Do Campo River (subtelocentric pairs 11 and 12, short and long arm, respectively). C-banding was efficient in differentiating the type locality population from the others. Cytogenetic data revealed that populations from Pirapó and Do Campo rivers, although treated until now as Hypostomus aff. tietensis, represent a cryptic species, and those morphological analyses are necessary to differentiate and for describing this new species.

摘要 下口鲶属（Hypostomus）是下口鲶亚科（Hypostominae）物种多样性最高的属，包含多个种内形态与体色模式差异显著的物种，这使其物种鉴定成为复杂难题。其中鉴定存疑的物种之一为蒂氏下口鲶（Hypostomus tietensis），该物种仅基于单一标本发表，导致其体色模式与正确鉴定方法均存在不确定性。为解决上述问题，本研究对上巴拉那河流域的3个蒂氏下口鲶推定种群开展了细胞遗传学分析，其中一个种群采自模式产地。结果显示3个种群的细胞遗传学特征存在显著差异：模式产地种群的二倍体染色体数为2n=72，其余两个种群则为2n=76。通过银染（Ag-）与18S核糖体DNA荧光原位杂交（18S rDNA-FISH）均检测到末端核仁组织区（NORs）：模式产地种群仅在第23对近端着丝粒染色体的长臂上存在单一核仁组织区，皮拉波河种群则在第11对亚近端着丝粒染色体的短臂上存在单一核仁组织区，而多坎普河种群的核仁组织区为多重分布，分别位于第11对亚近端着丝粒染色体的短臂与第12对亚近端着丝粒染色体的长臂上。C带染色可有效区分模式产地种群与其余两个种群。细胞遗传学数据表明，尽管皮拉波河与多坎普河种群此前被归为蒂氏下口鲶近缘种（Hypostomus aff. tietensis），但它们实际代表一个隐存种，未来需开展形态学分析以区分该新种并完成其正式描述。