Integrative cytogenetic and molecular analyses reveal chromosomal polymorphism in Rineloricaria langei (Siluriformes: Loricariidae): reporting the highest diploid number in the genus

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Abstract The scarcity of cytogenetic and genetic data on Rineloricaria langei highlights the need for comprehensive investigations. This study provides the first cytogenetic characterization of the species and reports new genetic data based on a population sampled from its type locality. Cytogenetic analyses revealed two karyomorphs with diploid numbers of 2n = 72, the highest reported for the genus, and 2n = 70. Despite differences in karyotype composition, both share similar structural features, including predominantly pericentromeric heterochromatin, a single 18S rDNA site, and multiple 5S rDNA sites, as detected by fluorescence in situ hybridization (FISH). Molecular analyses identified three haplotypes (H1–H3) of R. langei, with a maximum divergence of 0.4%. The inclusion of public COI sequences increased the total to 18 haplotypes and revealed interspecific distances exceeding 4%, identifying R. zawadzkii as the sister species. Species delimitation analyses (ASAP, PTP, and GMYC) recovered 10–12 OTUs for the genus but consistently delimited a single OTU for R. langei. These findings reveal intrapopulational chromosomal polymorphism associated with low mitochondrial genetic divergence and reinforce the value of integrative approaches to understanding genetic and structural variability in R. langei.