Cytogenetic and molecular identification of Chaetostoma bifurcum (Siluriformes: Loricariidae) from the Pacific Coast of Ecuador

This study presents an integrated molecular and cytogenetic characterization of Chaetostoma bifurcum from Ecuador, which belongs to the Hypostominae subfamily. Morphological identification was confirmed by molecular data using cytochrome c oxidase I and cytochrome b sequences. Karyotype analysis was performed with conventional staining and fluorescence in situ hybridization (FISH). The karyotype comprises 48 metacentric/submetacentric and 6 subtelocentric/acrocentric chromosomes, maintaining the chromosome number (2n = 54) considered ancestral in Loricariidae, though uncommon in Hypostominae. No differences were observed between female and male karyotypes, suggesting absence of heteromorphic sex chromosomes. Faint interstitial telomeric sequences (ITSs) were detected on a submetacentric pair, whose origin remains unclear and may represent remnants of translocations resulting from DNA repair mechanisms or be part of satellite DNA. The 18S rDNA and 5S rDNA loci are localized on distinct chromosomes (pair 11 and 6, respectively), deviating from the plesiomorphic synteny observed in Loricariidae. This pattern may reflect both evolutionary history but also the effect of ecological pressures. Thus, comparative analyses across other Chaetostoma species are essential to determine whether these karyotypic traits reflect their phylogenetic position or specific adaptive processes since this study corresponds to the first characterization for the genus.