Chromosomal dynamics and evidence of differentiation of the proto-sex chromosomes XY in Farlowella oxyrryncha (Loricariidae; Loricariinae).

Farlowella is the second most species-rich genus within Loricariinae; however, cytogenetic data for its species remain scarce. In this study, we employed conventional and molecular cytogenetic procedures on Farlowella oxyrryncha, a species widely distributed across the Amazon and Orinoco River basins. Karyotypic analysis revealed a conserved diploid number (2n = 58), with 12m+22sm+18st+6a, in addition to notable structural variation compared to other species of the genus. The pattern of constitutive heterochromatin (CH) facilitated the characterization of chromosome pairs. Furthermore, a conspicuous sex-linked heteromorphism was detected in chromosome pair 27, suggesting the presence of an XY sex chromosome system. Fluorescence in situ hybridization identified single site of 18S and 5S ribosomal DNA, with 18S colocalizing with U2 small nuclear DNA, simple sequence repeats, and CH. Interstitial telomeric sequences (ITSs) were found in five pairs, with dimorphic ITS patterns in pair 27, suggesting its role as a proto-sexual pair. The colocalization of repetitive sequences in pair 27 suggests that repetitive DNA plays a key role in the early differentiation of sex chromosomes. These findings indicate that F. oxyrryncha is a structurally dynamic lineage and provide new insights into karyotype evolution and sex chromosome differentiation in Loricariidae.