Robertsonian fusion site in Rineloricaria pentamaculata (Siluriformes: Loricariidae): involvement of 5S rDNA and satellite sequences.

Cytogenetic studies demonstrated that unstable chromosomal sites in armored catfishes (Loricariidae) triggered intensive karyotypic diversification, mainly derived from Robertsonian (Rb) rearrangements. In Loricariinae, the presence of ribosomal DNA (rDNA) clusters and their flanking repeated regions (such as microsatellites or partial transposable element sequences) were proposed to facilitate chromosomal rearrangements. Hence, this study aimed to characterize the numerical chromosomal polymorphism observed in Rineloricaria pentamaculata and to evaluate the chromosomal rearrangements which originated diploid chromosome number (2n) variation, from 56 to 54. Our data indicate a centric fusion event between acrocentric chromosomes of pairs 15 and 18, bearing 5S rDNA sites on their short (p) arms. This chromosome fusion established the numeric polymorphism, decreasing the 2n from original 56 (karyomorph A) to 55 in karyomorph B, and 54 in karyomorph C. Although vestiges of telomeric sequences were evidenced at the fusion point, no 5S rDNA was detected in this region. The acrocentric chromosomes involved in the origin of the fusion were enriched with (CA)n and (GA)n microsatellites. Repetitive sequences in the acrocentric chromosomes subtelomeres have facilitated the rearrangement. Our study thus reinforces the view on important role of particular repetitive DNA classes in promoting chromosome fusions which frequently drive Rineloricaria karyotype evolution.