Caracterização molecular e análise cromossômica comparativa de snRNAs em espécies de Ancistrus (Siluriformes:Loricariidae).

Abstract

Popularly known as “armored catfishes”, the fishes belonging to Loricariidae have their bodies covered by ossified dermal plates organized in series, in addition to a sucking mouth in the ventral position. Loricariidae presents a controversial taxonomy with a lot of undescribed species in the scientific literature. This family is divided in six subfamilies (Lithogeneinae, Delturinae, Rhinelepinae, Loricariinae, Hypoptopomatinae and Hypostominae), presenting species with extensive chromosome variation, due to a high variation in the diploid number (2n) and karyotype formulas. Diverging from the 2n = 52 chromosomes, considered basal for Ancistrini, the Ancistrus genus (Hypostominae) shows a 2n reduction in most species via centric fusions events (Robertsonian – Rb), in addition to inversions, transpositions and translocations events, which have contributed to the chromosome morphology variation. These rearrangements usually are associated to DNA double-strand breaks in highly repetitive unstable regions, considered “evolutionary breaking points”, which are considered hotspots for chromosomal rearrangements. These repetitive sequences can be organized in blocks (multigenic families, satellite, mini-satellites and micro-satellites DNAs) or are dispersed throughout the genome (transposable elements - TEs). The U snRNA multigenic family participates in pre-mRNA splicing process. Considering cytogenetic analyzes, U snRNAs are considered good markers for the understanding of chromosomal rearrangements and are useful to understand the evolutionary relationships among closely related species. Aiming to understand what types of repetitive DNA sequences are involved in chromosomal fission/fusion events, which probably generated the karyotype diversity present in the genus, it was performed the description and chromosomal mapping of U1, U2, U4, U5 and U6 snRNAs sequences on the genome of three species of Ancistrus (Ancistrus aguaboensis - 2n = 50, Ancistrus cf. multispinis - 2n = 52 e Ancistrus sp. - 2n = 50). The obtained nucleotide sequences and the predicted secondary structures showed similarities with its correspondents in fish genomes. The U2 and U5 snRNAs probes showed clustered and co-localized sequences in a single chromosome pair, whereas the U1 snRNA sites were present in a single pair. The in situ location of U4 snRNA probes showed sites dispersed along the chromosome pairs, however U6 snRNA markers were not detected by FISH. Due to their reuse in chromosomal rearrangements, the rDNAs sites, previously described as “evolutionary breaking points” in Loricariidae, were not co-located to the snRNAs sites. This is the first report describing the characterization and chromosome mapping of U snRNAs sequences in Ancistrus. According to the obtained cytogenetic data, probably this multigenic family is not involved to the chromosomal rearrangements that caused the karyotype diversification in species of Ancistrus.