Mapeamento cromossômico comparativo de ciclídeos amazônicos utilizando sequências repetitivas de DNA

Abstract

In addition to their importance as food sources and for ornamental purposes and aquiculture, cichlids are considered models in genetic and evolutionary studies. These fish are distributed throughout Central and South America, Africa, Madagascar, and southern India. All of the neotropical cichlids belonging to the subfamily Cichlinae, which is in turn subdivided into seven tribes: Cichlini, Retroculini, Astronotini, Chaetobranchini, Geophagini, Cichlasomatini, and Heroini. Classical cytogenetic investigations have focused on the neotropical group, but more information is still needed concerning their physical chromosomal mapping as well as comparative analyses of their repetitive sequences fundamental approaches for the identification of chromosomal rearrangements and for more precise interpretations of the genomic organization of this group of fish. In that sense, the present study undertook a comparative study of the genomic organization of specimens of the basal (Cichlini and Astronotini), intermediate (Geophagini), and derived (Heroini) tribes of Cichlinae using both classical and molecular cytogenetic techniques, utilizing 18 and 5S DNAr, telomeric sequences, retrotransposons Rex1, 3 and 6, microsatellite sequences, and repetitive Cot-1 DNA fractions as probes for chromosomal mapping. Analyses of nucleotide sequences were also performed. Variations were seen in diverse cytogenetic markers, although no tendency was confirmed in these tribes in terms of increasing, decreasing, or maintaining the basal diploid number 2n=48. Some species demonstrated genetic duplications of 18S DNAr, as well as simple or multiple sites. 5S DNAr was located in the interstitial region of a homologous chromosome pair in most of the taxa analyzed, although variations on this pattern were seen. Interstitial telomeric sites (ITS) were observed on the chromosomes of some species and were related to chromosomal rearrangement events. In terms of retroelements, physical mapping indicated abundant markers dispersed along the chromosomes of most species. Some species demonstrated conspicuous markings in their terminal and centromeric portions that were associated with heterochromatic as well as euchromatic chromosomal regions. The Rex1 element demonstrated greater numbers of markings in derived species. The nucleotide sequences of Rex1 and 3 were more conserved in basal species than in derived species, although this same pattern was not observed for Rex6. Evidence for different lineages was observed in Rex1, as well as possible horizontal transfer events marking the evolution of Rex1 and 3 in the cichlid genome. The mapping of the Cot-1 DNA repetitive fraction and microsatellite sequences indicated centromeric and telomeric sites in some species but dispersed sites in others. It was also possible to determine through Cot-1 DNA analyses that the phylogenetically more closely related species demonstrated greater similarity in the organizations of the repetitive fractions of their chromosomes. Our data indicated ample genomic and karyotypic diversity among Neotropical cichlids, suggesting that most of this diversity occurred due to repetitive sequences that influenced the evolution of these fish.