Comportamento cromossômico meiótico e miótica de acarás-disco (Symphysodon aequifasciatus e Symphysodon discus, Cichlidae, Perciformes) da Amazônia Central

Abstract

Meiotic and mitotic cytogenetics studies were carried out on species: Symphysodon aequifasciatus collected from the Manacapuru River- Manacapuru (AM) and Symphysodon discus from the Negro River- Novo Airão (AM). All individuals showed a modal number of 2n=60 chromosomes and there was no sex chromosomal heteromorphism in these species. Regarding the chromosomal formulae, S. aequifasciatus showed 46 M-SM + 14 microchromosomes and S. discus 50 M-SM + 10ST-A. However, some S. aequifasciatus individuals showed a Giemsa non-stained secondary constriction, looking like a chromosomal polymorphism. The constitutive heterochromatin was seen as large pericentromeric blocks on most chromosomes, of both species, however only S. aequifasciatus showed six wholly heterochromatic pairs. Multiple nucleolus organizing regions were identified in both species, S. aequifasciatus presented ribosomal sites on chromosomal pairs 4, 5, 11 and 21, and S. discus on the pairs 17 and 23, but never more than two sites showed to be active. In the meiosis, the greater difference observed between the two species was during diplotene/diacinesis, because S. aequifasciatus showed a chromosomal chain with 20 elements and 20 bivalents, while S. discus presented only 30 bivalents. Thus, 2n=60 chromosomes, in both species, and a meiotic chromosomal chain, in S. aequifasciatus, can be accounted for the Symphysodon polyploidy origin, since the ancestral number to the Ciclhidae family is 2n=48 chromosomes, most of them of the acrocentric type. Moreover, the S. aequifasciatus chromosomal chain points that this species is probably found in a transience period, while S. discus finished its diploidization process, on account of it being the older species.