Estudo da dinâmica de replicação cromossômica em Astyanax aff. scabripinnis (Teleostei: Characidae), com ênfase na natureza dos cromossomos B

Abstract

The B chromosomes are genomic components found in several taxonomic groups of plants and animals. So far no evidence of detectable phenotypic effects was found in specimens carrier chromosome B. In order to know the location, variability and nature of the chromatin of the B chromosome, and try to elucidate transcript regions, it is necessary, a priori, to obtain chromosomal bands patterns. After the determination of these replication bands (RBG bands), it is possible to know where the B chromosome begins to replicate, in the early S phase. These replication bands may have a local chromatin used as genome raw material for transcription. In this study 35 specimens of Astyanax aff. scabripinnis were analyzed and captured from rivers of Campos do Jordão regions, São Paulo, in populations previously identified by the presence of B chromosomes. The sample were made at one point of the Fazenda Lavrinha stream, Paraíba do Sul river basin. The specimens of the A. scabripinnis population presented a diploid number of 2n=50 and 2n=51, with a supernumerary metacentric B chromosome. It was possible to identify light and dark regions on B chromosome, at the chromosomal replication beginning, which confirm that replication does not happen in a homogeneous way. Thus, the B chromosome of A. scabripinnis has a chromosomal compartmentalization, similar to the isocores observed in other metazoan, which are involved in the genome compartmentalization and in the transcription regulation. By incorporating the analogue base BrdU in vivo, it was possible to find regions of B chromosome that replicate in the early S phase, different to the regions of late replication. In this perspective, the B chromosome of this species has available territory and chromatin to transcribe, especially in the initial replication of light bands, which are: p1.1; p1.3; p2.2 e q1.1; q1.3; q2.1; q2.3. The late replication regions match to constitutive heterochromatin blocks and to the As51 repetitive sequences location. GC-rich regions in a different contrast were evidenced with fluorochrome chromomycin CMA3, overlapping early replication regions, and confirming the replication bands and C-banding results. Proving and defining that the B chromosome has chromatin for transcription, thus it is necessary to continue this study to identify which informations are transcribed, and what possible phenotypic effects these potential genes located in the early replication regions may bring to the B chromosome keeper.