Citogenética de espécies de Brachiaria: contribuições para a construção de mapas físicos

Abstract

Brachiaria spp. (Poaceae) are forage species. Cytogenetic analyses exhibit different ploidy levels and intra-specific variability related to chromosome morphology, and to the number and position of rDNA sites. Repetitive DNA sequences have been widely used in cytogenetic analyses, however the use of single copy genes or low copy number sequences is poorly explored, likely because no Brachiaria genome has been sequenced. Studies of centromeric sequences in Brachiaria spp. chromosomes, as well as karyotypes detailing the location of specific sequences in Brachiaria decumbens are nonexistent. Regarding other species of the genus, data are sometimes conflicting. Therefore, this study aims to characterize the karyotype of Brachiaria brizantha, Brachiaria decumbens and Brachiaria ruziziensis with the location of repetitive sequences of 5S and 45S rDNA, Centromeric Retrotransposons (CR), CMA/DAPI bands, and single copy genes/low copy number sequences in the chromosome pairs. Useful loci were obtained from RNAseq and sequencing data of Setaria italica and Sorghum bicolor, and the useful loci were used in FISH, and also to assess the transcriptional activity of the nucleolar organizing regions (NORs) in Brachiaria species. Karyograms were detailed, heteromorphic chromosome pairs were identified, and intraspecific and interspecific differences in the number and size of ribosomal DNA sites were detected. The karyograms of the three analyzed species are considered to be symmetric. The transcriptional activity of NORs is variable, and one or two heteromorphic nucleoli were detected. Some of the single copy gene/low copy number sequences are in synteny with ribosomal genes sites, which is important to identify some chromosomes. In Brachiaria spp., conserved structure of chromosomes segments is observed. Possible chromosomal breaks that took place during the species diversification process in the genus are identified. The use of genomic sequencing data had great importance for the enrichment of cytogenetic analyses.