Citogenômica comparativa e aspectos epigenéticos em espécies de Brachiaria e híbridos interespecíficos

Abstract

The Brachiaria (Trinius) Grisebach genus consists of species with different ploidy levels and reproduction mode, some of great economic importance are used as forage. Studies about the genomic constitution and relationship within of the genus are restricted, as well as the evaluation of epigenetic marks that are involved in structural and functional organization of the genome. The aim of the first study was to investigate the genomic relationship among B. ruziziensis, B. decumbens and B. brizantha by means of Genomic in situ Hybridization (GISH). In addition, Fluorescence in situ Hybridization (FISH) with 45S and 5S rDNA and quantification of the nuclear genome size were performed. The hybrid 1863 (B. ruziziensis x B. brizantha) presented chromosome number (2n = 36), DNA amount (3.24 pg) and four sites of 45S rDNA and seven of 5S as expected from crossing between B. ruziziensis and B. brizantha. The hybrid cv. Mulato II [(B. ruziziensis x B. decumbens) x B. brizantha] also presented 2n=36 chromosomes, seven sites of 5S rDNA, four of 45S and 3.83 pg of DNA, higher than the average of the parents (3.43 pg). The hybrid 963 (B. ruziziensis x B. decumbens) all metaphases presented 2n = 38 chromosomes, exceeding 2 chromosomes to the expected, which represented an increase of 0.29 pg of DNA, considering the average of DNA content of the parents (3.33 pg ) and the presence of five sites of 45S rDNA and seven of 5S. The GISH analysis in the hybrid indicated the occurrence of rearrangements and differences in the contribution of parent genomes in the constitution of the hybrid. Based on GISH results, was proposed the genomic constitution of B. brizantha (BBB1B 1 ), B. decumbens (B1B 1B 2B 2 ) and B. ruziziensis (B2B 2 ). The genomes B, B1 and B2 were considered homeologous, with lower affinity between the genomes B e B2 . In a second approach, the aim was to evaluate the histone post-translational modifications and DNA methylation associated with chromatin modulation and gene silencing, in Brachiaria species and hybrids which have different ploidy levels and reproduction mode. Comparison of chromosomal distribution in histone modifications among species and hybrids showed greater variation of chromosomal types for the heterochromatic marks (H3K9me2) in B. decumbens and the 963 hybrid, while, for the euchromatin epigenetic mark (H3K4me2), the variation was higher in B. brizantha, B. decumbens and 963 hybrid. The chromosome distribution of 5-mCyt was similar between B. brizantha and B. decumbens, tetraploid and apomictic species, which differ from the distribution observed in B. ruziziensis, a diploid and sexual species. Significant alterations in DNA methylation were observed in the duplicated B. ruziziensis and in the interspecific hybrids, possibly as a result of hybridization and polyploidization process.