Avaliação do comportamento meiótico e da viabilidade polínica em hídridos de Brachiaria decumbens resistentes à degrana

Abstract

The objective of this research was to select hybrids with better performance in microsporogenesis and with lower percentage of unviable pollen to be used in the forage improvement programs of Embrapa Beef Cattle. The genetic diversity in the genus Brachiaria is restricted and intra- and interspecific crosses are being performed to create variability. Accessions of B. decumbens were crossed and in the F1 generation hybrids that retained seeds in the inflorescence were selected. Cytogenetic evaluations were then performed to identify meiotic abnormalities which could be detrimental to pollen viability and consequently to seed production. For the meiotic studies, young inorescences were collected from twenty seven intraspecific hybrids, xed in a mixture of6 ethanol: 3 chloroform: 2 propionic acid for 24 h and stored under refrigeration. The microsporocytes and the pollen grains were prepared by smear and staining with 1% propionic carmine and analyzed under light microscopy. For each hybrid, 100 cells in each phase of meiosis, between diakinesis and tetrad stages were analyzed. Chromosomal associations were analyzed in diakinesis and the counting of chromosome number in anaphase I. The chromosome behavior was analyzed in from metaphase I until the tetrad stage, and the percentages of irregularities were related to irregular segregation of chromosomes, micronuclei formation, microcytes and polyads. The percentages of abnormal tetrads and unviable pollen were subjected to analysis of variance with the SAS 9.2 software. Later, the Scott Knott test was applied at 5% probability of error with the GENES program. The hybrids are tetraploid (2n=4x=36). The frequency of meiotic abnormalities variedbetween 7% and 100%.Depending on the phases and hybrids, and the behavior of chromosomes during meiosis characterizes asynchrony of genomes.In the final product of meiosis, the main abnormalities identified were micronuclei, microcytesand polyads.The high frequency of irregularities in the segregation of chromosomes affects pollen viability. Fifteen of the 27 hybrids evaluated presented viability below 32%. Of the 27 intraspecific hybrids, the apomictic hybrids R158, R169 and R184 can be kept for the development of new cultivars process and also be used as pollen donors, whereas the most promising sexual hybrids that can be utilized in new crossings as pollen receptors are R161, R163, R165, R167, R181, R193, S47, Y21 and Z9