Microsporogenesis in Brachiaria dictyoneura (Fig. & De Not.) Stapf (Poaceae: Paniceae)

icrosporogenesis was analyzed in five accessions of Brachiaria dictyoneura presenting x = 6 as the basic chromosome number. All accessions were tetraploid (2n = 4x = 24) with chromosome pairing in bi-, tri-, and quadrivalents. The recorded meiotic abnormalities were those typical of polyploids, including precocious chromosome migration to the poles, laggard chromosomes, and micronucleus formation. The frequency of these abnormalities, however, was lower than those reported for other polyploid accessions previously analyzed for other Brachiaria species. Cell fusion and absence of cytokinesis were also recorded in some accessions, leading to restitutional nucleus formation in some cells. Genetically unbalanced microspores, binucleate, and 2n microspores were found among normal meiotic products as results from these abnormalities. The limitation in using these accessions as pollen donor in interspecific crosses with sexual species with x = 7 or x = 9 in breeding programs is discussed.

Evidence of allopolyploidy in Brachiaria brizantha (Poaceae: Paniceae) through chromosome arrangement at metaphase plate during microsporogenesis

In the hexaploid (2n = 6x = 54) accession B176 of Brachiaria brizantha, one cytological characteristic differentiated it from the other accessions previously analyzed with the same ploidy level. Nearly 40% of meiocytes displayed the chromosome set arranged at two metaphase plates at the poles of the cell, close to the membrane. In these cells, both metaphase plates were arranged in an angle to form a typical tripolar spindle. Therefore, cells did not show normal chromosome segregation at anaphase I. Only nine univalent chromosomes migrated from each plate to the opposite pole with the remainder staying immobile on the plate. As a result of such spindle orientation and chromosome behavior, trinucleate telophases I were recorded. After telophase, cytokinesis eliminated the small nuclei into a microcyte. The second division proceeded normally, with the presence of microcytes in all phases. The origin of such an abnormality was explained on the hexaploid level of the accession which could have resulted by chromosome doubling of a triploid derived from species that did not display the same behavior for spindle organization. The high percentage of meiotic abnormalities recorded in this accession compromises fertility and renders it inadequate for the breeding program.

Symmetric pollen mitosis I and suppression of pollen mitosis II prevent pollen development in Brachiaria jubata (Gramineae)

Microsporogenesis and pollen development were analyzed in a tetraploid (2n = 4x = 36) accession of the forage grass Brachiaria jubata (BRA 007820) from the Embrapa Beef Cattle Brachiaria collection that showed partial male sterility. Microsporocytes and pollen grains were prepared by squashing and staining with 0.5 percent propionic carmine. The meiotic process was typical of polyploids, with precocious chromosome migration to the poles and laggards in both meiosis I and II, resulting in tetrads with micronuclei in some microspores. After callose dissolution, microspores were released into the anther locule and appeared to be normal. Although each microspore initiated its differentiation into a pollen grain, in 11.1 percent of them nucleus polarization was not observed, i.e., pollen mitosis I was symmetric and the typical hemispherical cell plate was not detected. After a central cytokinesis, two equal-sized cells showing equal chromatin condensation and the same nuclear shape and size were formed. Generative cells and vegetative cells could not be distinguished. These cells did not undergo the second pollen mitosis and after completion of pollen wall synthesis each gave rise to a sterile and uninucleate pollen grain. The frequency of abnormal pollen mitosis varied among flowers and also among inflorescences. All plants were equally affected. The absence of fertile sperm cells in a considerable amount of pollen grains in this accession of B. jubata may compromise its use in breeding and could explain, at least in part, why seed production is low when compared with the amount of flowers per raceme.