Multiple spindles and cellularization during microsporogenesis in an artificially induced tetraploid accession of Brachiaria ruziziensis (Gramineae).

The genus Brachiaria is characterized by a majority of polyploid accessions--mainly tetraploid--and apomictic reproduction. Sexuality is found among diploids. To overcome incompatibility barriers, accessions with the same ploidy level are necessarily used in hybridization. Thus, sexual diploid accessions were tetraploidized to be used as female genitors. This paper reports microsporogenesis in an artificially induced tetraploid accession of Brachiaria ruziziensis. Chromosome pairing at diakinesis ranged from univalents to tetravalents, with predominance of bivalents. Irregular chromosome segregation was frequent in both meiotic divisions. During the first division, multiple spindles showing different arrangements were recorded. The spindle position determined the plane of first cytokinesis and the number of chromosomes determined the size of the cell. Meiotic products were characterized by polyads with spores of different sizes. Pollen sterility was estimated at 61.38%. The limitations of using this accession in the breeding program are discussed.