Asynchronous meiotic rhythm as the cause of selective chromosome elimination in an interspecific Brachiaria hybrid

This paper reports the occurrence of chromosome elimination during microsporogenesis in an interspecific hybrid between a sexual diploid accession (SEX) of Brachiaria ruziziensis (2n=2x=18) and an apomictic tetraploid accession (APO) of B. brizantha (2n=4x=36). Meiosis was very abnormal in the triploid hybrid (2n=3x=27); we observed a distinct asynchrony from metaphase I to the end of meiosis. The APO and the SEX genomes did not show the same meiotic rhythm. When the former, with nine bivalents, was in metaphase I, the nine SEX univalents were not yet aligned; when the latter reached the plate, the APO genome was already in anaphase. In subsequent stages, the APO genome had reached the poles while the SEX was undergoing sister-chromatid segregation. As the SEX genome always remained temporally behind, it gave rise to one extra-nucleus in each pole. In the second division, the behavior was the same but anaphase II did not occur for the SEX genome, and only one extra-nucleus was observed in each cell in telophase II. Chromosome elimination for the SEX genome ranged from partial to total. The importance of these findings with respect to Brachiaria breeding programmes is discussed.