Cytological evidence of natural hybridization in Brachiaria brizantha Stapf (Gramineae)

Abstract

Two accessions of Brachiaria brizantha under cytological analysis showed 2n = 5x = 45 chromosomes. Pentaploidy probably resulted from natural hybridization between two species that were not closely related: an apomictic tetraploid male (2n = 4x = 36), and a sexual diploid female (2n = 2x = 18). The lack of affinity between genomes was clearly indicated by asynchrony during meiosis. The haploid genome (n = 9) showed unique behaviour, remaining univalent during prophase I and metaphase I, and undergoing sister-chromatid segregation and lagging at anaphase I. The laggard genome did not always reach the poles in time to be included in the telophase nucleus. However, when the inclusion was effective, this genome was distributed peripherally, changing the otherwise spherical nucleus shape. In the second division, the haploid genome behaved similarly, but as there was sister-chromatid segregation during the first division, the chromatids were slow to reach the poles, forming several micronuclei at telophase II. The two accessions were characterized as allo-autopentaploids, with the tetraploid genome (2n = 4x = 36) designated as B (from B. brizantha) and the haploid genome as X, representing a species with a distinct genome having little affinity with the B genome. Thus, the hybrids’ genome composition is represented by BBBBX. By comparing their meiotic behaviour with that observed in synthetic hybrids between B. brizantha and B. ruziziensis analysed previously, B. ruziziensis is the putative diploid sexual parent species in these pentaploid accessions.