New Considerations on Meiosis, Affinity and Genomic Constitution in Aneuploids and Interploid Hybrids of Urochloa P. Beauv. ( Brachiaria )

The combination of polyploidy, apomixis and the formation of aneuploid and interploid hybrids makes the genus Urochloa an interesting system for investigating meiotic stability and genome behaviour in grasses. In this study, routine meiotic analysis and genomic in situ hybridization (GISH) were used to (i) evaluate meiosis in three hybrids of the Brizantha clade, (ii) characterize chromosome/genome behaviour throughout division and (iii) confirm the genomic constitution of the interploid hybrid 7F/153 (synthetic autotetraploid Urochloa ruziziensis × allopentaploid Urochloa brizantha , with B 2 and putative B/B 1 genomes, respectively) and its male parent, U. brizantha . The interploid hybrids (2 n = 40 and 41) showed broadly similar meiotic behaviour and differed from the aneuploid hybrid 1F/21 (2 n = 36 + 1), suggesting that increased chromosome number was accompanied by higher frequencies of multivalent configurations, meiotic abnormalities and lower pollen viability. GISH analysis in the hybrid 7F/153 revealed preferential bivalent pairing between chromosomes of the B 2 genome and between those of the B/B 1 genomes, whereas intergenomic associations between B/B 1 and B 2 genomes occurred predominantly in multivalent configurations. Most univalents identified belonged to the B 2 genome, and some showed chromatid segregation during Anaphase I. Chromosomes from the B 2 genome were also predominant in the micronuclei of tetrads. The parent U. brizantha (2 n = 5 x = 45) presented a set of nine B 2 chromosomes, confirming for the first time U. ruziziensis as a diploid ancestor. Despite the irregular meiotic behaviour of B 2 chromosomes in pentaploid U. brizantha , pollen viability and the chromosomal complement of the hybrid indicate that some gametes result from regular segregation and the transmission of four B 2 chromosomes and 18 B/B 1 chromosomes. The results provide important insights into meiotic behaviour, genomic interactions and chromosome transmission, contributing to the understanding of genome evolution in the Brizantha clade.