Microsporogenesis in Brachiaria brizantha (Poaceae) as a selection tool for breeding

Abstract

ABSTRACT. The genus Brachiaria comprises more than 100 species and is the single most important genus of forage grass in the tropics. Brachiaria brizantha, widely used in Brazilian pastures for beef and dairy production, is native to tropical Africa. As a subsidy to the breeding program underway in Brazil, cytological studies were employed to determine the chromosome number and to evaluate microsporogenesis in 46 accessions of this species available at Embrapa Beef Cattle (Brazil). Thirty-four accessions presented 2n = 36; seven had 2n = 45, and five had 2n = 54 chromosomes. Based on the higher level of chromosome association observed in diakinesis, in tetra-, penta-, and hexavalents, respectively, it was concluded that they are derived from x = 9; consequently, these accessions are tetra-(2n = 4x = 36), penta-(2n = 5x = 45), and hexaploids (2n = 6x = 54). The most common meiotic abnormalities were irregular chromosome segregation due to polyploidy. Chromosome stickiness, abnormal cytokinesis, noncongressed bivalents in metaphase I and chromosomes in metaphase II, and chromosome elimination were recorded at varying frequencies in several accessions. The mean percentage of meiotic abnormalities ranged from 0.36 to 95.76%. All the abnormalities had the potential to affect pollen viability by generating unbalanced gametes. Among the accessions, only the tetraploid ones with less than 40% of abnormalities are suitable as pollen donors in intra-and interspecific crosses. Currently, accessions with a high level of ploidy (5 and 6n) cannot be used as male genitors in crosses because of the lack of sexual female genitors with the same levels of ploidy.

Karyotype analysis and mode of reproduction of two species of Urochloa P. Beauv.

“…However, in our results, the chromosome sets were displayed in groups of two and three chromosomes, which suggests that the pentaploid genotype may be the product of crossings between a tetraploid and a hexaploid cytotype. The existence of 4 x and 6 x cytotypes in U. brizantha (Pagliarini et al., 2012) and the low frequency of unreduced gamete formation in Urochloa (Adamowski et al., 2007; Risso‐Pascotto et al., 2003) are evidence that supports our considerations.…”

“…Regarding U. brizantha , the occurrence of pentaploid cytotypes had been indicated by flow cytometry (Penteado et al., 2000), meiotic analyzes (Mendes et al., 2006; Pagliarini et al., 2012; Risso‐Pascotto et al., 2003), and chromosome counts (Akiyama et al., 2010). Risso‐Pascotto et al.…”

“…Specifically, cytogenetic analyses are essential tools to characterize species and cultivars (Davide et al., 2009), generate information for taxonomy and evolutionary studies, and contribute to breeding programs. This characterization may include karyotype traits (Bernini & Marin‐Morales, 2001; Moraes et al., 2019; Nani et al., 2016; Nani et al., 2018), meiotic behavior (Morais et al., 2018; Pagliarini et al., 2012; Paula et al., 2016), or genomic relationships (Corrêa et al., 2020; Paula et al., 2017). Greater refinement of the analyses can be achieved with the fluorescent in situ hybridization (FISH) technique, which allows the physical mapping of specific DNA sequences on chromosomes.…”

“…Despite the polyploid condition, this species was initially identified as sexual (Valle & Savidan, 1996). The germplasm of U. brizantha also offers alternatives containing a variety of diploid, tetraploid, pentaploid, and hexaploid cytotypes (Mendes‐Bonato et al., 2002; Pagliarini et al., 2012) that still need to be investigated in terms of variability and genomic and chromosomal constitution.…”

Genetic Diversity and Population Structure of Brachiaria (syn. Urochloa) Ecotypes from Uganda

“…Such differences among the ecotypes is anticipated due to the apomictic mode of reproduction in favor of maternal genotype regardless of the level of heterozygosity [56] and the polyploidy nature of Brachiaria species that's often associated with meiotic anomalies leading to reduced pollen fertility [57]. Many Brachiaria species are known to have variable ploidy levels, for example, presence of tetra-, penta-and hexaploidy in B. brizantha population [58]. Polyploidy benefits plants from heterosis, gene redundancy, and loss of self-incompatibility and gain of asexual reproduction [59].…”