2n Gamete formation in the genus Brachiaria (Poaceae: Paniceae)

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.

Section: Resultsmentioning

confidence: 62%

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

Pagliarini¹,

Valle²,

Santos³

et al. 2012

“…Polyploidy is predominant in the genus Brachiaria and is correlated to apomixis (Valle and Savidan, 1996;Valle and Pagliarini, 2009) and the role of unreduced (male and female) gametes in the phenomenon is unquestionable. The formation of unreduced male gametes, originated by different mechanisms of abnormal cytokinesis, has been reported in B. brizantha (Risso-Pascotto et al, 2003), B. nigropedata (Utsunomiya et al, 2005), B. jubata (Mendes-Bonato et al, 2006a), and B. humidicola Gallo et al, 2007;Calisto et al, 2008). In the accessions under analysis, one progenitor, probably apomictic contributed with an unreduced gamete (2n = 36) and the maternal contribution derived from a sexual diploid plant, with a reduced gamete (N = 9).…”

Section: Resultsmentioning

confidence: 89%

Genome elimination during microsporogenesis in two pentaploid accessions of Brachiaria decumbens (Poaceae)

Ricci¹,

Pagliarini²,

Valle³

2010

ABSTRACT. Polyploidy is a prominent and significant force in plant evolution, taking place since ancient times and continuing until today. Recent cytogenetic studies in the genus Brachiaria using germplasm collected from wild African savannas in the 1980s revealed that most species and accessions within species are polyploid. Diploid, tetraploid, and pentaploid accessions have been found. We found asynchronous meiosis during microsporogenesis, followed by genome elimination, in two pentaploid (2n = 5x = 45) accessions (D53 and D71) of a hardy, invasive pasture grass, introduced from Africa to Brazil, Brachiaria decumbens. In these accessions, chromosomes paired as 18 bivalents and nine univalents during diakinesis, suggesting that these accessions resulted from a recent event of natural hybridization. The lack of chromosome associations in the genomes suggests that these accessions resulted from hybridization between two genotypes that are not closely related, with low genome affinity and with different meiotic rhythms. This supposition is reinforced by the meiotic behavior of the nine univalents, which were always laggard in relation to the other chromosomes and eliminated as micronuclei Genome elimination in Brachiaria decumbens in microspores. The behavior of these accessions, which have an odd level of ploidy and confirmed genome elimination, supports the general assumption that a polyploid accession can undergo a new event of polyploidization by natural hybridization (neopolyploidyzation). This evidence for natural hybridization in Brachiaria shows that this is a wild genus in an ongoing evolutionary process.

“…Cell fusion was recorded in B. decumbens (Mendes-Bonato et al, 2001a) and in B. humidicola (Boldrini et al, 2006a). The absence of cytokinesis, observed at low frequency in the present accessions, is also a common abnormality among Brachiaria accessions (Utsunomiya et al, 2005;Risso-Pascotto et al, 2003Boldrini et al, 2006b;Gallo et al, 2007). Breeding of B. humidicola is warranted to increase nutritive value, seed and overall dry matter production, decrease seed dormancy and susceptibility to spittlebugs.…”

Section: Resultsmentioning

confidence: 60%

“…Dormancy and seed quality are, however, constant issues among producers and seed improvement. An approach to introduce sexuality into the hexaploid B. humidicola complex includes hybridization of the sexual genotype to an apomictic that produces 2n gametes identified cytologically (Gallo et al, 2007). Hybrids have not yet been produced due to the lack of flowering synchrony between the progenitors.…”

Section: Resultsmentioning

confidence: 99%

Meiotic behavior in nonaploid accessions of Brachiaria humidicola (Poaceae) and implications for breeding

Boldrini

Adamowski²,

Silva³

et al. 2011

ABSTRACT. Brachiaria humidicola is a grass adapted to seasonally swampy grasslands in Africa; two cultivars, 'common' and Llanero, are widely used in Brazilian pastures. New cultivars are in great demand in order to diversify current production systems to achieve improved quality and yield. Cytological analyses of 55 accessions of this species available from the Embrapa Beef Cattle germplasm collection revealed that 27 are apomictic and have 2n = 54 chromosomes. Chromosome pairing as bi-to nonavalent associations at diakinesis indicated a basic chromosome number in this species of x = 6, as found in other closely related Brachiaria species. Thus, these 27 accessions are nonaploid (2n = 9x = 54). Abnormalities were found in the meiosis of these accessions, at variable frequencies. The most common abnormalities were those related to irregular chromosome segregation, which led to unbalanced gamete formation; but chromosome stickiness, cell fusion, and absence of cytokinesis were also recorded. Although some accessions have a low frequency of meiotic abnormalities, ensuring potentially good pollen viability, these cannot be used in hybridization due to a lack of sexual accessions with the same ploidy level.