Selection based on meiotic behavior in <i>Urochloa decumbens</i> hybrids from non-shattered seed

Abstract

This study aimed to evaluate the end-products of meiosis in sexual and apomictic hybrids of Urochloa decumbens, so as to identify genotypes with good production of viable pollen for use in breeding programs to increase yields of pure viable seed and reduce degree of seed shattering. From 457 intraspecific hybrids of U. decumbens arising from crosses between 3 artificially tetraploidized sexual plants and the apomictic cultivar Basilisk, 27 hybrids from non-shattered seed were selected. Slides were prepared by smearing anthers and staining to determine the presence of abnormalities. The abnormalities found were micronuclei, microcytes and polyads. The data were compared by the Scott-Knott test at P&lt;0.05. Data obtained enabled separation of hybrids into 4 groups depending on the presence of micronuclei and formation of polyads and into 6 groups based on the presence of microcytes in the tetrads. Among the analyzed apomictic hybrids, R179 has the attributes for viable seed production to proceed to cultivar development. Among the sexual hybrids, R161, R181, R193 and S47 are recommended as female parents for use in crossing programs.

Microsporogenesis associated with seed yield in Urochloa sexual polyploid hybrids

“…Lutts et al (1991) studied the relationship between uni-, bi-, and multivalent in metaphase I with pollen viability and seed yield and concluded that chromosome associations cannot be used to evaluate the fertility level of F 1 hybrids from the crossing of U. ruziziensis × U. decumbens and U. ruziziensis × U. brizantha. Meanwhile, most studies report that a low frequency of meiotic abnormalities is associated with viable pollen production and consequent fertility in Urochloa, while the high frequency of irregularities is associated with inviable pollen featuring low production of fertile seeds (Adamowski et al 2008, Souza et al 2015, Paula et al 2016, Rocha et al 2019, Baldissera et al 2020. Souza et al (2015) reported that some hybrids of U. decumbens showed a peculiar behavior because 100% of the tetrads were affected, although pollen sterility was lower than expected.…”

“…The occurrence of one or two micronuclei in a tetrad would result in a ratio of 3:1 and 2:2 for viable and inviable pollen grains, respectively. According to Baldissera et al (2020), tetrads that presented micronucleus in three or four microspores were more detrimental to the production of viable pollen than those with micronuclei in only one or two microspores. However, the lack of association between the variables Mcn1 and Mcn2 with the seed set does not reinforce this hypothesis.…”

“…Apomixis is the dominant mode of reproduction in this species, with high commercial potential (Worthington et al 2019). However, this mode of reproduction, common in polyploid individuals, prevents self-fertilization and hybridization among existing elite genotypes or with sexual genotypes at different ploidy levels (Morais et al 2018, Baldissera et al 2020, Sales et al 2021.…”

“…There is extensive literature confirming the strong association between polyploidy and the occurrence of irregularities during meiosis in Urochloa spp. Tetraploid plants show high rates of segregational and non-segregational meiotic abnormalities (Mendes-Bonato et al 2007), which culminate in the formation of tetrads with abnormal microspores, suggesting the formation of sterile pollen grains (Pagliarini et al 2006, Baldissera et al 2020, Sales et al 2021. Several studies have been conducted on the relationship between the percentage of abnormal tetrads and pollen fertility (Souza et al 2015, Rocha et al 2019, Baldissera et al 2020, but none in relation to actual seed yield.…”

“…Tetraploid plants show high rates of segregational and non-segregational meiotic abnormalities (Mendes-Bonato et al 2007), which culminate in the formation of tetrads with abnormal microspores, suggesting the formation of sterile pollen grains (Pagliarini et al 2006, Baldissera et al 2020, Sales et al 2021. Several studies have been conducted on the relationship between the percentage of abnormal tetrads and pollen fertility (Souza et al 2015, Rocha et al 2019, Baldissera et al 2020, but none in relation to actual seed yield. In theory, when pollen mother cells exhibit irregular behavior during cell division and produce microspores with unbalanced genetic material, inviable pollen grains are formed (Mendes-Bonato et al 2007, Ricci et al 2010, Souza-Kaneshima et al 2010, Pagliarini et al 2012, which may result in infertile seeds.…”