Ricci G C L, Pagliarini M S, Valle C B
Departamento de Biologia Celular e Genética, Universidade Estadual de Maringá, Maringá, PR, Brasil.
Genet Mol Res. 2010 Dec 7;9(4):2364-71. doi: 10.4238/vol9-4gmr919.
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 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.
多倍体是植物进化中一股突出且重要的力量，自远古时代就已出现并一直持续至今。最近利用20世纪80年代从非洲野生稀树草原收集的种质对臂形草属进行的细胞遗传学研究表明，该属中的大多数物种及物种内的种质都是多倍体。已发现二倍体、四倍体和五倍体种质。我们在一种从非洲引入巴西的适应性强、具有入侵性的牧场草——俯仰臂形草的两个五倍体（2n = 5x = 45）种质（D53和D71）中发现，在小孢子发生过程中减数分裂不同步，随后发生基因组消除。在这些种质中，终变期染色体配对为18个二价体和9个单价体，这表明这些种质是近期自然杂交事件的结果。基因组中缺乏染色体联会表明，这些种质是由两个亲缘关系不密切、基因组亲和力低且减数分裂节奏不同的基因型杂交产生的。九个单价体的减数分裂行为进一步支持了这一推测，这些单价体相对于其他染色体总是滞后，并在小孢子中作为微核被消除。这些具有奇数倍体水平且证实发生基因组消除的种质的行为，支持了一般假设，即多倍体种质可以通过自然杂交经历新的多倍体化事件（新多倍体化）。臂形草属中自然杂交的这一证据表明，这是一个处于持续进化过程中的野生属。
