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

Abstract

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.

Meiotic behavior of Brachiaria decumbens hybrids

“…This may indicate that the abnormalities were not sufficiently severe to produce infertility. Similar results were reported for B. decumbens accessions which had apparently normal pollen but in which 100% of the tetrads had micronuclei or were fragmented into polyads (Ricci et al, 2010).…”

“…In Brachiaria, the various meiotic abnormalities observed in cytogenetic analyses compromise normal chromosome behavior and lead to the formation of infertile gametes (Risso-Pascotto et al, 2005;Mendes-Bonato et al, 2002Fuzinatto et al, 2007;Felismino et al, 2008, Ricci et al, 2010Souza-Kaneshima et al, 2010;Pagliarini et al, 2012a,b).…”

Microsporogenesis in Paspalum conspersum Schrad. (Virgata group) with different ploidy levels

“…Griseb., which is presently merged (Shirasuna, 2012) with the circumscription of Urochloa P. Beauv. (Ricci et al, 2010). Several polyploids were formed in the past by genome duplication events occurring millions of years ago (paleopolyploids), but according to Adams and Wendel (2005), polyploidy is an active and ongoing process; the meiotic behavior of the P. conspersum accessions analyzed here reinforces this proposition.…”

Complete asynapsis resulting in 2n pollen formation in Paspalum jesuiticum Parodi (Poaceae)

“…Such meiotic behavior is typical of recently formed natural hybrids resulting from parental genomes with dissimilar meiotic rhythms. This phenomenon has previously been found in 1 accession of P. subciliatum Chase (Adamowski et al, 1998), has been reported several times in species of Brachiaria, and is presently accepted (Shirasuna, 2012) within the circumscription of Urochloa (Risso-Pascotto et al, 2004;Mendes-Bonato et al, 2006;Boldrini et al, 2009Boldrini et al, , 2010Ricci et al, 2010). Although several polyploids were formed as a result of genome duplication events occurring several millions of years ago, Wendel and Doyle (2005) consider it an active and ongoing process, termed paleopolyploidization; the meiotic behavior of the P. jesuiticum accessions reported here reinforces this view.…”