Origin of a polyploid accession of Brachiaria humidicola (Poaceae: Panicoideae: Paniceae)

Abstract

ABSTRACT. Brachiaria humidicola, a species adapted to poorly drained and infertile acid soils, is widely used throughout the tropics. Cytological characterization of 54 accessions of B. humidicola for breeding purposes revealed 2n = 36, 42, and 54 chromosomes. One accession (H030), with 2n = 42 chromosomes, showed a different meiotic behavior. In most accessions from the genus Brachiaria previously studied, the basic chromosome number is x = 9, but the putative basic number in H030 appears to be x = 6. Since six univalent chromosomes were found in diakinesis and metaphase I, and these behaved as laggards in anaphase I, it was hypothesized that both genitors were derived from x = 6, and that this accession is a heptaploid 2n = 7x = 42. The parental genomes did not have the same meiotic behavior, particularly during anaphase, when one genome consisting of six univalents remained as laggards and underwent sister-chromatid segregation. At telophase, 18 segregated chromosomes were found at each pole. The laggard genome did not reach the poles at telophase I or II in time to be included in the nucleus and was eliminated as micronuclei.

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

“…Several polyploids were formed in the past by events of genome duplication that occurred many millions of years ago (paleopolyploids), but polyploidy is an active and ongoing process (Adams and Wendel, 2005). The present accessions, and others recorded in Brachiaria with odd level of ploidy and genome elimination (Mendes et al, 2006;Risso-Pascotto et al, 2006b;Boldrini et al, 2009aBoldrini et al, ,b, 2010 reinforce this assumption, showing that a polyploid accession can undergo a new event of polyploidization by natural hybridization (neopolyploidyzation). Abbott and Lowe (2004) maintained that the discovery of a new polyploid species in the wild soon after its origin presents an excellent opportunity to examine numerous phenomena concerning polyploid speciation and evolution.…”

“…Micronuclei remained present within released microspores (Figure 1k). The percentage of abnormal tetrads was very high, reaching 100% in D53 and 97.9% in D71 and the overall abnormality frequencies for each accession are presented in Table 1 The meiotic behavior described for D53 and D71 is also found in other polyploid accessions of Brachiaria with odd levels of ploidy, where one genome derived from x = 9 in B. brizantha (Mendes et al, 2006;Risso-Pascotto et al, 2006b) or derived from x = 6 in B. humidicola (Boldrini et al, 2009a(Boldrini et al, ,b, 2010 was eliminated in micronuclei by asynchrony during meiosis.…”

“…Accessions with odd levels of ploidy are not common in the genus Brachiaria and this is the first report of pentaploidy in B. decumbens. Another few pentaploids have also been recorded in B. brizantha (Mendes-Bonato et al, 2002;Mendes et al, 2006;Risso-Pascotto et al, 2003) and in B. humidicola (Boldrini et al, 2009a(Boldrini et al, ,b, 2010.…”

“…According to Soltis (2005) these studies indicate that probably all angiosperms, and maybe all plants, are polyploid to some extent. Polyploidy is common in the genus Brachiaria, and tetraploidy prevails (Valle and Savidan, 1996;Penteado et al, 2000;Utsunomiya et al, 2005;Mendes-Bonato et al, 2002Pagliarini et al, 2008;Risso-Pascotto et al, 2003Boldrini et al, 2009aBoldrini et al, ,b, 2010. It is estimated that most flowering plants are descendants of polyploid ancestors (Masterson, 1994).…”

“…Among the polyploid accessions there is evidence that some of them evolved from autopolyploidy (MendesBonato et al, 2002) and others from allopolyploidy (Mendes et al, 2006;Boldrini et al, 2009aBoldrini et al, ,b, 2010. Evidence of recent natural hybridizations has been found in some accessions with odd levels of ploidy, such as Brachiaria brizantha (Mendes et al, 2006;Risso-Pascotto et al, 2006b) and in B. humidicola (Boldrini et al, 2009a(Boldrini et al, ,b, 2010. In these accessions, the evidence of recent hybridization was provided by cytological phenomena, such as i) the formation of independent metaphase plates between genomes; ii) the organization of two nucleoli in hybrid meiocytes, and iii) genome elimination by asynchronous meiosis.…”