PrBn, a Major Gene Controlling Homeologous Pairing in Oilseed Rape (Brassica napus) Haploids

Abstract

Precise control of chromosome pairing is vital for conferring meiotic, and hence reproductive, stability in sexually reproducing polyploids. Apart from the Ph1 locus of wheat that suppresses homeologous pairing, little is known about the activity of genes that contribute to the cytological diploidization of allopolyploids. In oilseed rape (Brassica napus) haploids, the amount of chromosome pairing at metaphase I (MI) of meiosis varies depending on the varieties the haploids originate from. In this study, we combined a segregation analysis with a maximum-likelihood approach to demonstrate that this variation is genetically based and controlled mainly by a gene with a major effect. A total of 244 haploids were produced from F1 hybrids between a high- and a low-pairing variety (at the haploid stage) and their meiotic behavior at MI was characterized. Likelihood-ratio statistics were used to demonstrate that the distribution of the number of univalents among these haploids was consistent with the segregation of a diallelic major gene, presumably in a background of polygenic variation. Our observations suggest that this gene, named PrBn, is different from Ph1 and could thus provide complementary information on the meiotic stabilization of chromosome pairing in allopolyploid species.

Meiosis in Polyploids and Implications for Genetic Mapping: A Review

“…A key study by Sears (1976) [94] pointed out that homoeologous pairing at MI is suppressed by the Ph1 locus, increasing karyotypic stability and acting as a pairing regulator in wheat. Similar evidence has been reported for Avena sativa [142], Festuca arundinacea [143], B. napus [144], and Oryza sativa [145]. Moreover, a relevant indicator for the presence of pairing control genes (PCG) is the mendelian segregation of polymorphic meiotic behaviors, e.g., in B. napus [144], Lolium perenne [146,147], and Festuca pratensis [148].…”

“…Similar evidence has been reported for Avena sativa [142], Festuca arundinacea [143], B. napus [144], and Oryza sativa [145]. Moreover, a relevant indicator for the presence of pairing control genes (PCG) is the mendelian segregation of polymorphic meiotic behaviors, e.g., in B. napus [144], Lolium perenne [146,147], and Festuca pratensis [148].…”

“…Firstly, it was proposed that there is a PrBn dosage effect on the B. napus C genome. One copy of the gene(s) carried by the Yudal C genome would lead to fewer COs than one copy of the gene(s) carried by the Darmor-bzh C genome, but two copies of gene(s) carried by the C genomes would provide the same number of COs [144,[225][226][227]. Dosage effects have been shown to be common among the genes regulating chromosome pairing and recombination in polyploids [44,96], including Brassica [228,229].…”

Evolution of crossover interference enables stable autopolyploidy by ensuring pairwise partner connections in Arabidopsis arenosa

“… 69 In allopolyploids, the solution to the problem of pairwise CO or chiasma connectedness relies on preferential CO formation between the two more similar copies of each chromosome. 25 , 70 , 71 , 72 , 73 , 74 , 75 , 76 This allotetraploid solution cannot provide preadaptation to subsequent autopolyploidization, because genome doubling creates a novel situation in which each related pair of chromosomes (homeologs) is now present in multiple copies that cannot be distinguished by DNA sequence differences. By contrast, evolution of stable autotetraploidy via supercharged interference provides a mechanism that ensures pairwise interactions per se and thus among genetically near-identical copies.…”

Genomic and Meiotic Changes Accompanying Polyploidization

“…As it happens in allopolyploid wheats, in allohaploids (AC, 2n = 1x = 19) from oilseed rape ( Brassica napus , AACC; 2n = 4x = 38) homoeologous chromosome pairing during meiosis is genetically controlled by a major QTL named PrBn for PAIRING REGULATOR IN B. NAPUS [ 17 , 214 , 215 , 216 ]. PrBn has an effect on the frequency but not on the distribution of chiasmata between homoeologous chromosomes [ 217 ].…”