Repeated Polyploidy Drove Different Levels of Crossover Suppression between Homoeologous Chromosomes inBrassica napusAllohaploids  

Abstract

Allopolyploid species contain more than two sets of related chromosomes (homoeologs) that must be sorted during meiosis to ensure fertility. As polyploid species usually have multiple origins, one intriguing, yet largely underexplored, question is whether different mechanisms suppressing crossovers between homoeologs may coexist within the same polyphyletic species. We addressed this question using Brassica napus, a young polyphyletic allopolyploid species. We first analyzed the meiotic behavior of 363 allohaploids produced from 29 accessions, which represent a large part of B. napus genetic diversity. Two main clear-cut meiotic phenotypes were observed, encompassing a twofold difference in the number of univalents at metaphase I. We then sequenced two chloroplast intergenic regions to gain insight into the maternal origins of the same 29 accessions; only two plastid haplotypes were found, and these correlated with the dichotomy of meiotic phenotypes. Finally, we analyzed genetic diversity at the PrBn locus, which was shown to determine meiotic behavior in a segregating population of B. napus allohaploids. We observed that segregation of two alleles at PrBn could adequately explain a large part of the variation in meiotic behavior found among B. napus allohaploids. Overall, our results suggest that repeated polyploidy resulted in different levels of crossover suppression between homoeologs in B. napus allohaploids.

Homoeologous Chromosome Sorting and Progression of Meiotic Recombination inBrassica napus: Ploidy Does Matter!

“…At metaphase I ( Figures 2C and 2L), both univalents (i.e., chromosomes that failed to form chiasmata) and bivalents were observed, with variable numbers between genotypes. The number of chiasmata was also significantly higher in Darmor-bzh (9.25 on average) than in Yudal (5.70 on average) allohaploids (P = 0.0001; Table 1), which is consistent with Cifuentes et al (2010). In contrast to our observations in euploids, these chiasmata were formed between nonhomologous A and C chromosomes because allohaploids do not contain homologous chromosomes.…”

“…The production of allohaploid plants from Brassica napus cv Darmor-bzh and Yudal was described by Cifuentes et al (2010).The plants were cultivated in a greenhouse or growth chamber under a 16-h-light/8-h-night photoperiod, at 22°C day and 18°C night, with 65% humidity.…”

“…B. napus allohaploids also show significant variation in meiotic behavior at metaphase I, with two main meiotic phenotypes identified across a wide range of varieties (Nicolas et al, 2009;Cifuentes et al, 2010, and references therein). This variation relies on polygenic control, with one major quantitative trait locus, named Pairing regulator in Brassica napus (PrBn), being far more influential than the others (Cifuentes et al, 2010, and references therein).…”

“…As these approaches focused on the final products of meiotic recombination (chiasmata and genetic exchanges), they did not provide comprehensive information on the way(s) homologous bivalents form in euploid B. napus or the causes for the observed difference in CO rate between allohaploid plants. We do not even know whether the same strategy for homologous bivalent formation is implemented in B. napus euploid accessions originating from different polyploidization events (Cifuentes et al, 2010). The aim of this study, therefore, was to specifically address these questions.…”

The effect of different genome and cytoplasm on meiotic pairing in maize newly synthetic polyploids

“…Cytoplasm is predicted to play a vital role in meiosis behavior (Cifuentes et al 2010). In this study, although a close relationship exists between maize and Z. perennis (Tang et al 2005), chromosome paring is significantly different in two allotriploids or two allotetraploids, which reveals parental cytoplasm has distinct contribution for meiosis behavior.…”