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Molecular characterization of the PrBn locus and some other QTLs controlling homeologous recombination in Brassica napus

Final Report Summary - COGEPRBN (Molecular characterization of the PrBn locus and some other QTLs controlling homeologous recombination in Brassica napus)

Polyploidy is the process that gives rise to organisms with multiple complete sets of chromosomes. Polyploidy is commonplace in many eukaryotes lineages and particularly prominent in plants. All angiosperms are current polyploid or have experienced at least one round and usually several rounds of polyploidy during their evolution. Some of the world's most important crop plants, such as wheat, cotton or oilseed rape, are allopolyploids; i.e. they have a hybrid origin and combine different sets of related but not completely homologous chromosomes, called homeologues. These chromosomes were inherited from parental species that are ancient polyploids (paleopolyploids); they returned to a functionally diploid state by a massive elimination of some but not all duplicated genes. Given the prevalence of polyploidy in plants, an important direction that studies should take is to integrate and extend our knowledge of important biological processes into the field of polyploidy.

Meiosis and recombination are among the most important processes because they are essential to the life cycle of all sexual eukaryotes. Meiosis is the specialised programme of cell division responsible for production of gametes. Meiotic recombination is required in most organisms to ensure the correct segregation of chromosomes at meiosis I and to boost genetic diversity. These two outcomes are ensured by the exclusive formation of meiotic crossovers (CO) between homologous chromosomes. This process is particularly demanding in allopolyploid species because homeologous chromosomes usually retain sufficient similarity to be able to recombine with one another; if this would occur, multiple or illegitimate associations would result in chromosome misegregation and partial fertility. Suppression of COs between homeologous chromosomes is therefore a prerequisite for polyploidy speciation. Surprisingly little attention has been paid to the control of meiotic COs in polyploid species, except in wheat.

The overall objective of the COGEPRBN project was to advance understanding of the genetic architecture of CO suppression between homeologous chromosomes in oilseed rape (Brassica napus; AACC, 2n=38). Brassica napus is a young allopolyploid species that originated from multiple independent hybridisation events between ancestors of modern B. oleracea (CC, 2n=18) and B. rapa (AA, 2n=20). The genomes of B. oleracea and B. rapa share an ancestral whole-genome triplication so that most segments of the Arabidopsis genome can be aligned to 6 syntenic segments of the B. napus genetic map. Comparisons of meiosis among B. napus allohaploid plants, which carry one copy of each of the 10 A and 9 C B. napus chromosomes (AC), showed that allohaploids produced from different B. napus varieties displayed different levels of homeologous recombination. The quantitative trait loci (QTL) determining these phenotypes were mapped in a single population. A major locus (PrBn) localised to linkage group C9, and 4-6 other additive or epistatic loci were identified.

The specific objectives of this project were to decipher:
- if PrBn and at least some of the mapped QTLs are duplicated genes retained from successive rounds of polyploidisation that still cooperate to regulate homeologous recombination,
- if PrBn is still the key determinant for the level of homeologous recombination at the species level.
To address the first question, a comparative genetic analysis of all the QTL regions and their duplicates in the genome of B. napus was carried out. Combined bioinformatic and genetic approaches were used to determine which of the QTLs were located in syntenic regions of B. napus genome. Our results have demonstrated that no QTL is present in the regions that are related to that of PrBn, which suggests that this locus is not duplicated.