Genetic Mapping of a Major Locus Controlling Recombination Between Homoeologues in Brassica napus Using FISH and GISH

Allopolyploid Brassica napus (AACC) contains two genomes. Despite the close relationship between these genomes, there is only a low frequency of recombination between A and C genome homoeologues during meiosis in established cultivars. However, the frequency is higher in resynthesised B. napus lines, created by crossing B. rapa (AA) and B. oleracea (CC) and doubling the chromosome complement. We have used this difference to investigate the genetic basis of the control of homoeologous recombination, a trait crucial to the stability of the species. Chromosome spreads at meiotic metaphase I were obtained from pollen mother cells. Then, bivalents and multivalents formed as a result of recombination between homoeologues were detected by genomic in situ hybridisation (GISH), following fluorescence in situ hybridisation (FISH) with 45s rDNA and a BAC which hybridises to pericentromeric heterochromatin of 14 of the 19 chromosome pairs. Data were collected from a subset of lines of a doubled haploid (DH) population (SG; developed from DH12075 - an established B. napus line, and PSA12 - a resynthesised line), and analysed as a quantitative trait, using a genetic map constructed from RFLP data from a larger number of lines. One chromosomal region was strongly associated with this trait. This region was also identified previously (DJ Lydiate, unpublished) using different parental lines and a different technique for detecting the phenotype. We are now fine-mapping the trait locus, using additional lines screened for recombination in the target region. This, together with other experiments on meiotic material, will provide candidate genes to test.