description
- Brassica napus is an important species in world and UK agriculture. Oilseed rape (Canola) has become a major temperate oilseed crop over the past 40 years, and swede (rutabaga) has been established as a root vegetable and fodder crop for several centuries. We wish to develop a way of allowing breeders to more easily introduce useful genes. B. napus has 19 chromosomes, containing essentially the same 10 chromosomes as B. rapa ('A' genome; includes turnips, Chinese cabbage, pak choi) and the 9 chromosomes of B. oleracea ('C' genome; includes cabbage, cauliflower, broccoli, Brussels sprouts). The ploidy of a species refers to the typical number of copies of a given set of chromosomes in non-reproductive cells of an organism. Most organisms are normally diploid, meaning they have two sets of chromosomes - one set inherited from each parent. B. napus is an 'allopolyploid', since it contains chromosomes inherited from more than one species. Many other important crop species are allopolyploids, including wheat, cotton and potato. Genetic diversity in allopolyploids is often lower than in closely related diploid species and plant breeders frequently wish to introduce useful genes for traits such as disease resistance from diploids into the allopolyploid. Unlike B. rapa and B. oleracea, which diverged from each other about 5 million years ago, B. napus is not found in natural populations in the wild. It appears to have arisen, probably in or near human cultivation, on more than one occasion within the past 1-2000 years from a hybridisation between domesticated B. rapa and B. oleracea. Most forms of B. napus generally have stable pairing between corresponding parental copies of each of the 19 chromosomes. Such behaviour means they are called amphidiploids (a form of allopolyploid that has stable pairing between homologous chromosomes and so behaves genetically like a diploid). However, although recombination generally occurs between homologous chromosomes (i.e. specific A genome chromosomes or specific C genome chromosomes) within B. napus, a low incidence of recombination can occur between A and C genome homoeologues within oilseed rape cultivars. Homoeologues are similar but not identical chromosomes in each of the A and C genomes that correspond to common ancestral chromosomes and so still retain a generally conserved gene order since they diverged about 5 million years ago. Due to the relatively low genetic diversity within B. napus, plant breeders sometimes wish to introduce novel genes (e.g. conferring disease resistance) from one of the diploid progenitor species (e.g. B. rapa) into B. napus, or by resynthesising B. napus. It is possible to produce "resynthesized" B. napus by crossing the two diploid species, performing an operation called embryo rescue (early in seed development), and doubling the chromosome number with a treatment of a chemical colchicine. In resynthesized B. napus recombination between homoeologues occurs more frequently, leading to chromosomal rearrangements and genetically unbalanced gametes. The difference between established B. napus and resynthesized B. napus can be used to explore the genetical basis of pairing control. We have used a population derived from natural and resynthesized B. napus to locate a region of one chromosome (A9) that appears to contain a gene (or genes) that control the level of pairing between homoeologues. In this project we aim to characterise the effect that this locus has on the detailed process that occurs when chromosomes pair and recombine, and to pinpoint the likely candidate genes causing the differences in pairing. We will exploit the recently established genome sequence of B. rapa, and also try to identify subsets of the genepool of B. rapa that may have contributed stable pairing genes during the early hybridisation of B. napus. The information we generate will be of direct interest to plant breeders wishing to bring genes into modern varieties of oilseed rape.