Stabilization of Resistance to Leptosphaeria maculans in Brassica napus-B. juncea Recombinant Lines and Its Introgression into Spring-Type Brassica napus.

The value of Katanning Early Maturing (KEM) breeding lines from Western Australia, derived from Brassica napus × B. juncea crosses, was assessed as a source of germplasm for resistance to blackleg disease (caused by Leptosphaeria maculans) in spring-type oilseed rape cultivars. The stability of blackleg resistance in these KEM lines was related to key cytological characteristics to determine why there are poor levels of introgression of this resistance into progeny. Promising recombinant KEM lines were crossed with the spring-type B. napus cv. Dunkeld, which has useful polygenic resistance to blackleg, and screened for resistance. The lines were analyzed cytologically for pairing of bivalents in each generation to aid in the selection of stable recombinant lines. KEM recombinant lines showing regular meiotic behavior and a high level of blackleg resistance were obtained for the first time. We also showed that the stable introgression of the B. juncea resistance from the KEM lines into a 'Dunkeld' background was possible. Inoculation of selfing and backcross populations with isolates of L. maculans having different AvrLm genes indicated that the B. juncea resistance gene, Rlm6, had been introgressed into a B. napus spring-type cultivar carrying polygenic resistance. The combination of both resistances would enhance the overall effectiveness of resistance against L. maculans. This is clearly needed in Australia and France where cultivars relying upon single dominant gene-based resistance for their effectiveness have proved not durable.

Modelling gene flow between oilseed rape and wild radish. I. Evolution of chromosome structure.

The assessment of gene flow from crop species to weeds has found a new emphasis over the last years because of the marketing of transgenic crops and the possible selective advantage that crop (trans)gene may confer to the weeds. Several studies focused on the F1 interspecific hybrid production but few data are available on the factors affecting the genetic structure of advanced generations. It depends on the genomic structure of the species concerned as well as on the degree of their genome homology that affect the occurrence of intergenomic recombination. Oilseed rape (Brassica napus, AACC, 2n = 38)-wild radish (Raphanus raphanistrum, RrRr, 2n = 18), a distantly related weed, is a good model to address such questions. From seven male sterile oilseed rape lines carrying an herbicide tolerance transgene, F1 interspecific hybrids and four advanced generations were produced under field conditions with wild radish as pollinator. Observation of hybrid chromosome numbers across four generations revealed a high variability, especially in the "BC1" generation. A regression model was fitted in order to describe the relationship between parent and offspring chromosome numbers. The effects of generation, transgenic line and selection pressure on the mean relationship were investigated. The first two factors had an influence on the rate of decrease of chromosome numbers, whereas selection pressure resulted in the presence of an additional chromosome in the herbicide treated plants. The model provided a convenient framework for analysing how chromosome numbers evolve over successive hybridization events and it may prove useful as a basis for simulation-based approaches.

Pairing and recombination at meiosis of Brassica rapa (AA) x Brassica napus (AACC) hybrids.

Interspecific crosses contribute significantly to plant evolution enabling gene exchanges between species. The efficiency of interspecific crosses depends on the similarity between the implicated genomes as high levels of genome similarity are required to ensure appropriate chromosome pairing and genetic recombination. Brassica napus (AACC) is an allopolyploid, resulting from natural hybridization between Brassica rapa (AA) and Brassica oleracea (CC), both being diploid species derived from a common ancestor. To study the relationships between genomes of these Brassica species, we have determined simultaneously the pairing and recombination pattern of A and C chromosomes during meiosis of AAC triploid hybrids, which result from the interspecific cross between natural B. napus and B. rapa. Different AAC triploid hybrids and their progenies have been analysed using cytogenetic, BAC-FISH, and molecular techniques. In 71% of the pollen mother cells, homologous A chromosomes paired regularly, and usually one chromosome of each pair was transmitted to the progeny. C chromosomes remained mainly univalent, but were involved in homoeologous pairing in 21.5% of the cells, and 13% of the transmitted C chromosomes were either recombined or broken. The rate of transmission of C chromosomes depended on the identity of the particular chromosome and on the way the hybrid was crossed, as the male or as the female parent, to B. napus or to B. rapa. Gene transfers in triploid hybrids are favoured between A genomes of B. rapa and B. napus, but also occur between A and C genomes though at lower rates.