A detailed genetic linkage map of Brassica oleracea was constructed based on the segregation of 258 restriction fragment length polymorphism loci in a broccoli × cabbage F2 population. The genetic markers defined nine linkage groups, covering 820 recombination units. A majority of the informative genomic DNA probes hybridized to more than two restriction fragments in the F2 population. "Duplicate" sequences having restriction fragment length polymorphism were generally found to be unlinked for any given probe. Many of these duplicated loci were clustered non-randomly on certain pairs of linkage groups, and conservation of the relative linkage arrangement of the loci between linkage groups was observed. While these data support previous cytological evidence for the existence of duplicated regions and the evolution of B. oleracea from a lower chromosome number progenitor, no evidence was provided for the current existence of blocks of homoeology spanning entire pairs of linkage groups. The arrangement of the analyzed duplicated loci suggests that a fairly high degree of genetic rearrangement has occurred in the evolution of B. oleracea. Several probes used in this study were useful in detecting rearrangements between the B. oleracea accessions used as parents, indicating that genetic rearrangements have occurred in the relatively recent evolution of this species.
Resistance to Plasmodiophora brassicae Wor. race 7, the causal agent of the disease clubroot, was examined in an F2 population of a cross between a clubroot resistant broccoli (Brassica oleracea var . italica) and a susceptible cauliflower (B. oleracea var . botrytis) . A genetic linkage map was constructed in the same population based on the segregation of 58 dispersed restriction fragment length polymorphism (RFLP) markers . Associations between the inheritance of RFLP marker genotypes and segregation for disease resistance, morphological and maturity characteristics were examined . For each trait examined, several chromosomal regions marked by RFLP probes appeared to contain trait loci, suggesting that each trait was under polygenic control . RFLP marker linkage to a major factor imparting dominance for clubroot resistance from the broccoli parent was observed in this population . Additionally, RFLP marker linkage to an independently segregating factor contributing clubroot resistance from the cauliflower parent was observed, indicating that it should be possible to use RFLP markers to facilitate selection of transgressive segregants having the combined resistance from both parental sources . In some instances, RFLP markers from the same or closely linked chromosomal regions were associated with both clubroot resistance and morphological traits . Analysis of RFLP marker genotypes at linked loci should facilitate the selection of desired disease resistant morphotypes .
A cross between the open-pollinated Brassica oleracea cabbage cultivar 'Wisconsin Golden Acre' and the hybrid broccoli cultivar 'Packman' was used with molecular markers to investigate the genetic control of morphological variation. Twenty-two traits derived from leaf, stem, and flowering measurements were analyzed in 90 F2 individuals that were also classified for genotype by restriction fragment length polymorphism (RFLP) markers. Seventy-two RFLP loci, which covered the mapped genome at an average of 10 map-unit intervals on all nine linkage groups, were tested individually for associations to phenotypic measurements by single factor ANOVA, and markers with significant associations (P<0.05) were used to develop multilocus models. These data were utilized to describe the location, parental contribution of alleles, magnitude of effect, and the gene action of trait loci. Single marker loci that were significantly associated (P<0.05) with trait measurements accounted for 6.7-42.7% of the phenotypic variation. Multilocus models described as much as 60.1% of the phenotypic variation for a given trait. In some cases, different related traits had common marker-locus associations with similar gene action and genotypic class ranking. The numbers, action, and linkages, of genes controlling traits estimated with marker loci in this population corresponded to estimates based on classical genetic methods from other studies using similar, or similarly-wide, crosses. There was no evidence that genome duplication accounted for a significant portion of multiple genes controlling trait loci over the entire genome, but possible duplications of trait loci were identified for two regions with linked, duplicated marker loci.
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