Submitted to: Molecular Breeding
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/10/1998
Publication Date: N/A
Citation: Interpretive Summary: Downy mildew is a fungal disease of cucumber which can cause devastating losses in cucumber production. Breeders have successfully bred downy mildew resistance into cucumber varieties. Nevertheless, it is difficult to select resistant plants because the disease spread and intensity of infection (number of diseased plants) is dramatically effected by environmental conditions. Molecular markers are pieces of DNA that are made synthetically using biotechnology (new technologies using DNA techniques). Molecular markers are not affected by environment and can be used to determine the position of genes (factors of heredity) on chromosomes (DNA structures which contain genes). If a molecular marker is positioned close to a gene of interest (e.g., downy mildew resistance gene), then it can be used to detect the presence of that gene in plants. Since this association can exist for many generations and is not affected by environment, markers can be used as indirect selection tools for developing improved (i.e. resistance) varieties. A study was designed to identify molecular markers associated with downy mildew resistance. Five marker-downy mildew gene associations were identified. This research identified readily useable molecular markers which can be used by public and private breeders to increase their efficiency for selection of downy mildew resistant plants. The use of these tools will lead to more rapid development of disease resistant varieties.
Technical Abstract: Two F2 populations (WI 1983G x Straight 8 and Zudm1 x Straight 8) were used to identify and place random amplified polymorphic DNA marker linkage associations to the downy mildew gene (dm) on a previously constructed genetic map. Downy mildew resistance [causal agent Psuedoperonospora cubensis (Berk & Curt) Rostow] evaluations of 55 (WI 1983G x Straight 8 population) and 90 (Zudm1 x Straight 8 population) F3 families were conducted over five North American and European locations. Resistant and susceptible F3 families were identified and mean family resistance ratings were used to type individual F2 plants. No evidence for race differences P. cubensis in North America and Europe was found. Phenotypic correlations between locations ranged from 0.3 to 0.7. Of the 135 polymorphic RAPD markers identified from 960 primers, 1000. In the WI 1983G x Straight 8 population, G14800 was linked to dm at 16.5 cM, AS5800 at 32.8 cM, BC5191100 at 9.9 cM, and BC5261000 at 19.2 cM. Inthe Zudm1 x Straight 8 population, G14800 was linked at 20.9 cM, X151100 at 14.8 cM, AS5800 at 24.8 cM, and BC5261000 at 32.9 cM. Markers G14800 and 800, and BC5261000 were linked in coupling phase. Linkage analysis using pooled data yielded a map that closely resembled the Zudm1 x Straight 8 map. Multiple-marker linkage analysis revealed that a significant portion (> 80% depending on cross) of the total variance was explained by quantitative trait loci in the intervals surrounding the estimated position of the dm locus, thus lending support to the map placement of the dm locus in cucumber.