Progress 08/15/02 to 08/14/04
Outputs
The objectives of this grant were to develop a new recombinant inbred line (RIL) in lettuce from parents that were bred for the low desert production areas of Arizona and California. Our other major objective was to locate and map quantitative trait loci (QTL) that are associated with hydrotime and high temperature germination. We hypothesized that hydrotime parameters could be used as a quantitative method in which to screen the level of osmotic sensitivity and high temperature tolerance, and that molecular markers could be developed that are associated with these parameters. We developed an F8 RIL (142 families) from a cross between an iceberg lettuce type (cv. Diplomat) and a butterhead lettuce type (cv. Margarita). An initial screening of over 40 cultivars used in the lower desert production areas revealed these cultivars were at the phenotypic extremes in terms of their germination characteristics. Relative to Margarita, Diplomat germinates to higher final
percentages, and germination rate is faster under high temperatures and reduced water potentials. We constructed an amplified fragment-length polymorphism (AFLP) linkage map from bulked F2: 3 families. Our laboratory phenotyped the F2:3 seeds for each of the germination traits and located QTLs associated with each of these germination traits under stress. We have constructed a second AFLP linkage map on the F8 RIL population, and are in the process of mapping the germination phenotypes. Using these data we have developed a model that suggests light modulates sensitivity to water stress, as positive associations were detected between hydrotime parameters and high temperature germination in the dark, but, interestingly, not high temperatures in continuous red light. Germination under high temperatures in continuous far-red light to dark (20 oC) is negatively correlated with the ability to germinate under stress, and the same negative correlation was observed for dark 30 oC germination
conditions. We have two manuscripts in preparation, one from the F2:3 mapping and QTL analysis, and another with the F8 population which includes additional phenotyping data, and comparisons with the F2:3 mapping approach.
Impacts
The ability to germinate under environmentally stressful conditions is an important agronomic trait. Lettuce is the second most popular vegetable in the United States, and its production efficiency is often detrimentally affected by high temperatures and increased salinity in irrigation water. Information on the genetic mechanisms that control seed germination under environmental stress will lead to lettuce cultivars with improved tolerance to such conditions while simultaneously elucidating the mechanisms controlling seed dormancy.
Publications
- Hayashi E, Kim D-H, and Still DW. 2004. QTL mapping of seed germination characteristics in lettuce (Lactuca sativa),Plant Animal Genome XII Conference, San Diego CA 10-14 January 2004
- Hayashi E, Aoyama N, and Still DW. 2004. Quantitative trait loci (QTL) analysis of hydrotime, high temperatures, and light sensitivity in lettuce seed during germination. Annual meeting of American Society of Plant Biologists, Orland FL, 23-29 July 2004.
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Progress 08/15/02 to 08/15/03
Outputs
Objective 1. Develop recombinant inbred lines (RILs) from a cross between two low desert cultivars that exhibit phenotypic extremes in germination. Two common cultivars with contrasting germination phenotypes from the desert lettuce production areas were crossed and a F8 RIL containing 195 families were produced through single seed descent. This RIL population will provide a valuable mapping resource for improving germination and other horticultural traits for the desert production areas in California and Arizona. Objective 2. Construct an amplified fragment length polymorphism (AFLP) linkage map from RILs developed in Objective 1. Using a F2:F3 population, 180 AFLP primer combinations were screened. Ten primer combinations were selected based on their polymorphic distribution between the two parents. A total of 130 polymorphic markers were generated in 142 RILs; 69 and 61 markers were dominant to cv Diplomat, cv Margarita, respectively. For Diplomat, 23 markers were
assigned to 3 linkage groups and 2 pairs covering 183 cM with an average distance of 10.2 cM. For Margarita, 42 markers were assigned to 6 linkage groups and 4 pairs covering 284 cM with an average distance of 8.9 cM. A linkage map is now being created in the F8 RILs. This linkage map will allow placement of QTLs and other markers. Objective 3. Development of advanced recombinant inbred lines by directional selection using a modified hydrotime approach. Based on the ability to germinate in reduced water potentials, we used directional selection to improve germination under osmotic stress. We confirmed a selection gain between the F3 and F4 populations. Using hydrotime analyses we have determined that the improvement is attributed to lower psib50 values. The entire F4 population was screened and two QTLs associated with psib50 were identified. These results indicate heritability for the trait and establishes a genetic basis for hydrotime that should lead to lettuce germplasm that can
better tolerate high temperatures and salinity during germination. Objective 4. Test the hypothesis that hydrotime and high temperature germination are linked in lettuce. We hypothesized that selecting for the ability to germinate under conditions of reduced water potentials (such as increased salinity) would increase tolerance to higher temperatures as well. The F4 population was subjected to germination conditions of 20 C red (control), 30 C dark (30-D), 30 C red (30-R), and the FR to D at 20 C. We detected QTLs for 30-D (linkage group D-1, M-5), FR to D (linkage group D-1, D-5, M-2, M-3, and M-5), and hydrotime (psib50) values (linkage group D-3 and M-2). Although additional QTLs could not be mapped because of marker gaps, these data clearly demonstrate the feasibility of our approach. The increased resolution in the F8 linkage map will allow placement of these QTLs. The placement of the QTLs suggests separate mechanisms for each germination response. The ability to map QTLs
indicates a genetic mechanism controlling seed germination under stress, and that by using marker assisted selection, this trait can be improved.
Impacts
The ability to germinate under environmentally stressful conditions is an important agronomic trait. Lettuce is the second most popular vegetable in the United States, and its production efficiency is often detrimentally affected by high temperatures and increased salinity in irrigation water. Information on the genetic mechanisms that control seed germination under environmental stress will lead to lettuce cultivars with improved tolerance to such conditions while simultaneously elucidating the mechanisms controlling seed dormancy.
Publications
- No publications reported this period
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