Location: Vegetable Research2012 Annual Report
1a. Objectives (from AD-416):
1. Determine the genetic control of resistance to important diseases and pests of watermelon and release resistant breeding lines developed. 1.A. Determine genetic control of root-knot nematode (Meloidogyne incognita) resistance in watermelon; identify and map DNA-based markers closely linked to the resistance locus (loci). 1.B. Determine genetic control of Fusarium wilt (Fon race 2) resistance in watermelon; identify and map DNA-based markers closely linked to the resistance locus (loci). 2. Utilize genomic tools to develop genetic linkage maps for watermelon and diagnostic DNA-based markers for host-plant resistance to viruses and key watermelon fruit traits. 2.A. Map and isolate DNA sequences associated with Zucchini Yellow Mosaic Virus (ZYMV) resistance in watermelon. 2.B. Identify and map DNA sequences associated with watermelon fruit quality traits (e.g., fruit size and shape, flesh color and texture, carotenoid levels, and soluble content). 3. Develop lines of broccoli improved for economically important traits. 3.A. Develop inbred broccoli lines with tolerance to high temperature stress, elucidate the underlying genetic control of the tolerance, and identify quantitative trait loci (QTL) and associated DNA-based markers for the tolerance. 3.B. Breed high yielding, self-compatible inbred broccoli lines with high productivity and high levels of health-promoting compounds (e.g., glucoraphanin) as compared to that of standard hybrid cultivars. 4. Elucidate the genetic control of bacterial leaf spot resistance in leafy green Brassicas (B. juncea and B. rapa), identify DNA markers closely linked to the resistance locus (loci), and release resistant breeding lines developed in this project. 5. Develop pinkeye-type southernpea (cowpea) lines that exhibit yield potential equivalent to leading blackeye-type cultivars.
1b. Approach (from AD-416):
Select parental lines of watermelon, broccoli, or leafy green brasscias based on phenotypic expression of resistance, tolerance, or quality traits under study. Use the selected parents to construct conventional (i.e., F2, BC1, recombinant inbred) and doubled haploid (for broccoli only) populations that segregate for the traits of interest, and then employ those populations in studies to determine mode of inheritance of each character. Utilize PCR-based markers and other genomic technologies to identify sequences linked to the studied characters and to locate controlling genes on linkage maps. Use particular markers (i.e., SSR, SRAP, SNPs, or SCARs) closely associated with traits of interest to develop tools for marker-assisted selection. Based on knowledge gained through above studies, devise breeding strategies, and applications of marker technologies to use in the further development of horticulturally enhanced lines or hybrids that express resistances and other traits of interest and that also produce high quality vegetables. Make enhanced lines available through public releases or commercial licensing. Continue ongoing searches for new resistances and tolerances among watermelon and vegetable Brassica accessions from the U.S. PI and other collections. In addition to the above, the southernpea pinkeye-type cultivar GreenPack-DG and the high-yielding blackeye bean cultivar California Blackeye No. 46 will be used as the parental lines to initiate a plant breeding project with the major goal of breaking the apparent yield barrier in pinkeye-type southernpeas; the two cultivars will be crossed and pedigree and single-seed-descent breeding procedures will be employed to quickly advance progeny populations three generations per year (spring field cycle, fall field cycle, and a winter greenhouse cycle).
3. Progress Report:
For the watermelon portion of this project, the genome of the watermelon heirloom cultivar Charleston Gray has been sequenced using 454 technology. In collaboration with scientists at Cornell, we conducted Genotyping-by-Sequencing (using the watermelon genome sequence as reference) for 95 watermelon accessions and also for populations segregating for zucchini yellow mosaic virus (ZYMV)-resistance, discovering over 400,000 high quality single nucleotide polymorphic markers representing most regions of the genome. These single nucleotide polymorphism (SNP) data will be made available for the public through the International Cucurbit Genomic Initiative website (www.ICuGi.org). We also cooperated with a Research Plant Pathologist at Charleston to develop and release three Citrullus lanatus var. citroides lines that exhibit resistance to Fusarium wilt (FW). In cooperation with a Nematologist at Charleston, genetic populations of watermelon formed by crossing wild accessions exhibiting resistant and susceptible responses to infection by root knot nematodes (RKNs) and that segregate for these different responses are being evaluated in controlled inoculation trials. Inheritance of RKN resistance will be determined based on the collected results. A specific germplasm line that exhibits resistance to RKN has been recently released. In addition to the above, previously reported DNA markers linked to a gene associated with ZYMV resistance in watermelon have been tested in a marker-assisted selection scheme to develop new watermelon lines resistant to the virus. For the broccoli and leafy Brassica portion of this project, an additional cycle of breeding broccoli for heat tolerance was completed, and new tolerant selections were identified and advanced another generation. Replicated broccoli trials in the summer at Charleston are helping to identify the most tolerant inbreds and hybrids for release. In addition, four cooperating scientists are testing seven experimental hybrids from the Charleston program for warm season adaptation in NC, VA, NY, and ME. A large replicated evaluation of a doubled haploid broccoli population segregating for heat tolerance has been conducted; results indicate quantitative inheritance of this trait. In work aimed at developing leafy mustards and related greens resistant to bacterial leaf spot disease, resistant accessions found to date were tested in a second fall trial and results confirmed we now have mustard and Chinese cabbage accessions with good resistance even under heavy disease pressure. Broccoli families bred for high yield and quality heads were evaluated in fall nurseries and the best lines were selected and advanced. Relative to the objective on southernpeas, seed of advanced breeding lines of black-eye and pink-eye types harvested in 2011 were processed during winter, and replicated trials comparing these lines and standard cultivars were conducted in summer 2012 to identify the best line(s) for possible release as a high-yielding cultivar.
Levi, A., Wechter, W.P., Thies, J.A., Ling, K., Reddy, U.K., Xu, Y., Guo, S., Zhang, X. 2012. Watermelon In Wang, Y, H., Dehdra, T. K., and Kole, C. (eds) Genetics, genomics and breeding of cucurbits. 1st edition. CRC Press. Taylor & Francis Group, New York, NY. Pp. 309-334.