2008 Annual Report 1a.Objectives (from AD-416) To map and develop molecular markers linked to the fusarium wilt resistance (race. 1)gene in watermelon. 1b.Approach (from AD-416) The cooperator has developed parental lines and BC1 and F2 populations segregating for FW (race. 1)resistance. Previous experiments conducted by the cooperator with these populations showed that the FW (race. 1)resistance is controlled by a single dominant gene. The cooperator grew 200 F2 plants and collected leaf samples for DNA analysis from each of these F2 plants. The cooperator is self-pollinating each of the F2 plants in order to produce F3 families. These F3 families will be used by the cooperator for progeny evaluation and identifying the FW (race. 1)resistant and susceptible F2 plants. The cooperator will provide ARS the F3 progeny evaluation data and leaf samples of the F2 plants (and their resistant and susceptible parental lines). The ARS scientists will isolate DNA from each of the parental lines and F2 leaf samples, and conduct genetic mapping analysis to identify markers linked to the FW-resistance gene. 3.Progress Report This project is related to Objective 1 of this in-house project: To determine the genetic control of resistance to important diseases and pests of watermelon and release resistant lines. F3 populations were developed and screened by the cooperator for fusarium wilt (race. 1)resistance. Young leaves from each F2 plant were collected by the cooperator and were sent to USDA, ARS, U. S. Vegetable Laboratory (USVL), Charleston, SC. The leaf samples were kept by USVL scientists in a -80C freezer and DNA was isolated from each F2 sample. ARS tested over 1,100 Randomly Amplified Polymorphic DNA (RAPD) primers for polymorphism between the fusarim wilt resistant versus the susceptible parent and about 105 primers produced polymorphic markers. The ARS scientists tested the polymorphic markers with the F2 DNA samples using a procedure termed ‘bulk segeregant analysis’. However, none of the polymorphic markers tested showed linkage with the gene locus conferring fusarium wilt-resistance. At this time, Sequence-Related Amplified Polymorphism (SRAP) and Amplified Fragment Length Polymorphism (AFLP) markers are being tested by ARS for polymorphism between the Fusarium Wilt resistant and susceptible parents. The ARS scientists and the cooperators have been communicating frequently via telephone calls and emails, and met at the Cucurbitaecae 2008 conference in Avignon, France, to discuss the project progress and strategies.