An Extended Linkage Map for Watermelon Based on SRAP, AFLP, SSR, ISSR, and RAPD Markers

Seventy-one amplifi ed fragment length polymorphism (AFLP), 93 sequence related amplifi ed polymor- phism (SRAP), and 14 simple sequence repeat (SSR) markers were used to extend an initial genetic linkage map for watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai). The initial map was based on 151 randomly amplifi ed polymorphic DNA (RAPD) and 30 and inter-simple sequence repeat (ISSR) markers. A testcross population previ- ously used for mapping of RAPD and ISSR markers was used in this study: {plant accession Griffi n 14113 (C. lanatus var. citroide (L.H. Bailey) Mansf.) x the watermelon cultivar New Hampshire Midget (C. lanatus var. lanatus)} x PI 386015 (C. colocynthis (L.) Schrad.). The linkage map contains 360 DNA markers distributed on 19 linkage groups, and covers a genetic distance of 1976 cM with an average distance of 5.8 cM between two markers. A genomic DNA clone representing 1-amino-cyclopropane-1-carboxylic acid (ACC-) synthase gene, involved in ethylene biosynthesis, was also mapped. As in previous mapping studies for watermelon, a large number of AFLP and SRAP markers were skewed away from the 1:1 segregation ratio, and had to be excluded from the fi nal mapping analysis. The stringent mapping criteria (JoinMap 3.0 mapping program) produced linkage groups with marker order consistent with those reported in previous mapping study for watermelon. Cultivated watermelon accounts for 2% of the world area devoted to vegetable crops (Food and Agricultural Organization of the United Nations (FAO), 1995). In the United States, water- melon production has increased from 1.09 million Mg in 1980 to 3.54 million Mg in 2003, with a farm value of $310 million (U.S. Department of Agriculture (USDA), 2003). The traditional watermelon cultivars are diploids (n =11; Shimotsuma, 1963). However, in recent years, there has been an increased demand by consumers for seedless (triploid) watermelons, and produc- tion has increased considerably. During 2003, over 60% of the watermelons produced in the United States were seedless (USDA, 2003). There is a continuous need to enhance disease and pest resistance in watermelon cultivars, improve fruit quality, and develop new seedless watermelon cultivars suitable to consumer