The Genes of Watermelon

Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is a major vegetable crop in the world, accounting for 6.8% of the world area devoted to vegetable crops. Watermelon is a useful vegetable crop for genetic research because of its small genome size, and the many available gene mutants. The watermelon genes were originally organized and summarized in 1944, and have been expanded and updated periodically. However, the action of some watermelon genes has not been described clearly in some cases. Also, the interaction of multiple gene loci that control similar traits needs to be described more clearly. Finally, it is necessary to identify the inbred lines having each published gene mutant, for use as type lines in studies of gene action, allelism, and linkage. The objective of this work was to update the gene list, identify the cultivar or line having each gene mutant, and collect seeds of the lines for use by interested researchers. In addition, the gene descriptions were expanded and clarifi ed, information on gene interactions was added, and errors in naming or citing previously described genes were corrected. New genes that have not previously been described (cr, Ctr, dw-3, ms-2, Ti, ts and zym-FL) were added to the list, for a total of 163 watermelon gene mutants. Watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) is a major cucurbit crop that accounts for 6.8% of the world area devoted to vegetable crops (FAO, 2002). Watermelon is grown for its fl eshy, juicy, and sweet fruit. Mostly eaten fresh, they provide a delicious and refreshing dessert especially in hot weather. The watermelon has high lycopene content in the red-fl eshed cultivars: 60% more than tomato. Lycopene has been classifi ed as useful in the human diet for prevention of heart attacks and certain types of cancer. Watermelon is native to central Africa where it was domesticated as a source of water, a staple food crop, and an animal feed. It was cultivated in Africa and the Middle East for >4000 years, then introduced to China around 900 AD, and fi nally brought to the New World in the 1500s. There are 1.3 million ha of watermelon grown in the world, with China and the Middle Eastern countries the major consumers. China is the largest watermelon producer, with 68.9% of the total production. The other major watermelon producing countries are Turkey, Iran, Egypt, United States, Mexico, and Korea (FAO, 2002). In the United States, watermelon is used fresh as a dessert, or in salads. U.S. production is concentrated in Florida, California, Texas, and Georgia (USDA, 2002), increasing from 1.2 Mt in 1980 to 3.9 Mt in 2002, with a farm value of $329 million (USDA, 2002). Watermelon is a useful crop species for genetic research because of its small genome size, and the many available gene mutants. Genome size of watermelon is 424 million base pairs. DNA sequence analysis revealed high conservation useful for comparative genomic analysis with other plant species, as well as within the Cucurbitaceae. Like some of the other cultivated cucurbits, watermelon al., 1941), t (McKay, 1936), and w (Poole et al., 1941) for red, tan, and white seed coat, respectively. The genes interact to produce six phenotypes: black (RR TT WW); clump (RR TT ww); tan (RR tt WW); white with tan tip (RR tt ww); red (rr tt WW); and white with pink tip (rr tt ww). A fourth gene, d was suggested by Poole et al. (1941) as a modifi er, producing black dotted seed coat when dominant for r, t, and w but having no effect on other genotypes. The genes (s) and (l) for short and long seed length (sometimes called small and large seed size) control seed size, with s epistatic to l (Poole et al., 1941). The genotype LL SS gives medium size, ll SS gives long, and LL ss or ll ss gives short seeds. The Ti gene for tiny seed was reported by Tanaka et al. (1995). Tiny seed from ‘Sweet Princess’ was dominant over medium-size seed and controlled by a single dominant gene. The small seed gene behaved in a manner different from Poole’s medium-size seed cultivar. Tanaka et al. (1995) suggested that the Ti gene was different from the s and l genes. Unfortunately, the origin of short and long seed genes was not described in Poole’s paper. Tomato seed is shorter and narrower than the short seeded genotype, ll ss (width × length: 2.6 × 4.2 mm). It is controlled by the ts (Zhang et al., 1994a) gene, with genotype LL ss tsts. Cracked seed coat cr (El-Hafez et al., 1981) is inherited as a single gene that is recessive to noncracked seed coat. Egusi seed eg (Gusmini et al., 2003) has fl eshy pericarp covering the seeds. After washing and drying, the seeds are diffi cult to distinguish from normal. Vine mutants. Several genes control leaf or foliage traits of watermelon. Nonlobed leaf (nl) has the entire leaf rather than the lobed leaf type of the typical watermelon (Mohr, 1953). Seedling leaf variegation slv (Provvidenti, 1994) causes a variegation resembling virus infection on seedlings. It is linked or pleiotropic with Ctr for cool temperature resistance. The yellow leaf (Yl) gene results in yellow leaves, and is incompletely dominant to green leaves (Warid and Abd-El-Hafez, 1976). Delayed green leaf dg (Rhodes, 1986) causes pale green cotyledons and leaves for the fi rst few nodes, with later leaves developing the normal green color. Inhibitor of delayed green leaf (i-dg) makes leaves normal green even when they have dgdg genotype (Rhodes, 1986). The juvenile albino ja (Zhang et al., 1996b) gene causes reduced chlorophyll in seedling tissues, as well as leaf margins and fruit rind when plants are grown under short day conditions. The dominant gene Sp (Poole, 1944) causes round yellow spots to form on cotyledons, leaves and fruit, resulting in the fruit pattern called moon and stars. So far, four dwarf genes of watermelon have been identifi ed that affect stem length and plant habit: dw-1 (Mohr, 1956) and dw-1 (Dyutin and Afanas’eva, 1987) are allelic, and dw-1, dw-2 (Liu and Loy, 1972), and dw-3 (Hexun et al., 1998) are non-allelic. Dwarf-1 plants have short internodes due to fewer and shorter cells than the normal plant type. Plants with dw-1 have vine length intermediate between normal and dwarf, and the hypocotyls were somewhat has much genetic variability in seed and fruit traits. Genetic investigations have been made for some of those, including seed color, seed size, fruit shape, rind color, rind pattern, and fl esh color. This is the latest version of the gene list for watermelon. The watermelon genes were originally organized and summarized by Poole (1944). The list and updates of genes for watermelon have been expanded and published by Robinson et al. (1976), the Cucurbit Gene List Committee (1979, 1982, and 1987), Henderson (1991 and 1992), Rhodes and Zhang (1995), and Rhodes and Dane (1999). This current gene list provides an update of the known genes of watermelon, with 163 total mutants grouped into seed and seedling mutants, vine mutants, fl ower mutants, fruit mutants, resistance mutants, protein (isozyme) mutants, DNA (RFLP and RAPD) markers, and cloned genes. This gene list has been modifi ed from previous lists in that we have 1) added or expanded the description of the phenotypes of many of the gene mutants, 2) added descriptions for phenotypes of interacting gene loci, 3) identifi ed the type lines that carry each form of each gene, 4) identifi ed the gene mutant lines that are in the curator collections, and 5) added genes that have not previously been described: cr (El-Hafez et al., 1981), Ctr (Provvidenti, 1992), dw-3 (Hexun et al., 1998 ), ms-2 (Dyutin, and Sokolov, 1990), Ti (Tanaka et al., 1995), ts (Zhang et al., 1994a), and zym-FL (Provvidenti, 1991). We had intended to include a review of gene linkage, but few reports were found except for sets of molecular markers in wide crosses of Citrullus. Finally, we attempted to correct some of the errors in gene descriptions or references from previous lists.