Production of seedless watermelons in the United States has increased significantly in recent years, and there is a need to continue developing new seedless watermelon cultivars suitable to consumer needs. Seedless watermelons are triploids resulting from a cross between a tetraploid (female) plant and a diploid (male) plant (Kihara, 1951). The tetraploid plants are being produced by treating apical meristems of diploid (2n = 2x = 22; Shimotsuma, 1963; Shimotsuma and Matsumoto, 1957) watermelon plants with the alkaloid colchicine, which inhibits spindle formation and centromer detachment during cell division (Kihara, 1951). The triploid watermelon seeds are produced in isolation plots where tetraploid plants are pollinated by diploid plants. The seeds produced in isolation plots are mostly triploids. However, with some tetraploid lines, up to 30% of seeds produced might be tetraploids as a result of self-pollination (Loehrlein and Ray, 1999). Tetraploid seeds are thicker than triploid seeds (Shimotsuma and Matsumoto, 1957). Still, DNA markers can be useful in quality assurance tests to confirm sufficient production of triploid seeds in isolation plots.
DNA markers have been used in genetic studies and in breeding programs of different polyploidy crop plants, including sunflowers (Mokrani et al., 2002), coffee (Herrera et al., 2002), and alfalfa (Barcaccia et al., 2000). Low DNA polymorphism exists among American watermelon cultivars, indicating they are derived from common ancestors (Levi et al., 2001a, 2001b). A set of DNA markers representing different linkage regions of the watermelon genome and producing sufficient polymorphism among genotypes is needed in breeding programs aiming to produce elite triploid (seedless) watermelon lines derived from crosses between closely related diploid and tetraploid breeding lines.
An extended genetic linkage map was constructed for watermelon (Levi et al., 2006). Because of low DNA polymorphism among watermelon cultivars (C. lanatus var. lanatus) (Levi et al., 2001b), the map constructed using a wide cross [a watermelon cultivar (C. lanatus var. lanatus) and a U.S. Plant Introduction (PI) of the wild C. lanatus var. citroides]. The map contains 360 DNA markers distributed on 19 linkage groups (10 large and nine small linkage groups) and covers a genetic distance of 1976 cM with an average distance of 5.8 cM between two markers. The map consists mostly of randomly amplified polymorphic DNA (RAPD), intersimple sequence repeat (ISSR), amplified fragment length polymorphism (AFLP), and sequence-related amplified polymorphism (SRAP) markers (Levi et al., 2006).
In this study, markers from different linkage groups of the watermelon genetic linkage map (Levi et al., 2006) were examined for polymorphism among watermelon cultivars (diploids 2n = 22) sharing a narrow genetic background (Levi et al., 2001b). A set of markers representing most of the linkage groups and producing sufficient polymorphism among cultivars was assembled. The polymorphic markers can be useful in DNA fingerprinting of watermelon elite breeding lines, in differentiating among closely related diploid and tetraploid breeding lines, and in confirming the production of true F1 hybrid triploid (seedless) watermelon lines.
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Mokrani, L. Gentzbittel, L. Azanza, F. Fitamant, L. Al-Chaarani, G. Sarrafi, A. 2002 Mapping and analysis of quantitative trait loci for grain oil content and agronomic traits using AFLP and SSR in sunflower (Theor. Appl. Genet. Helianthus annuusL.) 106 149 156
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