Genetic diversity and relatedness were assessed among 46 American cultivars of watermelon (Citrullus lanatus var. lanatus), and 12 U.S. Plant Introduction accessions (PIs) of Citrullus sp. using 25 randomly amplified polymorphic DNA (RAPD) primers. These primers produced 288 distinct reproducible bands that could be scored with high confidence among cultivars and PIs. Based on the RAPD data, genetic similarity coefficients were calculated and a dendrogram was constructed using the unweighted pair-group method with arithmetic average (UPGMA). The cultivars and C. lanatus var. lanatus PIs differentiated at the level of 92% to 99.6% and 88% to 95% genetic similarity, respectively. In contrast, the C. lanatus var. citroides, and C. colocynthis PIs were more divergent and differentiated at the level of 65% to 82.5% and 70.5% genetic similarity, respectively. The low genetic diversity among watermelon cultivars in this study emphasizes the need to expand the genetic base of cultivated watermelon.

SummaryThe soil-borne fungus Fusarium oxysporum f.sp. melonis causes significant losses in the cultivated melon, a key member of the economically important family, the Cucurbitaceae. Here, we report the map-based cloning and characterization of the resistance gene Fom-2 that confers resistance to race 0 and 1 of this plant pathogen. Two recombination events, 75 kb apart, were found to bracket Fom-2 after screening approximately 1324 gametes with PCR-based markers. Sequence analysis of the Fom-2 interval revealed the presence of two candidate genes. One candidate gene showed significant similarity to previously characterized resistance genes. Sequence analysis of this gene revealed clear polymorphisms between resistant and susceptible materials and was therefore designated as Fom-2. Analysis of susceptible breeding lines (BL) presenting a haplotype very similar to the resistant cultivar MR-1 indicated that a gene conversion had occurred in Fom-2, resulting in a significant rearrangement of this gene. The second candidate gene which shared high similarity to an essential gene in Arabidopsis, presented an almost identical sequence in MR-1 and BL, further supporting Fom-2 identity. The gene conversion in Fom-2 produced a truncated R gene, revealing new insights into R gene evolution. Fom-2 was predicted to encode an NBS-LRR type R protein of the non-TIR subfamily. In contrast to most members of this class a coiled-coil structure was predicted within the LRR region rather than in the N-terminal. The Fom-2 physical region contained retroelement-like sequences and truncated genes, suggesting that this locus is complex.

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Seventy-one amplified fragment length polymorphism (AFLP), 93 sequence related amplified polymorphism (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 amplified polymorphic DNA (RAPD) and 30 and inter-simple sequence repeat (ISSR) markers. A testcross population previously used for mapping of RAPD and ISSR markers was used in this study: {plant accession Griffin 14113 [C. lanatus var. citroide (L.H. Bailey) Mansf.] × the watermelon cultivar New Hampshire Midget (C. lanatus var. lanatus)} × 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 final 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.