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  • Author or Editor: Amnon Levi. x
  • Journal of the American Society for Horticultural Science x
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Wide phenotypic diversity exists among American heirloom cultivars of watermelon (Citrullus lanatus var. lanatus). However, in published studies, low or no polymorphism was revealed among those heirlooms using isozyme or randomly amplified polymorphic DNA (RAPD) markers. In this study, experiments with inter-simple sequence repeat (ISSR) [also known as simple sequence repeat-(SSR-) anchored primers] and amplified fragment-length polymorphism (AFLP) markers produced high polymorphisms among watermelon heirloom cultivars. ISSR (111) and AFLP (118) markers (229 total) identified 80.2% to 97.8% genetic similarity among heirloom cultivars. The phylogenetic relations based on ISSR and AFLP markers are highly consistent with the parental records available for some of the heirloom cultivars, providing confidence in the dendogram constructed for heirlooms based on similarity values. As compared with RAPD markers, ISSRs and AFLPs are highly effective in differentiating among watermelon cultivars or elite lines with limited genetic diversity.

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A genetic linkage map was constructed for watermelon using 117 recombinant inbred lines (RILs) (F2S7) descended from a cross between the high quality inbred line 97103 [Citrullus lanatus var. lanatus (Thunb.) Matsum. & Nakai] and the Fusarium wilt (races 0, 1, and 2) resistant U.S. Plant Introduction (PI) 296341 (C. lanatus var. citroides). The linkage map contains 87 randomly amplified polymorphic DNA (RAPD) markers, 13 inter simple sequence repeat (ISSR) markers, and four sequenced characterized amplified region (SCAR) markers. The map consists of 15 linkage groups. Among them are a large linkage group of 31 markers covering a mapping distance of 277.5 cM, six groups each with 4 to 12 markers covering a mapping distance of 51.7 to 172.2 cM, and eight small groups each with 2-5 markers covering a mapping distance of 7.9 to 46.4 cM. The map covers a total distance of 1027.5 cM with an average distance of 11.7 cM between two markers. The map is useful for the further development of quantitative trait loci (QTLs) affecting fruit qualities and for identification of genes conferring resistance to Fusarium wilt (races 0, 1 and 2). The present map can be used for further construction of a reference linkage map for watermelon based on an immortalized mapping population with progenies homozygous for most gene loci.

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A genetic linkage [randomly amplified polymorphic DNA (RAPD)-based] map was constructed for watermelon [Citrullus lanatus (Thunb.) Matsum and Nakai] using a BC1 population [PI 296341-fusarium wilt resistant × New Hampshire Midget (fusarium susceptible)] × `New Hampshire Midget'. The map contains 155 RAPD markers, and a 700-base pair sequenced characterized amplified region (SCAR) marker that corresponds to a fragment produced by the RAPD primer GTAGCACTCC. This marker was reported previously as linked (1.6 cM) to race 1 fusarium wilt resistance in watermelon. The markers segregated to 17 linkage groups. Of these, 10 groups included nine to 19 markers, and seven groups included two to four markers. The map covers a genetic linkage distance of 1295 cM. Nine of the 10 large linkage groups contained segments with low (or no) level of recombination (0 to 2.6 cM) among markers, indicating that the watermelon genome may contain large chromosomal regions that are deficient in recombination events. The map should be useful for identification of markers linked closely to genes that control fruit quality and fusarium wilt (races 1 and 2) resistance in watermelon.

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Citrullus colocynthis (CC) is a viable source of genes for enhancing disease and pest resistance in common cultivated watermelon [Citrullus lanatus var. lanatus (CLL)] cultivars. However, there is little information about genetic diversity within CC or the relationship of CC accessions to C. lanatus. In this study, we examined genetic diversity and relationships among 29 CC accessions collected in northern Africa, the Middle East, and Asia, and their relationships to 3 accessions and 3 cultivars of CLL, 12 accessions of citron melon [C. lanatus ssp. lanatus var. citroides (CLC)], and 1 accession representing the desert perennial Citrullus ecirrhosus (CE). Twenty-three high-frequency oligonucleotides-targeting active gene (HFO–TAG) primers were used to produce a total of 431 polymorphic fragments that target coding regions of the genome. Cluster and multidimensional scaling plot analysis, separated the CC into five groups, in general agreement with their geographic origins. CC genotypes admixed with CLL and CLC also were identified. Major reproductive barriers resulted in significantly reduced fertility in CC × CLL hybridizations. However, several of the U.S. PIs of CC were successfully crossed with watermelon cultivars using traditional breeding procedures, and the seeds produced from these crosses were viable. This suggests that CC can be a viable source to introduce biotic and abiotic stress resistance genes into cultivated watermelon.

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