An intraspecific genetic map for watermelon was constructed using an F2 population derived from ‘Arka Manik’ × ‘TS34’ and transcript sequence variants and quantitative trait loci (QTL) for resistance to powdery mildew (PMR), seed size (SS), and fruit shape (FS) were analyzed. The map consists of 14 linkage groups (LGs) defined by 174 cleaved amplified polymorphic sequences (CAPS), 2 derived-cleaved amplified polymorphic sequence markers, 20 sequence-characterized amplified regions, and 8 expressed sequence tag-simple sequence repeat markers spanning 1,404.3 cM, with a mean marker interval of 6.9 cM and an average of 14.6 markers per LG. Genetic inheritance and QTL analyses indicated that each of the PMR, SS, and FS traits is controlled by an incompletely dominant effect of major QTLs designated as pmr2.1, ss2.1, and fsi3.1, respectively. The pmr2.1, detected on chromosome 2 (Chr02), explained 80.0% of the phenotypic variation (LOD = 30.76). This QTL was flanked by two CAPS markers, wsb2-24 (4.00 cM) and wsb2-39 (13.97 cM). The ss2.1, located close to pmr2.1 and CAPS marker wsb2-13 (1.00 cM) on Chr02, explained 92.3% of the phenotypic variation (LOD = 68.78). The fsi3.1, detected on Chr03, explained 79.7% of the phenotypic variation (LOD = 31.37) and was flanked by two CAPS, wsb3-24 (1.91 cM) and wsb3-9 (7.00 cM). Candidate gene-based CAPS markers were developed from the disease resistance and fruit shape gene homologs located on Chr.02 and Chr03 and were mapped on the intraspecific map. Colocalization of these markers with the major QTLs indicated that watermelon orthologs of a nucleotide-binding site-leucine-rich repeat class gene containing an RPW8 domain and a member of SUN containing the IQ67 domain are candidate genes for pmr2.1 and fsi3.1, respectively. The results presented herein provide useful information for marker-assisted breeding and gene cloning for PMR and fruit-related traits.

No abstract

No abstract

The genetic relationships among 27 watermelon cultigens (Citrullus lanatus var. lanatus) from different countries of origin and with different horticultural characteristics and 5 related wild-type species and subspecies (Citrullus colocynthis, Citrullus lanatus var. citroides, and Citrullus rehmii) were assessed using amplified fragment length polymorphism (AFLP) and expressed sequence tag-simple sequence repeat (EST-SSR) markers. AFLPs were evaluated using 16 EcoRI-MseI primer combinations, and 862 alleles (an average of 53.8 alleles per primer combination) were scored. Polymorphisms were found in 806 (93.4%) alleles, whereas 56 monomorphic alleles were identified. Using 16 EST-SSR primer sets, 103 alleles were scored, and all 103 alleles were polymorphic among the 32 genotypes with an average of 6.4 polymorphic alleles per primer pair. However, the high polymorphic ratio among the AFLPs and EST-SSRs was largely due to the wild-type species, while little diversity was observed among the adapted cultivars. Genetic similarity coefficients were calculated based on the 965 polymorphic AFLP and EST-SSR alleles, and a phenetic tree was constructed. The dendrogram contained 2 major clusters. Cluster I included all adapted watermelon cultivars, and the similarity among these cultigens was very high (0.94-0.98), demonstrating cross relationships and a narrow genetic background. Cluster II was composed of 4 wild-type species. The genetic distance between the nodes that comprise these 2 clusters was approximately 0.63, indicating a high level of genetic dissimilarity between the adapted watermelon species and the other related species. The low level of marker polymorphism among the adapted cultivars implies that a severe bottleneck in genetic diversity existed in watermelon during the initial breeding practices.

No abstract