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  • Author or Editor: Angela Davis x
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Genetic diversity among 95 watermelon (Citrullus lanatus) ecotypes was evaluated and compared with representative Chinese, American, Japanese, and Russian watermelon cultigens, landraces, and a wild watermelon relative (Trichosanthes kirilowii). Open-pollinated, hybrid, and inbred lines were included for most of the ecotypes and are hereafter collectively referred to as cultigens unless an ecotype group is being discussed. Morphological characteristics (including days to flower, female to male flower ratio, branch number, fruit length and diameter ratio, fruit soluble solid content, fruit yield, and simple sequence repeat (SSR) markers were used to estimate genetic diversity. Of 398 watermelon primer pairs tested, 9.5% (38) produced polymerase chain reaction amplicons in watermelon. Of these 38 primer pairs, the average number of polymorphic bands among the 96 cultigens was 2.4, even with 12 primer pairs demonstrating monomorphic banding patterns. Based on the SSR data, the genetic similarity coefficients were calculated and a dendrogram constructed. All cultigens were clustered to six groups. The wild species and landraces formed distant clusters from the cultivated watermelon. The genetic similarity coefficients within the Chinese cultigens ranged from 0.37 to 0.99, but except for a wild relative to watermelon, most cultigens were closely related. The genetic distance among non-Chinese cultigens ranged from 0.67 to 0.91 with an average of 0.88. When combined morphological traits and molecular traits were assessed, Russian and U.S. fruit were more genetically similar to each other than to Chinese and Japanese cultigens. Crossing Russian and/or U.S. cultigens with Chinese or Japanese cultigens should thus improve genetic diversity and introduce new traits for the resulting watermelon cultigens.

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Citrullus lanatus (watermelon) is an excellent daily source of dietary lycopene and β-carotene. To investigate the transcriptional regulation of carotenoid biosynthesis genes relative to lycopene and β-carotene accumulation in watermelon fruit, six watermelon accessions with different flesh colors were examined in this study: white-fleshed PI 459074, pale-yellow-fleshed ‘Cream of Saskatchewan’, light-pink-fleshed PI 482255, orange-yellow-fleshed ‘WM-Clr-1’, and red-fleshed ‘LSW177’ and ‘MSW28’. The expression patterns of eight genes (PSY1, PSY2, PDS, ZDS, CRTISO, LCYB, NCED1, and NCED7) involved in lycopene and β-carotene biosynthesis and biodegradation were analyzed. The results confirmed the accumulation of large quantities of lycopene in red-fleshed ‘LSW177’ and ‘MSW28’, reflecting the elevated expression of PSY1 and the low transcriptional expression of NCED1. The relative expression levels of NCED1 likely play an important role in the color development of the light-pink-fleshed PI 482255, whereas the reduced transcriptional expression of PSY1 and the increased expression of NCED1 appear to be the main factors contributing to the formation of white flesh in the fruit of PI 459074. Low transcriptional expression of PSY1 results in the pale-yellow flesh of the ‘Cream of Saskatchewan’ fruit.

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A cDNA library was assembled using mRNA of watermelon fruit. The cDNA library was normalized and subtracted by hybridization with leaf cDNA of the same watermelon cultivar (Illini Red). 1,046 cDNA clones were sequenced to identify genes associated with fruit development and quality. Of 1,046 cDNA clones sequenced, 832 were unique sequences and designated as expressed sequenced tags (ESTs). Of the 832 ESTs, 205 (24.6%) have not been reported in any other plant species. Additionally, 186 ESTs (22.4%) correspond to genes with unknown function, while 441 ESTs (53.0%) correspond to genes with known function in other plant species. These ESTs are mainly associated with primary metabolism, membrane transport, cytoskeleton synthesis and structure, cell wall and cell division, signal transduction, nucleic acid binding and transcription factors, and defense and stress response. Differential expression of the ESTs was examined using microarray analysis. About 200 (24%) of the 832 ESTs showed differential expression during the development and ripening of watermelon fruit. The ESTs were also screened for simple sequence repeat (SSR) motifs. Of 832 ESTs screened, 177 contain SSR motifs. Primer pairs are being designed for these ESTs, and will be used for development of EST-SSR markers and for mapping on a genetic linkage map constructed for watermelon. This study provides valuable information on genes controlling watermelon fruit development and quality.

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Information on the mode of inheritance of powdery mildew resistance in watermelon is important for designing a breeding strategy for the development of new cultivars. Resistance in the watermelon accession PI 270545 was investigated by generation means analysis by crossing it with susceptible PI 267677. The analyses showed involvement of two genes, a recessive resistance gene, pmr-1, and a dominant gene for moderate resistance, Pmr-2. Resistance to powdery mildew in the leaf had a large dominance effect and a heritability of 71%. The additive-dominance model was inadequate in explaining variation in leaf resistance as revealed by the joint scaling test. However, nonallelic interactions could not be detected by the nonweighted six-parameter scaling test. For stem resistance, the additive-dominance model was adequate, and inheritance was controlled mainly by additive effects. A high narrow-sense heritability of 79% suggested that selection for stem resistance in early generations would be effective.

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The primary purpose of grafting vegetables worldwide has been to provide resistance to soilborne diseases. The potential loss of methyl bromide as a soil fumigant combined with pathogen resistance to commonly used pesticides will make resistance to soilborne pathogens even more important in the future. The major disease problems addressed by grafting include fusarium wilt, bacterial wilt, verticillium wilt, monosporascus root rot, and nematodes. Grafting has also been shown in some instances to increase tolerance to foliar fungal diseases, viruses, and insects. If the area devoted to grafting increases in the future, there will likely be a shift in the soil microbial environment that could lead to the development of new diseases or changes in the pathogen population of current diseases. This shift in pathogen populations could lead to the development of new diseases or the re-emergence of previously controlled diseases. Although grafting has been demonstrated to control many common diseases, the ultimate success will likely depend on how well we monitor for changes in pathogen populations and other unexpected consequences.

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Two loci, C and i-C, were previously reported to determine flesh colors between canary yellow and red watermelon (Citrullus lanatus). Recently, lycopene β-cyclase (LCYB) was found as a color determinant gene for canary yellow (C) and a codominant cleaved amplified polymorphic sequence (CAPS) marker was developed to identify canary yellow and red alleles. The inhibitor of canary yellow (i-C), as reported in a previous work, was not detected in our original family derived from a cross between canary yellow and red parents. To identify additional genetic determinants such as i-C, we prepared a new family using ‘Yellow Doll’ (canary yellow) and ‘Sweet Princess’ (red), which was reported to carry the inhibitor gene i-C as parents. A new distinct class of flesh color, pale yellow, was identified in the progeny from the new canary yellow × red cross. The predominant carotenoid in canary yellow and pale yellow phenotypes was neoxanthin, followed by violaxanthin and neochrome; pale yellow contained less total carotenoids, but had more minor carotenoids compared with canary yellow. The chi-square goodness-of-fit test indicated that there are two genes involved in determining flesh color among canary yellow, pale yellow, and red, but the segregation pattern did not fit the pattern as reported for an i-C gene. When the genotype of the family ‘Yellow Doll’ × ‘Sweet Princess’ was analyzed with our LCYB CAPS marker, the flesh color of every individual perfectly cosegregated with the marker. The new pale yellow phenotype also cosegregated with the marker linked to the C allele, indicating that the recessive py phenotype (pale yellow) must carry at least one of the C alleles for expression. Therefore, we propose to designate py for a pale yellow determinant along with C as a canary yellow determinant. A homozygous recessive py gene resulted in pale yellow flesh color in the presence of a dominant C.

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In this study, we report a simple procedure for developing and using new types of polymerase chain reaction (PCR) primers, named “high-frequency oligonucleotides–targeting active genes” (HFO-TAG). The HFO-TAG primers were constructed by first using a “practical extraction and report language” script to identify oligonucleotides (8, 9, and 10 bases) that exist in high frequency in 4700 expressed sequence tag (EST)-unigenes of watermelon (Citrullus lanatus) fruit. This computer-based screening yielded 3162 oligonucleotides that exist 32 to 335 times in the 4700 EST-unigenes. Of these, 192 HFO-TAG primers (found 51 to 269 times in the 4700 EST-unigenes) were used to amplify genomic DNA of four closely related watermelon cultivars (Allsweet, Crimson Sweet, Charleston Gray, and Dixielee). The average number of DNA fragments produced by a single HFO-TAG primer among these four watermelon cultivars was considerably higher (an average of 5.74 bands per primer) than the number of fragments produced by intersimple sequence repeat (ISSR) or randomly amplified polymorphic DNA (RAPD) primers (an average of 2.32 or 4.15 bands per primer, respectively). The HFO-TAG primers produced a higher number of polymorphic fragments (an average of 1.77 polymorphic fragments per primer) compared with the ISSR and RAPD primers (an average of 0.89 and 0.47 polymorphic fragments per primer, respectively). Amplification of genomic DNA from 12 watermelon cultivars and two U.S. Plant Introductions with the HFO-TAG primers produced a significantly higher number of fragments than RAPD primers. Also, in PCR experiments examining the ability of primers to amplify fragments from a watermelon cDNA library, the HFO-TAG primers produced considerably more fragments (an average of 6.44 fragments per primer) compared with ISSR and RAPD primers (an average of 3.59 and 2.49 fragments per primer, respectively). These results indicate that the HFO-TAG primers should be more effective than ISSR or RAPD primers in targeting active gene loci. The extensive EST database available for a large number of plant and animal species should be a useful source for developing HFO-TAG primers that can be used in genetic mapping and phylogenic studies of important crop plants and animal species.

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