Watermelon, Citrullus lanatus, is an important cucurbit crop grown throughout the world. Here we report a high-quality draft genome sequence of the east Asia watermelon cultivar 97103 (2n = 2x = 22) containing 23,440 predicted protein-coding genes. Comparative genomics analysis provided an evolutionary scenario for the origin of the 11 watermelon chromosomes derived from a 7-chromosome paleohexaploid eudicot ancestor. Resequencing of 20 watermelon accessions representing three different C. lanatus subspecies produced numerous haplotypes and identified the extent of genetic diversity and population structure of watermelon germplasm. Genomic regions that were preferentially selected during domestication were identified. Many disease-resistance genes were also found to be lost during domestication. In addition, integrative genomic and transcriptomic analyses yielded important insights into aspects of phloem-based vascular signaling in common between watermelon and cucumber and identified genes crucial to valuable fruit-quality traits, including sugar accumulation and citrulline metabolism

No abstract

Fruit characteristics of sweet watermelon are largely the result of human selection. Here we report an improved watermelon reference genome and whole-genome resequencing of 414 accessions representing all extant species in the Citrullus genus. Population genomic analyses reveal the evolutionary history of Citrullus, suggesting independent evolutions in Citrullus amarus and the lineage containing Citrullus lanatus and Citrullus mucosospermus. Our findings indicate that different loci affecting watermelon fruit size have been under selection during speciation, domestication and improvement. A non-bitter allele, arising in the progenitor of sweet watermelon, is largely fixed in C. lanatus. Selection for flesh sweetness started in the progenitor of C. lanatus and continues through modern breeding on loci controlling raffinose catabolism and sugar transport. Fruit flesh coloration and sugar accumulation might have co-evolved through shared genetic components including a sugar transporter gene. This study provides valuable genomic resources and sheds light on watermelon speciation and breeding history.

No abstract

Vegetable grafting began in the 1920s using resistant rootstock to control soilborne diseases. This process is now common in Asia, parts of Europe, and the Middle East. In Japan and Korea, most of the cucurbits and tomatoes (Lycopersicon esculentum Mill.) grown are grafted. This practice is rare in the United States, and there have been few experiments to determine optimal grafting production practices for different geographical and climatic regions in America. This is beginning to change as a result of the phase out of methyl bromide. The U.S. cucurbit and tomato industries are evaluating grafting as a viable option for disease control. Because reports indicate that type of rootstock alters yield and quality attributes of the scion fruit, some seed companies are investigating grafting as a means to improve quality. It has been reported that pH, flavor, sugar, color, carotenoid content, and texture can be affected by grafting and the type of rootstock used. Reports vary on whether grafting effects are advantageous or deleterious, but it is usually agreed that the rootstock/scion combination must be carefully chosen for optimal fruit quality. Additionally, it is important to study rootstock/scion combinations under multiple climatic and geographic conditions because many rootstocks have optimal temperature and moisture ranges. This report gives an overview of the effect of grafting on vegetable quality.

Oxygen isotopic ratios (5180) of precipitation samples collected over several years at three meteorological stations on the northern Tibetan Plateau were used to conduct the first investigation of the relationship between i•lgO and contemporaneous air temperatures (To). Inferring past temperatures from i•180 measured in recently acquired Tibetan ice cores necessitates establishing whether a i•lgO-To relationship exists. For each station a strong temporal relationship is found between i•180 and To, particularly for monthly averages which remove synoptic-scale influences such as changes in condensation level, condensation temperature, and moisture sources. Moisture source is identified as a major factor in the spatial distribution of i•180, but air temperature determines the temporal fluctuations of i•lSO at individual sites on the northern Tibetan Plateau. The 30-year records of annually averaged i•lSO from three different ice coring sites are not correlated significantly with contemporaneous air temperature records from their closest meteorological station (150 to 200 km). However, since 1960 the three air temperature records reveal a modest warming trend, while the three contemporaneous i•lSO records show a modest •O enrichment. 1. Introduction Oxygen isotopic ratios (5•80) have been used extensively in palcoclimatic reconstructions [Dansgaard, 1953; Craig, 1961; Lorius et al., 1985; Thompson et al., 1989, 1995a; Yao et al., 1991; Jouzel et al., 1994] as a proxy for air temperature. The relationship between •80 and air temperature (To) has been investigated most thoroughly in the polar regions. In Antarctica, various studies [Aldaz and Deutsch, 1967; Lorius and Merlivat, 1977; Kotlyakov et al., 1982; Jouzel and Merlivat, 1984; Mosley-Thompson et al., 1990; Peel, 1992] have demonstrated a strong linear relationship between •O and To with slopes ranging from 0.76 to 0.92. In Greenland snow, Dansgaard et al. [1973] and Johnsen et al. [1989] obtained slopes from 0.62 to 0.67. Palcoclimatic reconstructions rely on the robustness of the assumption that the 5•80-To relationship can be used to reconstruct accurately the climatic record (e.g,. temperature trends) preserved in ice cores [Jouzel et a/.,1983]. A recent study by Joussaume and Jouzel [1993] demonstrated that using currently measured 5•O-To relationships to reconstruct temporal variations in To from 5•O may result in errors of 20% to 30% over Greenland and Antarctica. Outside the polar regions these studies are more limited. A comprehensive study of the long-term 5•80-To relationship was conducted in midlatitude, low-elevation regions {Rozanski et al., 1992] using the relatively long •O and meteorological records from stations in the International Atomic Energy Agency (IAEA)/World Meteorological Organization (WMO) precipitation network. Three different changes in the 5•O-To relationship were identified: spatial changes, short-term changes, and long-term changes. Spatially, the 5l•O/Ta slope values varied from 0.719/oo per

No abstract

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.