Watermelon Accessions Research Articles

Development of branchless watermelon near isogenic lines by marker assisted selection

Pruning is time-consuming and laborious in watermelon cultivation, which can not meet the needs for simplified cultivation in the future. The development of branchless lines will provide important germplasms for breeding watermelon varieties and is an important method for genetic improvement. In this study, the watermelon accession, Wu Cha Zao (WCZ) is a branchless inbred line that carries the branchless gene Clbl, which was used as the donor parent to develop branchless near isogenic lines (NILs). To construct the NILs of Clbl, WCZ crossed with the normal branching watermelon inbred line WT20 which was used as the recurrent parent. The co-segregating markers dCAPS10 and Indel1 with Clbl were used for foreground selection, and a total of 108 SSR markers was selected with good polymorphism between two parental lines for background selection which had relatively uniform distribution across 11 chromosomes. Using these markers to select individuals from the BC1F1, BC2F1, and BC2F2 generations, three NILs with a proportion of recurrent parent genome (PRPG) > 99% were finally obtained. The lateral branch and plant height phenotypes did not significantly differ between the NILs and WCZ, indicating that the NILs of Clbl under the genetic background of WT20 has been successfully developed. These results provide ideal materials for further in-depth analysis of the genetic mechanisms of lateral branch development and ideal plant architecture breeding in watermelon.

Characterization of genetic diversity of citron watermelon landrace accessions of South Africa through simple sequence repeat markers

ABSTRACT Profiling the genetic diversity of germplasm collections is fundamental for selection programs and strategic conservation. Citron watermelon (Citrullus lanatus var. citroides [L.H. Bailey] Mansf. ex Greb.) is an under-researched cucurbit crop despite providing food and nutritional security in sub-Saharan Africa. There is limited information on genetic relationships of South African citron watermelon accessions for effective breeding. The objective of this study was to determine the extent of genetic diversity among 48 South African citron watermelon landrace accessions using selected simple sequence repeat (SSR) markers to identify genetically divergent accessions for trait integration and variety development. The SSR markers amplified a total of 24 alleles, with a mean expected heterozygosity value of 0.38, indicating moderate genetic diversity among the studied accessions. The mean polymorphic information content value of 0.21 indicated low discriminatory power of the SSR markers. Analysis of molecular variance attributed 8%, 75%, and 17% of the molecular variation between populations, among accessions and within accessions, respectively. Three distinctive genetic groups were identified based on cluster analysis, which aided the selection of genetically diverse citron watermelon accessions for breeding and cultivar development.

Drought tolerance assessment of citron watermelon (Citrullus lanatus var. citroides (L.H. Bailey) Mansf. ex Greb.) accessions based on morphological and physiological traits

Long-term cultivation of citron watermelon under water-constrained environments in sub-Saharan Africa resulted in the selection and domestication of highly tolerant genotypes. However, information on the magnitude of variation for drought tolerance in citron watermelon is limited for the effective selection of suitable genotypes for breeding. The objective of this study was to determine variation for drought tolerance among South African citron watermelon landrace accessions for selection and use as genetic stock for drought-tolerance breeding in this crop and closely-related cucurbit crops. Forty genetically differentiated citron watermelon accessions were grown under non-stress (NS) and drought-stress (DS) conditions under glasshouse environment. Data of physiological (i.e., leaf gas exchange and chlorophyll fluorescence parameters) and morphological traits (i.e., shoot and root system architecture traits, and fruit yield) were collected and subjected to various parametric statistical analyses. The accessions varied significantly for assessed traits under both NS and DS conditions which aided classification into five groups, namely; A (highly drought-tolerant), B (drought-tolerant), C (moderate drought-tolerant), D (drought-sensitive) and E (highly drought-sensitive). Drought-tolerant genotypes produced more fruit yield with less water compared with drought-sensitive genotypes. Several physiological and morphological parameters correlated with fruit yield under DS condition namely: instantaneous water-use efficiency (r = 0.97), leaf dry weight (r = 0.77), total root length (r = 0.46) and root dry weight (r = 0.48). The following accessions, namely: WWM-46, WWM-68, WWM-41(A), WWM-15, WWM-64, WWM-57, WWM-47, WWM-37(2), WWM-79, WWM-05 and WWM-50) were identified as highly drought-tolerant and recommended for drought-tolerance breeding in this crop or related cucurbit crops such as sweet dessert watermelon.

Genetic diversity analysis among different horticultural groups of indigenous and exotic Citrullus landraces using microsatellite markers

The study utilised a total of twenty simple sequence repeat (SSR) markers to investigate the genetic diversity of 53 watermelon accessions from different groups of Citrullus i.e., citroides, lanatus, and colocythis. Twelve of the twenty SSR markers were shown to be highly polymorphic and were statistically analyzed using Power Marker and NTSYSPc softwares. A total of 33 alleles were generated by polymorphic SSR markers, on an average of 2.6 alleles per loci. Heterozygosity among accessions for individual loci varied from zero to 0.17 (BVWS02306 marker).The gene diversity and PIC values varied between 0.08 to 0.72 and 0.46 to 0.90, respectively and the primer which had the highest gene diversity and PIC values was BVWS02422. The UPGMA-based dendrogram classified all the 53 accessions into two major clusters at 48 per cent similarity. All 41 cultivated watermelon accessions were grouped in cluster I and the remaining 12 accessions from wild types were grouped in cluster II. Thus, this study highlights the importance of molecular markers in the identification of wild and cultivated lines and their exploitation in future breeding programs. Keywords: Watermelon, genetic diversity, microsatellite, Jaccard similarity coefficient &nbsp

Genome-wide association analysis provides molecular insights into the natural variation of watermelon seed size.

Watermelons used for seed consumption tend to have larger seeds, whereas watermelons used for flesh consumption often require relatively small seeds. Therefore, watermelon seed size has received extensive attention from consumers and breeders. However, the natural variation and genetic mechanism of watermelon seed size remain unclear. In the present study, 100-seed weight, seed hilum length, seed length, seed width, and seed thickness were examined in 197 watermelon accessions. Furthermore, association analysis was performed between seed size traits and high-quality SNP data. The results revealed that there were strong correlations among the five seed traits, and seed enlargement was an important feature during watermelon seed size domestication. The seed-consumed biological species Citrullus mucosospermus and the edible seed watermelon Citrullus lanatus had significantly larger seeds than the other species. Eleven non-repeating significant SNPs above the threshold line were obtained from GWAS analysis. Four SNPs on chromosome 5 were considered to be closely associated with seed size traits (S5:32250307, S5:32250454, S5:32256177, and S5:32260870) and could be used as potential molecular markers for the breeding of watermelon cultivars with a target seed size. In addition, based on gene annotation information and previous reports, five genes near the four significant SNPs may regulate seed size. qRT-PCR analysis showed that two genes that may be involved in abscisic acid metabolism, Cla97C05G104360 and Cla97C05G104380, may play an important role in regulating watermelon seed size. Our findings provide molecular insights into natural variation in watermelon seed size and valuable information for molecular marker-assisted breeding.

Inheritance of resistance to damping-off (Rhizoctonia solani) in watermelon

This study aimed to find sources of resistance to damping-off and to determine the inheritance of resistance in watermelon. Firstly, 72 watermelon accessions were inoculated with CMM 1053 isolate of Rhizoctonia solani. Only two accessions (BGH 29 and BGH 76) were considered moderately resistant to the disease. The inoculated plants without symptoms from accession BGH-29 identified as resistant were self-fertilized and the inbred line S1 was used for the crosses. The reaction to R. solani after inoculation of the segregating populations of the cross and backcrosses between susceptible cultivar Crimson Sweet and the inbred line BGH-29 was evaluated. In other experiment eight S1 lines of watermelon were inoculated with six different isolates of R. solani and the reaction was evaluated by partial diallel analysis using the Griffing’s method IV to obtain estimates of general resistance ability (GRA), general aggressiveness ability (GAA), and specific interaction ability (SIA). According to ratings of the susceptible and resistant parents, generations F1, F2, BC1 (F1 × P1) and BC2 (F1 × P2), resistance to damping-off was conferred by at least nine genes with additive effect and low heritability which indicates polygenic inheritance. The results of the diallel analysis corroborate with the analysis of segregating population, indicating inheritance of resistance was horizontal, since 46.1% of the total sum of squares of the variation observed in the crosses was due to GRA, while the SIA corresponded to only 20.39% of variation. This is the first report about inheritance of resistance to damping-off caused by R. solani in watermelon.

Genome-wide association study and population structure analysis of seed-bound amino acids and total protein in watermelon.

BackgroundWatermelon seeds are a powerhouse of value-added traits such as proteins, free amino acids, vitamins, and essential minerals, offering a paleo-friendly dietary option. Despite the availability of substantial genetic variation, there is no sufficient information on the natural variation in seed-bound amino acids or proteins across the watermelon germplasm. This study aimed to analyze the natural variation in watermelon seed amino acids and total protein and explore underpinning genetic loci by genome-wide association study (GWAS).MethodsThe study evaluated the distribution of seed-bound free amino acids and total protein in 211 watermelon accessions of Citrullus spp, including 154 of Citrullus lanatus, 54 of Citrullus mucosospermus (egusi) and three of Citrullus amarus. We used the GWAS approach to associate seed phenotypes with 11,456 single nucleotide polymorphisms (SNPs) generated by genotyping-by-sequencing (GBS).ResultsOur results demonstrate a significant natural variation in different free amino acids and total protein content across accessions and geographic regions. The accessions with high protein content and proportion of essential amino acids warrant its use for value-added benefits in the food and feed industries via biofortification. The GWAS analysis identified 188 SNPs coinciding with 167 candidate genes associated with watermelon seed-bound amino acids and total protein. Clustering of SNPs associated with individual amino acids found by principal component analysis was independent of the speciation or cultivar groups and was not selected during the domestication of sweet watermelon. The identified candidate genes were involved in metabolic pathways associated with amino acid metabolism, such as Argininosuccinate synthase, explaining 7% of the variation in arginine content, which validate their functional relevance and potential for marker-assisted analysis selection. This study provides a platform for exploring potential gene loci involved in seed-bound amino acids metabolism, useful in genetic analysis and development of watermelon varieties with superior seed nutritional values.

Nucleotide variation in the phytoene synthase (ClPsy1) gene contributes to golden flesh in watermelon (Citrullus lanatus L.).

A gene controlling golden flesh trait in watermelon was discovered and fine mapped to a 39.08 Kb region on chromosome 1 through a forward genetic strategy, and Cla97C01G008760 (annotated as phytoene synthase protein, ClPsy1 ) was recognized as the most likely candidate gene. Vitamin A deficiency is a worldwide public nutrition problem, and β-carotene is the precursor for vitamin A synthesis. Watermelon with golden flesh (gf, which occurs due to an accumulated abundance of β-carotene) is an important germplasm resource. In this study, a genetic analysis of segregated gf gene populations indicated that gf was controlled by a single recessive gene. BSA-seq (Bulked segregation analysis) and an initial linkage analysis placed the gf locus in a 290-Kb region on watermelon chromosome 1. Further fine mapping in a large population including over 1000 F2 plants narrowed this region to 39.08Kb harboring two genes, Cla97C01G008760 and Cla97C01G008770, which encode phytoene synthase (ClPsy1) and GATA zinc finger domain-containing protein, respectively. Gene sequence alignment and expression analysis between parental lines revealed Cla97C01G008760 as the best possible candidate gene for the gf trait. Nonsynonymous SNP mutations in the first exon of ClPsy1 between parental lines co-segregated with the gf trait only among individuals in the genetic population and were not related to flesh color in natural watermelon panels. Promoter sequence analysis of 26 watermelon accessions revealed two SNPs in the cis-acting element sequences corresponding to MYB and MYC2 transcription factors. RNA-seq data and qRT-PCR verification showed that two MYBs exhibited expression trends similar to that of ClPsy1 in the parental lines and may regulate the ClPsy1 expression. Further research findings indicate that the gf trait is determined not only by ClPsy1 but also by ClLCYB, ClCRTISO and ClNCED7, which play important roles in watermelon β-carotene accumulation.

Fine-mapping reveals that the bHLH gene Cla011508 regulates the bitterness of watermelon fruit

The bitterness of the fruit seriously affects the taste of watermelon. Therefore, genetic research on bitterness is of great significance to improve watermelon fruit quality. However, there is no research on fine mapping of watermelon fruit bitterness. In the present study, an F2 population was developed, and two inbred lines, ‘9904’ (bitter flesh) and ‘Handel’ (non-bitter flesh) as parental materials. Genetic analysis indicated that the bitterness of watermelon was a dominant trait. The initial mapping revealed a 6.16 Mb region on chromosome 1. SNP and CAPS markers were used to narrowed the candidate region to 116.7 kb with four candidate genes (Cla011507, Cla011508, Cla011509, and Cla011510). Notably, Cla011508, and Cla011510 have high homology with gene Csa5G156220 and Csa5G157230, which regulate the bitterness trait in cucumber, and the mutation base at the locus 3,168,882 on chromosome 1 of Cla011508 led to the premature termination of the translation process; at the same time, a non-synonymous mutation caused the amino acid sequence change of Cla011510. The results of qRT-PCR showed that Cla011508 was highly expressed in watermelon flesh of ‘9904’, while no such significant difference was found in Cla011510. Furthermore, the analysis of 16 watermelon accessions showed that 8 bitterness accessions with base C at the locus 3,168,882 had higher gene expression level of Cla011508, which was significantly different from that of the 8 non-bitter accessions with base T. In conclusion, the bitterness of watermelon fruit may be controlled by Cla011508, and the important mutation loucs in this gene provide molecular insights for marker-assisted breeding of target trait.

Fine mapping a ClGS gene controlling dark-green stripe rind in watermelon

The stripe pattern is an important agronomic trait in watermelon, which determines the fruit rind pattern and consumer choice. However, the genes controlling these traits are still largely unknown. In the present study, a dark-green stripe inbred line WT2 and a netted stripe inbred line WM204 were used for genetic analysis, which revealed that the dark-green stripe is controlled by a single dominant gene ClGS. By bulked-segregant analysis (BSA), the ClGS was primarily mapped on watermelon chromosome 6 by using F2 plants developed from a cross between WT2 and WM204. Next-generation sequencing-aided marker discovery and a large mapping population consisting of 1206 F2 plants was used for fine mapping of the ClGS gene, and it was further mapped into a 107 kb candidate region. There were 11 genes predicated in this candidate region and 10 of them were differentially expressed in the 1-DAP fruit rind between two parental lines. Furthermore, 64 SNPs and 3 Indels were detected in the CDS region of these candidate genes. To further confirm the candidate gene of ClGS, we investigated the sequence variations among 74 re-sequenced natural watermelon accessions by in silico bulk segregant analysis. A 3-bp insertion was identified in all the non-dark green stripe watermelon accession group, which was located on the 8th exon of Cla019205. A Indel marker developed harboring the insertion showed co-segregation with the phenotype in the F2 mapping population, and it was also in completely agreement with another 25 watermelon accessions by electrophoretic analysis. These evidences suggested Cla019205 is probably the candidate gene controlling dark-green stripe in watermelon. The results of this study will be helpful for better understanding of the stripe formation and marker-assisted selection in watermelon.

Determination of Some Agronomic and Fruit Quality Characteristics of Some Watermelon Accessions from Turkish Watermelon Germplasm

This study has been conducted to determine the fruit quality parameters such as sugar and carotenoid content as well as plant and fruit characteristics of 11 local watermelon genotypes from watermelon genetic resources collection of Cukurova University, Department of Horticulture. First and 50% male and female flowering period, main stem length, main stem diameter, number of nodes on main stem, total yield, fruit weight, fruit length, fruit diameter, fruit rind thickness, total soluble solids (TSS), sugar and carotenoid contents were examined. Although there is no significant difference for main stem diameter and number of nodes on the main stem, significant differences were obtained for total yield, sugar and carotenoid composition of the genotypes. Carotenoid and sugar analysis were performed with a high-performance liquid chromatographic method coupled with diode-array detector (HPLC-DAD) and HPLC coupled with refractive index detector (RID), respectively. In all studied genotypes, cis-13-lycopene and β-carotene were the most abundant compounds. As expected for watermelon genotypes, the main sugar found in all studied genotypes was fructose. According to PCA analyses, genotypes were characterized by physical and chemical composition. Overall evaluation of results revealed that Kar 147 had better potential with carotenoid, sugar contents and fruit characteristics.

Effect of different water deficiency levels on some enzymatic activities of watermelon accessions

Effect of different water deficiency levels on some enzymatic activities of watermelon accessions

QTL Mapping for Gummy Stem Blight Resistance in Watermelon (Citrullus spp.).

Watermelon (Citrullus lanatus) is an economically important fruit crop worldwide. Gummy stem blight (GSB) is one of the most damaging diseases encountered during watermelon cultivation. In the present study, we identified quantitative trait loci (QTLs) associated with GSB resistance in an F2 population derived from a cross between maternal-susceptible line ‘920533’ (C. lanatus) and the paternal-resistant line ‘PI 189225’ (C. amarus). The resistance of 178 F2 plants was assessed by two different evaluation methods, including leaf lesion (LL) and stem blight (SB). To analyze the QTLs associated with GSB resistance, a linkage map was constructed covering a total genetic distance of 1070.2 cM. QTL analysis detected three QTLs associated with GSB resistance on chromosome 8 and 6. Among them, two QTLs, qLL8.1 and qSB8.1 on chromosome 8 identified as major QTLs, explaining 10.5 and 10.0% of the phenotypic variations localizing at same area and sharing the same top markers for both LL and SB traits, respectively. A minor QTL, qSB6.1, explains 9.7% of phenotypic variations detected on chromosome 6 only for the SB trait. High-throughput markers were developed and validated for the selection of resistant QTLs using watermelon accessions, and commercial cultivars. Four potential candidate genes were predicted associated with GSB resistance based on the physical location of flanking markers on chromosome 8. These findings will be helpful for the development of watermelon cultivars resistant to GSB.

Variation in South African citron watermelon (Citrullus lanatus var. citroides [L.H. Bailey] Mansf. ex Greb.) landraces assessed through qualitative and quantitative phenotypic traits

Citron watermelon [Citrullus lanatus var. citroides (L.H. Bailey) Mansf. ex Greb.] is an important genetic resource serving for food, feed, and breeding dessert watermelon (C. lanatus var. lanatus) and other cross-compatible species. Wide phenotypic variation exists among South African citron watermelon landraces which are yet to be systematicaly assessed for diverse uses and cultivar design. The objective of this study was to assess phenotypic diversity of citron watermelon landrace accessions of South Africa and to select desirable genotypes with suitable agronomic and horticultual traits for direct production, breeding and conservation. Thirty six citron watermelon landrace accessions were evaluated under field conditions in two seasons using a 6 × 6 alpha lattice design with three replicates. Data on key qualitative and quantitative traits were collected and subjected to non-parametric and parametric statistical analyses. The accessions showed wide phenotypic variation and unique traits for genetic improvement. Positive and significant correlations (p < 0.001) were recorded between total fruit yield per plant with plant height (r = 0.64), number of harvestable fruits (r = 0.70), number of marketable fruits (r = 0.73) and marketable fruit yield (r = 0.96). Seed yield per plant positively and significantly (p < 0.001) correlated with number of male flowers (r = 0.68), plant height (r = 0.61) and total fruit yield (r = 0.79). Prinicipal component analysis identified nine components which accounted for 86.38% of total variation amongst accessions for assessed phenotypic traits. Citron watermelon accessions WWM16, WWM67 and WWM79 were high-yielding suitable for fruit production for food or livestock feed. Accessions namely WWM03, WWM09, WWM16, WWM39, WM64 and WWM76 produced high fruit number and marketable fruit yield per plant which are recommended as parental genotypes for breeding. Furthermore, accessions WWM04, WWM15, WWM24, WWM28, WWM46, WWM66, WWM68, WWM76 and WWM79 were selected with higher seed production for the food or feed industry. The study recommends citron watermelon accessions such as WWM14, WWM16, WWM39, WWM41, WWM67 and WWM79 for use as leafy vegetables owing to their profuse branching ability and longer vine production. Whereas accessions including WWM03, WWM17, WWM35, WWM40, WWM50, WWM67, WWM79 and WWM85 are selected with larger fruit size. Accessions WWM05 and WWM09 are sour-flesh types which are suitable genetic stocks for breeding sweet-and-sour dessert watermelons. Orange-fleshed accessions such as WWM03, WWM04, WWM46, WWM64, WWM66 and WWM67 are recommended for fresh consumption, cooking, processing or variety design. Accessions WWM02, WWM03, WWM08, WWM14, WWM16, WWM23, WWM38, WWM40, WWM41 and WWM67 have red and white seed coat colour which are superior selections to prepare roasted citron watermelon seed snack.

Morph-agronomic characterization of watermelon accessions with resistance to Fusarium Wilt.

Fifty-five watermelon accessions were inoculated in pots using Fusarium oxysporum f. sp. niveum chlamydospores in a completely randomized design with five replications. Among the 42 accessions that were resistant, twelve accessions with a mean score ≤1 were selected for morpho-agronomic characterization. 'Sugar Baby' and 'Charleston Gray' were used as controls. Field research was conducted over two production cycles in a randomized block design with three replications and five plants per plot. For the characterization 13 quantitative agronomic traits were used. The dissimilarity measures were obtained by the sum of the matrices of standardized mean Euclidean distance. Coefficients of variation ranged from 5.33% (number of days until 50% of plants with at least one female flower) and 17.20% (mass of characterized fruit). In the second cycle, a reduction in days was observed for the flowering. For soluble solids content, the accession BGH-UNIVASF 40 was statistically equal to the commercial cultivars in the second cycle. Accessions were grouped similarly using two grouping methods, Tocher and the farthest neighbor method. The most promising accessions were BGH-UNIVASF 40, 169, 177 and 210 for use in future hybridizations. BGH-UNIVASF 76, 128 and 185 (Citrullus lanatus var. citroides) can be used as resistant Fusarium wilt rootstocks.

Comparative varieties characteristics of watermelon accessions of ukrainian and foreign breeding

Aim. Evaluation of the common watermelon gene pool in terms of yield, marketable fruit weight and dry soluble matter content and isolation of the most valuable accessions for use in breeding programs. Results and Discussion. The article presents the results of studies on the assessment of 153 samples of common watermelon of Ukrainian and foreign selection in terms of yield, weight of marketable fruit and the content of dry soluble substances. One accession with very high level of fruit yield (&gt; 113% to the standard) was selected – accession Lad (62,3 t/ha) from Ukraine. With high level of yield (&gt; 101% to the standard), samples from Russia were selected: Solntsedar (55,7t/ha), Podarok Solntsa (58t/ha) and Samurai (58,3t/ha). With yield closed to the standard (97,7%) there was a sample from Russia – Shapka Imperatora (53,5t/ha). The mass of marketable fruit, 5 samples were the best: Solntsedar (2,5 kg), Lad (2,6 kg), Yatum (2,86 kg), Samurai (3,04 kg), Podarok Solntsa (3,99 kg). According to the content of dry soluble matter, samples from Ukraine were selected as a source for breeding: Alyi solodkyi (14%) and accession from Czech Republic Sugar baby (15%). According to the complex of features (consistently high yield, high taste), the following samples are of practical value for breeding: Lad, Yatum, Alyi solodkyi (Ukraine); Solntsedar, Podarok Solntsa, Samurai (Russia), Sugar baby (Czech Republic). Conclusions. Based on the results of the study of 153 collection accessions of common watermelon during 2018–2020. Accessions were selected: 4 by yield (55,7-62,3t/ha), 5 by fruit weight (2,51-3,99 kg), 2 by content of dry soluble matter (14-15 mg%), 7 by complex of valuable features.

Response of Some Watermelon Accessions to a Puerto Rico Strain of Zucchini yellow mosaic virus

One of the most important members of the Potyviridae is Zucchini yellow mosaic virus (ZYMV). It affects watermelon [Citrullus lanatus (Thunb.) Matsum. &amp; Nakai] as well as other cucurbits in most parts of the world. Although several genotypes have been reported as having resistance to ZYMV, differential responses to ZYMV strains are known to occur. Using a Puerto Rico strain of ZYMV (ZYMV-PR, GenBank accession number MN422959), we tested the response of 11 genotypes [PIs from the U.S. Department of Agriculture (USDA) National Genetic Resources Program] previously reported as having resistance to this virus. In two greenhouse trials, the first three leaves of seedlings of each genotype were mechanically inoculated with ZYMV-PR. An enzyme-linked immunosorbent assay (ELISA) was done on each seedling’s fourth leaf and symptom severity was rated on the first, third, fifth, and seventh leaves. There were significant genotype × trial interactions for most variables, but some genotypes performed consistently in both trials. All seedlings of PI 537277 tested negative for ELISA (absorbance &lt; 0.200) across both trials. PI 537277, PI 595200, PI 595201, and PI 595203 were generally among the accessions with the lowest symptom severity scores. Overall, symptom severity correlated poorly with ELISA readings. But all plants of PI 537277, and most plants of PI 595201 and PI 595203, had low ELISA readings and low severity scores. Despite having low severity scores, PI 595200 was among the genotypes with the highest ELISA readings in trial 2. For the plant breeder, the most useful genotypes are those that exhibit reduced severity as well as low ELISA. PI 537277, PI 595201, and PI 595203 met those criteria in this study. Of these three accessions, PI 595203 would be the most useful in a breeding program because it has shown resistance to the Puerto Rico, Florida, and China strains of ZYMV.

Decreased Protein Abundance of Lycopene β-Cyclase Contributes to Red Flesh in Domesticated Watermelon.

Red-fleshed watermelons (Citrullus lanatus) that accumulate lycopene in their flesh cells have been selected and domesticated from their pale-fleshed ancestors. However, the molecular basis of this trait remains poorly understood. Using map-based cloning and transgenic analysis, we identified a lycopene β-cyclase (ClLCYB) gene that controls the flesh color of watermelon. Down-regulation of ClLCYB caused the flesh color to change from pale yellow to red, and ClLCYB overexpression in the red-fleshed line caused the flesh color to change to orange. Analysis of ClLCYB single-nucleotide polymorphisms using 211 watermelon accessions with different flesh colors revealed that two missense mutations between three haplotypes (ClLCYB red , ClLCYB white , and ClLCYB yellow ) were selected and largely fixed in domesticated watermelon. Proteins derived from these three ClLCYB haplotypes were localized in plastids to catalyze the conversion of lycopene to β-carotene and showed similar catalytic abilities. We revealed that ClLCYB protein abundance, instead of ClLCYB transcript level, was negatively correlated with lycopene accumulation. Different amounts of ClLCYB protein degradation among the ClLCYB haplotypes were found in ClLCYB transgenic Arabidopsis (Arabidopsis thaliana) lines. After treatment with the proteasome inhibitor MG132, the concentration of ClLCYBred increased noticeably compared with other ClLCYB proteins. These results indicate that natural missense mutations within ClLCYB influence ClLCYB protein abundance and have contributed to the development of red flesh color in domesticated watermelon.

Discovery of the Genomic Region and Candidate Genes of the Scarlet Red Flesh Color (Yscr ) Locus in Watermelon (Citrullus Lanatus L.).

The flesh color of watermelon (Citrullus lanatus) is an important fruit quality trait that helps to determine fruit attractiveness and is potentially beneficial to human health. Previous inheritance analyses determined that a single dominant gene, Yscr, produces the scarlet red flesh color rather than the coral red flesh color in watermelon. However, no genomic region or gene-based molecular markers for the locus Yscr have been reported thus far. In the present study, two high-density genetic maps and whole-genome variation detection aided by genome resequencing were first map the flesh color locus Yscr to a small region on chromosome 6 based on two independent populations derived from two scarlet red-fleshed lines and two coral red-fleshed lines. Two major quantitative trait loci located in the same genomic regions were identified in the F2 and BC1P2 populations and explained 90.36% and 75.1% of the phenotypic variation in flesh color, respectively. Based on the genetic variation in the two parental lines, newly developed PCR-based markers narrowed the Yscr region to 40 Kb. Of the five putative genes in this region, four encoded glycine-rich cell wall structural proteins, which implied that a new regulatory mechanism might occur between scarlet red- and coral red-fleshed in watermelon. Moreover, the genotypes of two newly developed InDel markers (InDel27_fc6 and InDel28_fc6) were completely consistent with the phenotypes in the F2 and BC1P2 populations and all 56 scarlet red-fleshed watermelon accessions. The results presented here provide valuable information for marker-assisted selection of flesh color breeding and the functional validation of candidate genes in watermelon.

Characterization of Watermelon (Citrullus lanatus) Accessions Introduced from USDA

Characterization of Watermelon (<i>Citrullus lanatus</i>) Accessions Introduced from USDA