A three-locus model for rind phenotypes in watermelon (Citrullus lanatus) was previously proposed based on genetic analysis. These three loci, S (foreground stripe pattern), D (depth of rind color), and Dgo (background rind color), segregate in a Mendelian manner. Whole genome sequencing of watermelon offers a new strategy for marker development in these rind phenotype-related loci. A genotype analysis using subsets of 188, 273, 287 and 113 probes was performed for the ‘0901’, ‘10909’, ‘109905’ and ‘90509’ rind trait-segregating F2 populations, respectively. A total of 26, 34, 30 and 15 linkage groups with 175, 254, 269 and 79 probes were constructed for the ‘0901’, ‘10909’, ‘109905’ and ‘90509’ populations, respectively. The genetic order of the probes was mostly collinear with the physical order on the reference genome, except for some probes on chromosomes 1, 3 and 11. The three rind-related loci, S, D, and Dgo were anchored near chr6_25767 on chromosome 6, chr8_26061 on chromosome 8 and chr4_150/chr4_249 on chromosome 4, respectively. The three loci are located on different chromosomes, and the three-locus model was therefore verified through molecular genetic analysis. We suggest a rapid and practical marker development strategy that can be used not only for rind traits but also for other agriculturally important traits in watermelon and applied for conventional breeding.

Rapid and practical molecular marker development for rind traits in watermelon

Citron watermelon (Citrullus lanatus var. citroides) is an important tropical crop belonging to the Cucurbitaceae family. It possesses novel genes for biotic and abiotic stress tolerance and horticultural traits useful for breeding including sweet dessert watermelon. There is a lack of research priority limiting the genetic improvement and cultivar deployment of citron watermelon. Genetically diverse citron watermelon genetic resources are important to identify and select suitable genotypes for cultivar development incorporating horticultural attributes according to the needs and preferences of clients and marketplace. The objective of this review was to present progress on genetic improvement of citron watermelon with emphasis on cultivar development with biotic and abiotic stress resistance and desired horticultural attributes. Further, the review highlighted genetic and molecular resources in citron watermelon and other cross compatible cucurbits that may aid in accelerated breeding and cultivar release. Finally, the paper summarized research focus areas with potential to enhance effective utilization of citron watermelon genetic resources for targeted breeding of this crop or related taxa such as dessert watermelon.

Progress in genetic improvement of citron watermelon (Citrullus lanatus var. citroides): a review

The impaired biosynthetic networks in defective tapetum lead to male sterility in watermelon.

Watermelon (Citrulluslanatus) 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.

/pdf/qtl-mapping-for-gummy-stem-blight-resistance-in-watermelon-2vaxzzqsh1.pdf

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

Genetic polymorphism amid plant species is a crucial factor for plant improvement and maintaining their biodiversity. Evaluation of genetic diversity amongst plant species is significant to deal with the environmental stress conditions and their effective involvement in the breeding programs. Hence, in present study, an attempt has been made towards the genetic assessment of individual and bulked populations of 25 watermelon genotypes, belonging to Citroides (citron watermelon) and Lanatus (dessert watermelon) group from Konya, Thrace, Turkmenistan, Saudi Arabia and Turkey. The employed Random Amplified Polymorphic DNA (RAPD) and Inter-Simple Sequence Polymorphism (ISSR) marker systems provided 69.4 and 95.4% polymorphisms, respectively. Different clustering methods showed clear grouping of the genotypes based on the geographical origin and species. Citron genotypes from Turkmenistan stood apart from all the Turkish Lanatus genotypes. However, Saudi Arab Lanatus genotype grouped with native Turkish varieties indicating the genetic linkage. Among all the Turkmenistan Citron genotypes, Turkmenistan-11 was the most distinct form. Moreover, sufficient genetic variation was found between the commercial and native Lanatus genotypes of Turkey as well as Citron genotypes of Turkmenistan. Hence, it will be beneficial to include these genotypes in the future breeding programs to transfer disease-resistant alleles from Citron to Lanatus genotypes.

Genetic diversity and population structure of watermelon (Citrullus sp.) genotypes.

Backcross breeding programs are widely used for rapid cultivar development by introgression of valuable traits from wild-type germplasm into elite lines. For efficient introgression from a distant cross, a genetic map is useful to select markers for maintaining the genetic background of elite lines. Construction of a genetic map in watermelon has been limited on one hand, by the relatively narrow genetic background within watermelon cultigens that results in little genetic polymorphism, and on the other hand, by high segregation distortion rates (SDR) of highly polymorphic markers found in a distant cross. In this study, we tried to develop a genetic map using expressed sequence tag (EST)-based simple sequence repeat (SSR) markers; however, only 33 EST-SSRs were anchored to the map, mainly because of multi-copies of repetitive sequences and SDR in the F2 mapping population derived from a cross of PI 189225 (Citrullus amarus; previously C. lanatus ssp. citroides) and ‘TS’ (Citrullus lanatus ssp. lanatus). To obtain more candidate markers, the whole genome of both parental lines was resequenced, and 2.5 and 0.3 million singlenucleotide polymorphisms (SNPs) were identified in PI 189225 and ‘TS’, respectively. By comparing these SNPs to the reference genome, we developed 200 high-resolution melting (HRM) candidate markers for genotyping the F2 progeny. Ultimately, 103 HRM markers were located on the genetic map, and 42 EST-SSR markers were consolidated in the map. The total map length was 1178.3 cM, the average length of the linkage groups was 107.1 cM, and the average inter-marker distance was 8.24 cM. Although these markers are not evenly distributed along the genome, they are considered to be successfully anchored. The EST-SSR and HRM markers in this genetic map will be useful in breeding programs as frame markers for foreground and background selection.

Construction of a genetic linkage map using a frame set of simple sequence repeat and high-resolution melting markers for watermelon ( Citrullus spp.)

The whole-genome sequence of watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai), a valuable horticultural crop worldwide, was released in 2013. Here, we compared a de novo-based approach (DBA) to a reference-based approach (RBA) using RNA-seq data, to aid in efforts to improve the annotation of the watermelon reference genome and to obtain biological insight into male-sterility in watermelon. We applied these techniques to available data from two watermelon lines: the male-sterile line DAH3615-MS and the male-fertile line DAH3615. Using DBA, we newly annotated 855 watermelon transcripts, and found gene functional clusters predicted to be related to stimulus responses, nucleic acid binding, transmembrane transport, homeostasis, and Golgi/vesicles. Among the DBA-annotated transcripts, 138 de novo-exclusive differentially-expressed genes (DEDEGs) related to male sterility were detected. Out of 33 randomly selected newly annotated transcripts and DEDEGs, 32 were validated by RT-qPCR. This study demonstrates the usefulness and reliability of the de novo transcriptome assembly in watermelon, and provides new insights for researchers exploring transcriptional blueprints with regard to the male sterility.

/pdf/de-novo-based-transcriptome-profiling-of-male-sterile-and-fvrhiq7723.pdf

De novo-based transcriptome profiling of male-sterile and fertile watermelon lines.

Watermelon production is often limited by powdery mildew in areas with a large daily temperature range. Development of resistant watermelon cultivars can protect against powdery mildew; however, little is known about the characteristics of its causal agents. Here, we identified the genus and race of a causal pathogen of powdery mildew in Ansung province of South Korea, and developed molecular markers for the generation of resistant watermelon cultivars. The causal pathogen was determined to be Podosphaera xanthii based on multiple sequence alignments of internal transcribed spacers (ITS) of rDNA. The physiological race was identified as 1W, and the Ansung isolate was named P. xanthii 1W-AN. Following inoculation with the identified P. xanthii 1W-AN, we found inheritance of the resistant gene fitting a single dominant Mendelian model in a segregated population (‘SBA’ × PI 254744). To develop molecular markers linked to fungus-resistant loci, random amplified polymorphic DNA (RAPD) was accomplished between DNA pooled from eight near-isogenic lines (NILs; BC₄F₆), originated from PI 254744 and susceptible ‘SBB’ watermelon. After sequencing bands from RAPD were identified in all eight NILs and PI254744, 42 sequence-characterized amplified-region (SCAR) markers were developed. Overall, 107 F₂ plants derived from BC₄F₆ NIL-1 × ‘SBB’ were tested, and one SCAR marker was selected. Sequence comparison between the SCAR marker and the reference watermelon genome identified three Nco I restriction enzyme sites harboring a single nucleotide polymorphism, and codominant cleavage-amplified polymorphic site markers were subsequently developed. A CAPS marker was converted to a high-resolution melt (HRM) marker, which can discriminate C/T SNP (254PMR-HRM3). The 254PMR-HRM3 marker was evaluated in 138 F 2:3 plants of a segregating population (‘SBA’ × PI254744) and was presumed to be 4.3 cM from the resistance locus. These results could ensure P. xanthii 1W-AN resistance in watermelon germplasm and aid watermelon cultivar development in marker-assist breeding programs.

https://koreascience.or.kr:443/article/JAKO201608366218867.pdf

Identification of a causal pathogen of watermelon powdery mildew in Korea and development of a genetic linkage marker for resistance in watermelon (Citrullus lanatus).

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Construction of a genetic linkage map using a frame set of simple sequence repeat and high-resolution melting markers for watermelon ( Citrullus spp.)

De novo-based transcriptome profiling of male-sterile and fertile watermelon lines.

Identification of a causal pathogen of watermelon powdery mildew in Korea and development of a genetic linkage marker for resistance in watermelon (Citrullus lanatus).