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Title: Genetic diversity in the desert watermelon Citrullus colocynthis and its relationship with Citrullus species as determined by high-frequency oligonucleotides-targeting active gene markers

Author
item Levi, Amnon
item Simmons, Alvin
item Massey, Laura
item Coffey, John
item Wechter, William - Pat
item Jarret, Robert - Bob
item TADMOR, Y - Agricultural Research Organization Of Israel
item NIMMAKAYALA, P - West Virginia State University
item REDDY, U - West Virginia State University

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2016
Publication Date: 2/1/2017
Citation: Levi, A., Simmons, A.M., Massey, L.M., Coffey, J., Wechter, W.P., Jarret, R.L., Tadmor, Y., Nimmakayala, P., Reddy, U. 2017. Genetic diversity in the desert watermelon Citrullus colocynthis and its relationship with Citrullus species as determined by high-frequency oligonucleotides-targeting active gene markers. Journal of the American Society for Horticultural Science. 142(1):47–56. doi:10.21273/JASHS03834-16.

Interpretive Summary: The desert watermelon, known as Citrullus colocynthis, is indigenous to the hot and dry deserts of northern Africa, the Middle East, and Central Asia. It is known to be resistant to major diseases and pests of watermelon. However, there is little knowledge about the genetic diversity and relationships of this desert species with cultivated watermelon. In this study, ARS scientist from Charleston, SC, collaborated with a team of scientists at West Virginia State University and at the Agricultural Research Organization (ARO), Israel, on using advanced genomic technologies to evaluate genetic diversity and relationships among desert watermelon accessions collected in different parts of the world. The analysis identified several distinct taxonomic groups and showed that colocynthis watermelon accessions can be cross-pollinated with cultivated watermelon types and be used in breeding programs working to enhance disease, pest, heat, or drought resistance in watermelon cultivars. The results from this study should be useful for seed company breeders and for scientists interested in studying and utilizing the wild desert watermelon in their breeding and research programs.

Technical Abstract: Citrullus colocynthis (L.) Schrad. is a viable source of genes for enhancing disease and pest resistance in the cultivated watermelon. However, there is little information in the literature about genetic diversity within C. colocynthis (CC) or the relationship of specific genotypes of CC to C. lanatus. In this study, we examined genetic diversity and relationships among 29 accessions of CC collected in northern Africa, the Middle East, and Asia, and their relationships to three genebank accession and three cultivars of Citrullus lanatus var. lanatus (Thunb.) Matsum. & Nakai, (CLL), 12 genebank accessions of C. lanatus (Thunb.) Matsum. & Nakai subsp. lanatus var. citroides (Bailey) Mansf. ex Greb. (CLC), and an accession representing the desert perennial C. ecirrhosus Cogn. (CE). Twenty-three high-frequency oligonucleotides - targeting active gene (HFO-TAG) primers were used to produce a total of 431 polymorphic fragments that target coding regions of the genome. Cluster and multidimensional scaling plot analysis separated the CC into five clades based on a unique set of alleles. Admixed genotypes were also identified. The analyses indicated a slightly closer genetic relationship of CC to CLL than to CLC. Since considerable genetic and morphological overlap exists between CLC and CLL we propose that CLC still be considered a subspecies of Citrullus lanatus (as originally named by Thunberg in 1773). The desert perennial CE contained alleles present in CC, CLL, and CLC, indicating that it may have evolved from a common ancestor. The clustering pattern of the CC into five subgroups was in general agreement with their geographic origins. Reproductive barriers resulted in significantly reduced fertility in CC x CLL hybridizations. However, several of the CC PIs were successfully crossed with multiple watermelon cultivars using traditional breeding procedures and the seeds produced from these crosses are viable, indicating that CC can be used to introduce disease, pest, and abiotic stress resistance genes into cultivated watermelon.