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Title: High frequency oligonucleotides – targeting active gene (HFO-TAG) markers reveal wide genetic diversity among Citrullus spp. accessions useful for enhancing disease or pest resistance in watermelon cultivars

Author
item Levi, Amnon
item Thies, Judy
item Wechter, William - Pat
item Harrison Jr, Howard
item Simmons, Alvin
item REDDY, U - West Virginia State University
item NIMMAKAYALA, P - West Virginia State University
item FEI, Z - Cornell University

Submitted to: Genetic Resources and Crop Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2012
Publication Date: 4/17/2012
Citation: Levi, A., Thies, J.A., Wechter, W.P., Harrison Jr, H.F., Simmons, A.M., Reddy, U., Nimmakayala, P., Fei, Z. 2012. High frequency oligonucleotides – targeting active gene (HFO-TAG) markers reveal wide genetic diversity among Citrullus spp. accessions useful for enhancing disease or pest resistance in watermelon cultivars. Genetic Resources and Crop Evolution. 60:427-440.

Interpretive Summary: The USDA, ARS maintains a large number of wild watermelon accessions that were collected in different parts of the world. These wild watermelon accessions are considered a valuable genetic material, useful for enhancing resistance to diseases and pests in commercial watermelon cultivars. However, there is little information about the genetic diversity and genetic relationships among these wild watermelon accessions, which is essential for plant breeders who wish to use these wild watermelon accessions in their breeding programs. ARS scientists at Charleston, SC, evaluated a group of wild watermelon accessions that were collected in southern Africa. The scientists used a new type of DNA markers (named high frequency oligonucleotides-targeting active genes; HFO-TAG) to examine genetic diversity among the accessions and were able to classify them into several major groups. They identified several accessions in each group that are resistant to diseases or pests of watermelon. The scientists selected the accessions that are resistant to Fusarium wilt (a major soil-borne disease of watermelon) or to root-knot nematodes (microscopic parasitic roundwarms that attack roots of many crop plants) for future genetic studies to identify the genes that confer resistance in watermelon. The information in this study will be useful for plant breeders and scientists interested in utilizing the wild watermelon accessions in their breeding programs, aiming to enhance disease or pest resistance in commercial watermelon cultivars.

Technical Abstract: There is a continuous need to enhance watermelon cultivars for disease and pest resistance. Different U.S. Plant Introductions (PIs) of Citrullus lanatus subsp. lanatus var. lanatus [also known as C. lanatus (Thunb.) Matsum. et Nakai subsp. lanatus var. citroides (Bailey) Mansf. ex Greb.] (CLC) collected in southern Africa are a useful source for enhancing disease or pest resistance in watermelon cultivars. They are also valuable as rootstocks for grafted watermelon, particularly in fields infested with root-knot nematodes or Fusarium wilt. However, there is little information about genetic relationships among these PIs. In this study, genetic diversity was examined among 74 CLC PIs collected from their center of origin in southern Africa. Also, 15 Citrullus lanatus subsp. lanatus (CLL) PIs and the American heirloom cultivars Charleston Gray and Black Diamond (Citrullus lanatus subsp. vulgaris Schrader ex Eckl. Et Zeyh. Fursa) (CLV) and five Citrullus colocynthis (L.) Schrader (CC) PIs collected in different locations throughout the world were used as out-groups in the phylogenetic analysis for the CLC PIs. Twenty-three high frequency oligonucleotides - targeting active gene (HFO-TAG) primers were used in polymerase chain reaction (PCR) experiments to produce a total of 562 polymorphic markers among the Citrullus PIs and cultivars. Cluster and multidimensional scaling plot analysis produced distinct groups of CLC, CLL, and CC PIs. Several PIs that were designated as CLC or CLL were in transitional positions, indicating that they are the result of gene flow between the major Citrullus groups or subgroups. Population structure analysis indicated that CLC comprises two subgroups; each containing a set of unique alleles. Also, unique alleles exist in the CLL and the CC genotypes. Overall, broad genetic diversity exists among the Citrullus PIs. The data in this study should be useful for identifying PIs with a wide genetic distance between them that could be used in breeding programs aiming to develop heterotic F1 hybrid rootstock lines for grafted watermelon.