干旱区生态环境知识资源中心

There is a continuous need to enhance watermelon cultivars for disease and pest resistance. U.S. Plant Introductions (PIs) representing the different groups of watermelon (Citrullus spp.), including the desert watermelon Citrullus colocynthis (L.) Schrader (CC), the "cow" or Tsamma watermelon Citrullus lanatus subsp. lanatus var. lanatus [also known as C. lanatus (Thunb.) Matsum. et Nakai subsp. lanatus var. citroides (Bailey) Mansf. ex Greb.] (CLC), and Citrullus lanatus subsp. lanatus (CLL) are considered a useful source for enhancing disease or pest resistance in watermelon cultivars. However, the genetic relationships among these species and subspecies need further investigation. In this study, we examined genetic diversity among Citrullus spp. accessions representing CC, CLC, and CLL groups collected in their center of origin in Africa. High frequency oligonucleotides targeting active gene (HFO-TAG) primers were used in polymerase chain reaction (PCR) experiments to produce over 560 polymorphic markers among the Citrullus genotypes. 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. The results here are consistent with our single nucleotide polymorphism (SNP) results developed in our recent genotyping-by-sequencing (GBS) study and our recent findings using fluorescence in situ hybridization (FISH) technology showing major differences in configuration of ribosomal DNA markers between the CLC and the CLL or CC genotypes. Overall, a wide genetic diversity exists among Citrullus spp. and different sources should be useful for enhancing disease or pest resistance in elite watermelon cultivars.