Genetic diversity among watermelon (Citrullus lanatus and Citrullus colocynthis) accessions

Genetic diversity was estimated among 42 U.S. Plant Introduction (PI) accessions of the genus Citrullus (of these, 34 PIs are reported to have disease resistance), and 5 watermelon cultivars, using 30 RAPD primers. These primers produced 662 RAPD markers that could be rated with high confidence. Based on these markers, genetic similarity coefficients were calculated and a dendrogram was constructed using the unweighted pair-group method with arithmetic average (UPGMA). The analysis delineated three major clusters. The first cluster consisted of a group of five watermelon cultivars, a group of C. lanatus var. lanatus accessions, and a group of C. lanatus var. lanatus accessions that contained some C. lanatus var. citroides genes. The second cluster consisted of the C. lanatus var. citroides accessions, while the third cluster consisted of the C. colocynthis accessions. The two C. lanatus clusters differentiated from each other and from the C. colocynthis cluster at the level of 58.8% and 38.9% genetic similarity, respectively. Assessment of genetic diversity among accessions that have been reported to have disease resistance indicated that resistance to either anthracnose, downy mildew, powdery mildew, or watermelon mosaic virus is found among all major groups of Citrullus Pls. Additionally, resistance to gummy stem blight or Fusarium wilt may exist among C. lanatus var. citroides PIs. This study demonstrates that molecular markers can be useful in assessing genetic diversity, and in sorting Citrullus PIs into phylogenetic groups prior to their evaluation for disease or pest resistance.