Submitted to: Journal American Society Hortscience
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/18/2003
Publication Date: 2/15/2004
Citation: Levi, A., Thomas, C.E., Newman, M.L., Reddy, O., Zhang, X. 2004. ISSR and AFLP markers sufficiently differentiated among American watermelon cultivars with limited genetic diversity. Journal American Society Horticultural Science 129:553-558. Interpretive Summary: Watermelon is an important vegetable crop grown in 44 states in the U.S. Florida, California, Texas, Georgia, and Arizona have the largest production of this crop. U.S. watermelon production has increased from 1.2 M tons in 1980 to 3.9 M tons in 1999 with a farm value of $272 million. Seed companies and growers have great interest in continuing to improve watermelon by developing new types with high fruit combined with better resistance to diseases and pests. Advanced DNA technologies are used by researchers in genetic studies to improve many crop plants. This study examines genetic (DNA) differences among American watermelon heirloom varieties using advanced technologies. The technologies used detected minute genetic (DNA) differences among watermelon varieties that were not detected with other scientific methods in previous studies. Thus, they will be effective in finding genes that affect watermelon quality or genes that enhance disease or pest resistances in this crop.
Technical Abstract: Forty-four randomly amplified polymorphic DNA (RAPD) markers and twenty-one anchored simple sequence repeat (ISSR) markers were tested for polymorphism among heirloom watermelon cultivars (Citrullus lanatus var. lanatus) that had limited genetic diversity. The markers tested for polymorphism represent chromosomal regions with low or no recombination events (0-1.2 cM), or chromosomal regions with high recombination events (3.5-29.8 cM) on a linkage map constructed for watermelon. Twelve (18.4%) of the 65 markers tested were polymorphic among cultivars. One (5.2%) of 19 markers located on regions with no/low recombination events, and eleven (23.4%) of 46 markers located on regions with high recombination events were polymorphic among cultivars. Low polymorphism is predominant across most major linkage groups of the watermelon genome, except for one linkage group that showed high polymorphism. Four (50%) of eight markers tested for this linkage group were polymorphic among cultivars. Among molecular markers tested RAPD markers produced low polymorphism, while ISSR markers were four times more polymorphic. In contrast with RAPDs all 128 AFLP markers produced in this study were polymorphic among cultivars. Thus, genetic diversity and relatedness were assessed among cultivars using 58 ISSR and 128 AFLP markers and a dendrogram was constructed using the unweighted pair-group method with arithmetic average (UPGMA). The dendrogram delineated phylogenetic relationships that are consistent with parentage data known for some of the heirlooms. The cultivars differentiated at the level of 79-98.3% genetic similarity, which is significantly higher than that (92.5-99.6%) produced by a comparable number (278) of RAPD markers. The results of this study indicate that AFLP and ISSR markers produce higher resolution than RAPD markers, and provide accurate differentiation among watermelon genotypes with limited genetic diversity.