Quantitative Trait Loci Mapping of Resistance to Fusarium oxysporum f. sp. niveum race 2 in Citrullus lanatus var. Citroides using Genotyping-by-Sequencing (GBS)

One of the most devastating watermelon diseases worldwide, Fusarium wilt, is caused by Fusarium oxysporum f. sp. niveum (Fon). Spread of the particularly virulent Fon race 2 in the United States, coupled with the lack of resistance in edible cultivars of the sweet cultivated watermelon Citrullus lanatus var. lanatus (Cll) poses a serious problem. Years of selection for desirable horiticultural traits in Cll has eroded genetic diversity, including the loss of many disease resistance genes. The wild subspecies of watermelon, C. lanatus var. citroides (Clc), harbors a wealth of genetic diversity including resistance to many diseases and is therefore essential to continued watermelon improvement. We used GBS to generate the first intra-variety Clc genetic map with an F_2:3 population (N=173) segregating for Fon race 2 resistance. A total of 2,495 binned SNPs formed 11 linkage groups with an average distance of 0.7 centiMorgans (cM) between SNPs and a total map length of 1,798.9 cM. One major and four minor QTL explained 69.3% of the variation in resistance. The major QTL was delimited to a 1.2Mb interval of chromosome 9, which collocated with one putative disease resistance gene.