Mapping and validation of Fusarium wilt race 2 resistance QTL from Citrullus amarus line USVL246-FR2

Authors: GANAPARTHI, VENKATA - Clemson University; WECHTER, WILLIAM - Clemson University; Levi, Amnon; BRANHAM, SANDRA - Clemson University

Submitted to: Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/7/2024
Publication Date: 3/31/2024
Citation: Ganaparthi, V., Wechter, W., Levi, A., Branham, S. 2024. Mapping and validation of Fusarium wilt race 2 resistance QTL from Citrullus amarus line USVL246-FR2. Theoretical and Applied Genetics. 137/91. https://doi.org/10.1007/s00122-024-04595-z.
DOI: https://doi.org/10.1007/s00122-024-04595-z

Interpretive Summary: Watermelon is an important vegetable crop in the U.S.A. and throughout the world. Fusarium wilt (Fon) is a major disease of watermelon, causing significant crop damage and monetary losses to growers. Identifying genetic resources with resistance to Fon and utilizing them for developing resistant varieties is the most effective strategy in reducing the presence of this disease in watermelon fields. ARS Scientists along with Clemson University cooperators used advanced genomic technologies and identified gene loci conferring resistance to the virulent race 2 of Fon. The scientists developed DNA markers useful for incorporating the Fon-resistance gene loci into watermelon cultivars. These DNA markers will be useful to seed company and public breeders aiming to improve resistance to Fon in elite watermelon cultivars.

Technical Abstract: Fusarium oxysporum f. sp. niveum (Fon) race 2 causes Fusarium wilt in watermelon and threatens watermelon production worldwide. Grafting to resistant rootstocks can reduce economic losses but increases the cost of cultivation. Available fungicides coupled with management practices lower disease severity but do not prevent financial losses. Development of resistant cultivars is an effective and important component of disease management, but no edible watermelon cultivars have been released. Implementation of marker-assisted selection to develop resistant cultivars requires identifying sources of resistance and the underlying QTL, developing molecular markers associated with the QTL, and validating marker-phenotype associations with an independent population. An intraspecific Citrullus amarus recombinant inbred line (RIL) population from a cross of resistant USVL246-FR2 and susceptible line USVL114 was used for mapping Fon race 2 resistance QTL. KASP markers were developed (N=51) for the major QTL on chromosome 9 and minor QTL on chromosomes 1, 6, and 8. An interspecific F2:3 population was developed from resistance donor USVL246-FR2 (C. amarus) and a susceptible cultivar ‘Sugar Baby’ (Citrullus lanatus) to validate the utility of the markers for introgression of resistance from the wild crop relative into cultivated watermelon. While 45 KASP markers were polymorphic in the intraspecific C. amarus population, only 16 segregated in the interspecific C. amarus/lanatus validation population. Four of the polymorphic markers (two on chromosome 9 and one each on chromosomes 1 and 8) showed significant differences in the separation of genotypes based on family mean disease severity, but together explained only 16% of the phenotypic variance. Genotypes that inherited homozygous resistant parental alleles at three KASP marker loci on chromosomes 9 and 8 had 42% lower family mean disease severity than genotypes with homozygous susceptible parental alleles. Thus, haplotype analysis was more effective at predicting the mean disease severity of families than single markers. The haplotype identified in this study will be valuable for developing Fon race 2 resistant watermelon cultivars.