Distinct genomic loci underlie quantitative resistance to Meloidogyne enterolobii galling and reproduction in Citrullus amarus

Meloidogyne enterolobii is a virulent species of root-knot nematode that threatens watermelon (Citrullus lanatus) production in the southeast United States. There are no known sources of root-knot nematode resistance in cultivated C. lanatus. Specific genotypes of a wild watermelon relative, C. amarus, are resistant against Meloidogyne incognitaM. incognita, but the genetics that underly this resistance are still unknown and it is not clear that this same resistance will be effective against M. enterolobii. To identify and characterize new sources of resistance to M. enterolobii, we screened 108 diverse C. amarus lines alongside a susceptible C. lanatus cultivar (‘Charleston Gray’) for resistance against M. enterolobii. Different C. amarus genotypes ranged from resistant to susceptible for the three resistance phenotypes measured; mean percent root system galled ranged 10 - 73%, mean egg mass counts ranged 0.3 - 64.5, and mean eggs per gram of root ranged 326 - 146,160. We used each of these three resistance phenotypes combined with whole-genome resequencing data to conduct a genome wide association scan that identified significant associations between M. enterolobii resistance and 11 SNPs (single nucleotide polymorphisms) within the C. amarus genome. Interestingly, SNPs associated with reduced galling and egg masses were located within a single quantitative trait locus (QTL) on Chromosome Ca03, while reductions in nematode eggs per gram of root were associated with separate QTL on chromosomes Ca04 and Ca08. The results of this study suggest multiple genes are involved with M. enterolobii resistance in C. amarus and the SNPs identified will assist with efforts to breed for M. enterolobii resistance in watermelon.