Mapping quantitative trait loci for resistance to watermelon bud necrosis orthotospovirus in watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai]

Watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] is an important cucurbit crop cultivated worldwide. In an effort to map WBNV resistance, the current study consisted of two populations derived from a WBNV resistant accession, IIHR-82 belonging to Citrullus lanatus var. citroides (Syn. Citrullus amarus). The first population is a backcross inbred population (BC1F6) of 141 families (Pop I) that was developed by crossing IIHR-82 to an elite cultivar Arka Manik. A resistant family, BIL-53 belonging to Pop I was crossed to IIHR-140 to develop F3 population, consisting of 112 families (Pop II). Each of these populations was evaluated for WBNV incidence for two years under natural epiphytotic conditions. A significant correlation was observed for various resistance traits across years and populations. The frequency distribution graphs of Area Under Disease Progress Curve for WBNV incidence in both the populations exhibited a continuous distribution, indicating that trait is quantitative in nature. A total of 163 markers were mapped for the Pop I which spread onto 15 linkage groups (LGs) spanning a total map length of 3310.17 cM with a mean marker interval of 20.31 cM. The linkage map for the Pop II was constructed with 135 markers spread over 12 linkage groups spanning a total length of 1965.53 cM with an average interval between markers of 14.56 cM.The QTL analysis for WBNV resistance related traits revealed 14 major QTL's for Pop I and 19 for Pop II with a maximum PVE upto 21.20%. Multi-trait QTL regions were observed on LG 3b for Pop I and LG 2, LG 4, LG 7 and LG 8 for Pop II. Importantly, we observed common QTL regions for plant survival on LG 2 and PDI on LG 3 and LG 8 for both the populations. As this is the first attempt to map QTL's for WBNV resistance, the results obtained in the present study may provide a guide for fine mapping multi-trait QTL regions.