Genetic mapping and development of molecular markers for a candidate gene locus controlling rind color in watermelon

Rind color is an important agronomic trait in watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai], but the underlying molecular mechanism for this trait is not fully known. In the present study, we identified a single locus on chromosome 8 accounting for watermelon rind color (dark green vs. light green). Genetic analysis of F-1, F-2, and BC1 populations derived from two parental lines (9904 with dark green rind and Handel with light green rind) revealed that the watermelon rind color (dark green vs. light green) is controlled by a single locus, and dark green is dominant to light green rind. Initial mapping revealed a region of interest spanning 2.07 Mb on chromosome 8. Genetic mapping with CAPS and SNP markers narrowed down the candidate region to 31.4 kb. Gene annotation of the corresponding region in the reference genome revealed the ClCG08G017810 gene sequence encoding the 2-phytyl-1,4-beta-naphthoquinone methyltransferase protein. The sequence alignment of the candidate gene with the two parental lines suggested a nonsynonymous SNP mutation in the coding region of ClCG08G017810, converting an arginine (R) to glycine (G). The SNP might be associated with rind color of 103 watermelon germplasm lines investigated in this study. The qRT-PCR analysis revealed higher expression of ClCG08G017810 in dark green rind than in light green rind. Therefore, ClCG08G017810 is a candidate gene associated with watermelon rind color. The present study facilitates marker-assisted selection useful for the development of cultivars with desirable rind color.