Meta-analysis of qualitative and quantitative trait variation in sweet watermelon and citron watermelon genetic resources

Sweet watermelon (Citrullus lanatus var. lanatus (Thunb.) Matsum. & Nakai) is an economically important cucurbit species despite its relatively limited genetic variation for some desirable horticultural attributes. Conversely, citron watermelon (C. lanatus. var. citroides [L.H. Bailey] Mansf. ex Greb.) displays marked genetic variation for economic traits that can be integrated in genetic analysis and breeding of C. lanatus var. lanatus. Exploring the genetic resources of citron watermelon will aid gene discovery, mapping, introgression and genetic modification of sweet watermelon cultivars. The objective of this study was to document the qualitative and quantitative phenotypic trait variation present in sweet watermelon and citron watermelon genetic resources through a comparative meta-analysis as a guide for variety design with market-preferred attributes, genetic and genomic analysis. Data were assembled across 27 selected studies, and comparative meta-analyses were made. The metadata for qualitative traits included fruit traits namely: shape, colour, pattern of stripes and endocarp colour, and seed coat colour, all resolved significant genetic variations between the two species. Furthermore, significant (p < 0.001) variations were detected for quantitative phenotypic traits such as leaf length, days to the appearance of first male and female flowers, male and female flower count per single plant, plant height, branches per plant, fruit number per single plant, fruit dimensions (i.e., length and diameter), single fruit yield per plant, rind thickness, total soluble solutes, seed length, number of seeds per fruit and seed yield per fruit. The findings suggest that, compared with sweet watermelon, citron watermelon has a relatively early flowering ability, and possesses higher proportion of male and female flowers, a higher total number of branches, taller plants, a higher proportion of fruits per single plant, thicker fruit rind, low total soluble solutes, a high seed count and yield per single fruit. The quantitative and qualitative trait variation present in citron watermelon makes it a valuable genetic stock to transfer novel genes into sweet watermelon. Current and future watermelon genetic improvement programs should integrate fruit quality and yield promoting traits, local adaptation, postharvest and other consumer and industrially essential food and feed attributes for multiple utilities. The novel and contrasting traits can be introgressed through conventional breeding methods or genetic and genomic techniques such as genomics-assisted breeding, including genome editing.