Inter- and Intracultivar Variation of Heirloom and Open-pollinated Watermelon Cultivars

Abstract

Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] cultivars exhibit diverse phenotypic traits, yet are derived from a narrow genetic base. Heirloom cultivars, and to a lesser extent modern open-pollinated (OP) cultivars, are perceived to contain vital genetic variation that is critical for biodiversity conservation and crop improvement. The objective of this study was to characterize the diversity of six heirloom and open-pollinated watermelon cultivars that are popular among U.S. organic, direct-market, and home gardeners. An additional evaluation was conducted to determine whether significant phenotypic and genotypic variation existed among seed lots sourced from different commercial seed vendors. Important horticultural traits such as days to germination, days to first flower, yield, and fruit quality were measured over two field seasons. Genetic diversity was estimated using 32 simple sequence repeat (SSR) markers. Significant differences in horticultural traits among seed lots in both years were observed only in days to germination and first male flower, which may be a consequence of vendor differences in seed storage and quality control. Heirloom ‘Moon and Stars’ and modern OP ‘Sugar Baby’ were the most genetically distinct from the other cultivars and heirloom ‘Georgia Rattlesnake’ was determined to be highly related to the modern OP ‘Charleston Gray’. The two heirloom cultivars were observed to have lower average gene diversity than the modern cultivars. Heirloom ‘Moon and Stars’ contained significant genetic variation among seed lots, yet heirloom ‘Georgia Rattlesnake’ contained none. These findings suggest that genetic variation can be more accurately attributed to pedigree and foundation seed maintenance practices than to the “heirloom” designation per se. The variation reported in this study can be used to inform conservation and breeding efforts.