Genetic relatedness and phenotype are important factors that govern the expression of heterosis in hybrid progeny of many cross-pollinating plant species. Since this relationship is important but not well understood in melon (Cucumis melo L.), one monoecious and two andromonoecious melon lines of diverse Chinese [Peoples Republic of China (PRC)] origin were crossed to the andromonoecious U.S. Western Shipping market type 'Top Mark' (TM) and the andromonoecious, highly branched line H-16 to determine parental combining ability and heterosis for five yield component traits in three test environments [open-field (USA), and energy-saving (PRC) and plastic greenhouses (PRC)]. Random amplified polymorphic DNA (RAPD)-and simple sequence repeat (SSR)-based genetic distances (GD) among and between parents (5) and their hybrids (6) were calculated and compared to phenotypic trait values. These germplasms were evaluated for lateral branch number (LBN), days to 50% flower (DF), fruit number and weight per plant, and fruit length:diameter (L:D) ratio in each of three test environments. General combining ability was significant for all characters, except for L:D in all locations, and LBN and DF in the plastic greenhouse environment. Both descriptors of difference (genetic marker and phenotype) were discriminatory, and provided similar assessments of relationships among parents and hybrids. Although dramatic performance differences were detected between parents and among F 1 hybrid progeny, a strong relationship between GD and heterotic effects was not consistently detected.

Genetic diversity among 95 watermelon (Citrullus lanatus) ecotypes was evaluated and compared with representative Chinese, American, Japanese, and Russian watermelon cultigens, landraces, and a wild watermelon relative (Trichosanthes kirilowii). Open-pollinated, hybrid, and inbred lines were included for most of the ecotypes and are hereafter collectively referred to as cultigens unless an ecotype group is being discussed. Morphological characteristics (including days to flower, female to male flower ratio, branch number, fruit length and diameter ratio, fruit soluble solid content, fruit yield, and simple sequence repeat (SSR) markers were used to estimate genetic diversity. Of 398 watermelon primer pairs tested, 9.5% (38) produced polymerase chain reaction amplicons in watermelon. Of these 38 primer pairs, the average number of polymorphic bands among the 96 cultigens was 2.4, even with 12 primer pairs demonstrating monomorphic banding patterns. Based on the SSR data, the genetic similarity coefficients were calculated and a dendrogram constructed. All cultigens were clustered to six groups. The wild species and landraces formed distant clusters from the cultivated watermelon. The genetic similarity coefficients within the Chinese cultigens ranged from 0.37 to 0.99, but except for a wild relative to watermelon, most cultigens were closely related. The genetic distance among non-Chinese cultigens ranged from 0.67 to 0.91 with an average of 0.88. When combined morphological traits and molecular traits were assessed, Russian and U.S. fruit were more genetically similar to each other than to Chinese and Japanese cultigens. Crossing Russian and/or U.S. cultigens with Chinese or Japanese cultigens should thus improve genetic diversity and introduce new traits for the resulting watermelon cultigens.