Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus; 2n = 2x = 22] is an economically important, cross-pollinated vegetable crop that is grown throughout the world. Watermelon is grown over 3.5 million ha worldwide with production of 104 million Mg (Food and Agriculture Organization, 2012). The United States is the fourth largest producer after China, Iran, and Turkey (Kumar and Wehner, 2011a). Total area has decreased from 76,000 ha in 1998 to 56,000 ha in 2011 (U.S. Department of Agriculture, 2012). However, production has increased from 1.9 million Mg in 1998 to 2 million Mg in 2011. Over 80% of production is concentrated in the southern United States where temperatures are high: Arizona, California, North Carolina, Florida, Texas, and Georgia.
Watermelon has been bred to improved fruit yield, fruit quality, disease resistance, seedlessness, short vine length, and adaptation to production areas around the world. The first genetic studies on watermelon were published in the 1930s and 1940s and involved pure-line cultivars developed in the previous few decades. Those studies focused on traits such as rind pattern, flesh color, seedcoat color, fruit shape, fruit size, and sex expression (Poole, 1944; Poole and Grimball, 1945; Poole et al., 1941; Porter, 1933, 1937; Weetman, 1937).
Yield varies among watermelon accessions, old cultivars, and modern elite cultivars (Gusmini and Wehner, 2005). Growers are currently getting ≈50 Mg·ha−1 of marketable yield (Maynard, 2001). Many have studied the inheritance of qualitative genes in watermelon (Cucurbit Gene List Committee, 1979, 1982, 1987; Guner and Wehner, 2004; Henderson, 1991; Rhodes and Dane, 1999; Rhodes and Zhang, 1995). However, there are few quantitative genetic studies, especially for important traits such as fruit yield and size. Fruit yield was reported to be correlated with component traits such as fruit count and fruit size (Kumar and Wehner, 2011b). Heterosis for watermelon fruit yield and its component traits has been reported (Brar and Sidhu, 1977; Brar and Sukhija, 1977; Chhonkar, 1977; Sidhu and Brar, 1978; Thakur and Nandpuri, 1974). However, fewer studies have examined the inheritance of fruit yield and its component traits in watermelon (Gusmini and Wehner, 2007; Kumar and Wehner, 2011a).
Gusmini and Wehner (2005) screened a diverse set of 80 watermelon cultivars for fruit yield, fruit count, and fruit size and reported a large amount of genetic variation. Yield ranged from 114.2 Mg·ha−1 in ‘Mountain Hoosier’ to 36.4 Mg·ha−1 in ‘Minilee’. The highest yielders were the inbreds ‘Legacy’, ‘Mountain Hoosier’, ‘Hopi Red Flesh’, ‘Early Arizona’, ‘Stone Mountain’, ‘AU-Jubilant’, ‘Sweetheart’, ‘Calhoun Gray’, ‘Big Crimson’, ‘Moon & Stars’, ‘Cole Early’, ‘Yellow Crimson’, and ‘Blacklee’ and the F1 hybrids ‘Starbrite’ and ‘Stars-N-Stripes’. These high yielders included cultivars producing an intermediate number of fruit of medium size (9 to 12 kg/fruit), except ‘Early Arizona’, ‘Stone Mountain’, ‘Sweetheart’, and ‘Cole Early’, which had small (6 to 9 kg/fruit) fruit. ‘Sweet Princess’, ‘Calsweet’, and ‘Minilee’ were the lowest yielders.
To improve complex (quantitative) traits like yield, understanding variances and heritability behaviors of yield and its components is paramount. Genetic variance and heritability can be estimated using parent–offspring regression (Holland et al., 2003; Kumar and Wehner, 2011b; Nyquist, 1991), North Carolina Design I, NC Design II (Comstock and Robinson, 1948), and North Carolina Design III (Comstock and Robinson, 1952). Kumar and Wehner (2011b) used parent–offspring regression to measure heritability of yield in watermelon. However, populations that were used to calculate heritability estimates in their study were developed by half-diallel using diverse set of old and new cultivars. Those estimates were low (0.02 to 0.09) and were applicable to those populations. That study indicated that genetic gain will be small and replicated progeny rows were required to select for yield improvement. Among other methods, a design based on the measure of variance from six generations (PaS1, PbS1, F1, F2, BC1Pa, and BC1Pb) can be used to estimate environmental, genetic, additive, dominance, and phenotypic variances and heritability in biparental populations (Lyimo et al., 2011; Zalapa et al., 2006). To improve yield by pedigree selection, biparental populations can be developed by crossing high- with low-yielding cultivars. If the heritability estimates are high for yield, individual plant selection may be practiced in early generations to make genetic gain. If heritability estimates are low, selection for yield should be based on replicated plot trials at multiple locations in more advanced generations.
Genetic information related to yield improvement in watermelon is limited. The present study was designed to determine genetic variance and inheritance of fruit yield, fruit count, and fruit size from the cross of high-yielding ‘Mountain Hoosier’ with low-yielding ‘Minilee’.
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