The fruit of the cultivated watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus] vary in weight from 1 kg to over 100 kg depending on cultivar and environmental conditions. In the United States, commercial fruit are usually classified into five categories: icebox (less than 5.5 kg), small or pee wee (5.5–8.0 kg), medium (8.1–11.0 kg), large (11.1–14.5 kg), and giant (greater than 14.5 kg) (Maynard, 2001). One of the smallest-fruited watermelon cultivars is ‘New Hampshire Midget’, released by the University of New Hampshire in 1951. This cultivar has fruit with oval shape, thin rind, gray skin, red flesh, and black seeds and produces very early fruit of icebox size (Wehner, 2002). Fruit of smaller size (less than 0.5 kg) can be found in wild relatives of the cultivated watermelon such as C. colocynthis, which are typically used in Africa as animal forage.
In recent years, consumers in the United States have been increasingly interested in seedless watermelons weighing 7 to 10 kg. In 2003, a new fruit size category was introduced under the name of mini watermelon. Cultivars produce fruit that are round, have a thin rind, and weigh between 1.5 and 4.0 kg. Leading cultivars among those currently available are ‘Petite Perfection’, ‘Precious Petite’ (Syngenta Seeds–Rogers Brand, Boise, ID), and cultivars of the Bambino trademark (Seminis Vegetable Seeds, St. Louis) (Schultheis et al., 2005). Although mini watermelons occupy a small portion of the market, their introduction and appreciation by consumers has increased the interest of watermelon breeders in developing cultivars with reduced fruit size.
Fruit weight in watermelon production is an important descriptor of fruit type, although it can also be considered a yield component. Yield is defined as the total weight per unit area. In the United States, growers expect to harvest at least “one load of fruit” per acre of land, corresponding to 50.5 Mg·ha−1 (metric ton·ha−1) of marketable fruit (Maynard, 2001). Marketable fruit must be free of defects and fall into the weight classes most desired by consumers. Currently, smaller sizes are preferred over the traditional large watermelon as a dessert for parties and picnics.
The genetics of watermelon have been studied widely, and many genes have been described (Cucurbit Gene List Committee, 1979, 1982; Henderson, 1991, 1992; Rhodes and Dane, 1999). However, single genes or quantitative trait loci have not been identified for watermelon fruit weight. In two preliminary studies on the inheritance of fruit weight, significant additive, dominance, and epistatic effects were reported, dominance and dominance-by-dominance being the largest gene effects (Brar and Nandpuri, 1974; Sharma and Choudhury, 1988).
Several methods of estimating heritability and predicting selection response are available. Primarily, these methods partition the total variance into genetic and environmental variances, and the genetic variance into additive and dominance components and interallelic interaction effects, whenever the population structure and composition allows (Holland et al., 2003; Nyquist, 1991). Among others, a design based on the measure of variance from six generations (Pa, Pb, F1, F2, BC1Pa, and BC1Pb) can be used to estimate environmental, genetic, and additive variances. The variance of the F2 provides an estimate of phenotypic variance, whereas the mean variance of the nonsegregating generations (Pa, Pb, and F1) gives an estimate of environmental effects (Wright, 1968). The additive variance is derived by subtracting the variances of the backcrosses from twice the phenotypic (F2) variance as an extension of the single locus model under the hypothesis of absence of linkage and genetic-by-environment interactions (Warner, 1952). The broad- and narrow-sense heritability and the predicted gain from selection can then be calculated from the available estimates of genetic, additive, and phenotypic variances.
The objective of this experiment was to estimate the heritability and genetic variances of fruit weight in watermelon using a set of crosses of giant by icebox-type inbreds using measures of variances of six generations (Pa, Pb, F1, F2, BC1Pa, and BC1Pb) for each cross. The information would be of great help to watermelon breeders in choosing appropriate breeding methods aimed at modifying fruit weight of their cultivars. A confirmation of the quantitative nature of this trait would indicate the need for techniques such as recurrent selection in improvement of populations specifically dedicated to a weight class of interest.
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