Cucurbita pepo, C. moschata, and C. maxima are the most economically important three (out of five) cultivated species within the Cucurbita genus that include squashes, pumpkins, and gourds, which represent several species in the same crop (Blanca et al., 2011; Robinson, 1995). These species are remarkably diverse in morphology, disease resistance, and environmental adaptability (Loy, 2004; Saade and Hernandez, 1994; Whitaker and Bemis, 1964). For a long time, breeders have attempted to use variability in the genus for crop improvement through interspecific breeding yet overcoming crossing barriers, the male sterility, and incompatibility of the interspecific F1 and early succeeding generations of distant crosses has been a major challenge for Cucurbit breeders (Chekalina, 1974; Hiroshi, 1963; Rhodes, 1959; Shifriss, 1987; Wall, 1961). Based on the species crossability, Whitaker and Davis (1962) concluded that C. moschata occupies a central position among the annual species and can be crossed with difficulty with C. maxima, C. pepo, and C. mixta. Fertile seeds from a series of interspecific crosses were successfully obtained in the past few decades (Baggett, 1979; Castetter, 1930; Erwin and Haber, 1929; Kanda, 1984; Shifriss, 1987; Wall, 1961). While making the crosses, fruit set is generally quite low for many crosses and the occasional fruit produced may have few seed or none (Baggett, 1979; Cheng et al., 2002; Robinson, 1999). To obtain fruits and fertile seeds from the F1 plants of interspecific crosses, additional techniques like repeated pollination, bud pollination, mixed pollen pollination, embryo culture and /or amphidiploidy, and the adjustment of florescence and environmental conditions are frequently used (Bemis, 1973; Cheng et al., 2002; Hiroshi, 1963; Shifriss, 1987).
To overcome species barriers, a wild species (for example, C. argyrosperma) with a wide cross compatibility have been used as a genetic bridge to transfer genes between other less-compatible cultivated species (McCandless, 1998; Wessel-Beaver et al., 2004) or used to create genetic bridge lines by crossing with an interspecific F1 (Chetelat and DeVerna, 1991; Finkers et al., 2007). A sterile F1 from two distant species can be retrieved by embryo and ovule culture and directly used as a bridge line for gene transfer (Pico et al., 2000; Poysa, 1990; Wang et al., 2002) or subsequently chromosome doubled to produce a fertile amphidiploid. This amphidiploid or the derivatives therefrom offer a possible genetic bridge between the incompatible species (Chen et al., 2011; Parisi et al., 2001; Staub, 2002). However, although a wild species, interspecific F1, amphidiploidy, or induced polyploidy as a genetic bridge plays an important role in overcoming species barriers and the male sterility of interspecific F1 for gene transfer, none of these genetic bridges can solve the male sterile, incompatible, and infertile problems in the later generations (Stebbins, 1956; Wang et al., 2002). Moreover, during the transfer of important characteristics with the bridges, unfavorably species-specific traits are frequently carried along to subsequent populations from initially interspecific hybridization (Whitaker and Robinson, 1986). Nevertheless, the disadvantages may be removed by intervarietal hybridization and selection (Munoz et al., 2004; Singh et al., 2009; Stebbins, 1956).
The objective of this study is to develop interspecific inbred lines with normal compatibility by varietal recombination among the three species and successive selection through different mating and selection methods. Meanwhile, some important traits such as plant habits, fruit types, multiple disease resistance, and heat and cold tolerance are integrated into the lines for the purpose of developing new Cucurbit types or varieties. To realize the objective, the removal of the male sterility and sexual incompatibility of interspecific F1 and subsequent generations was determined as a main task in this study.
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