Cucurbits. James R. Myers
href="#litres_trial_promo">1994). Also, the major allele (B) for high carotenoid content has various pleiotropic, and not always favourable, effects on fruit and foliage. Direct selection for high carotene content has proved to be more effective. The B gene is not a factor in C. maxima and C. moschata, and selection for high carotenoid content and sometimes specifically for carotenes has been more successful in these species.
Gene linkage, and also pleiotropy, has been a problem in breeding monoecious melons. The use of monoecious melons in F1 hybrid seed production is desirable because emasculation of the female parent is easier with the pistillate flowers of monoecious inbred lines than with the perfect flowers of andromonoecious inbred lines. However, selection for monoecy has been limited by its genetic association with elongated fruit shape. Elongated shape is dominant and F1 hybrids with a monoecious parent generally have undesirably long fruit. Other genes can influence the shape of melons with the monoecious gene, and H.M. Munger of Cornell University was successful in breeding monoecious selections with nearly-round fruit.
Many objectives in cucurbit breeding involve the improvement of traits having complex inheritance. Earliness, yield, adaptation to certain environmental conditions and fruit quality are quantitative traits. Consequently, large populations, efficient experiment designs and multivariate analyses are useful for evaluation of breeding material for future selections. Recurrent selection and pedigree selection have been used to improve cucurbits for quantitative traits. Backcrossing has been used especially for improvement of sex expression and disease resistance (Munger et al., 1993).
Studies with cucumber, melon, watermelon and squash (Whitaker and Davis, 1962; Wehner, 1999) indicate that, in general, there is little or no inbreeding depression, but there is some heterosis for certain morphological traits. Increasingly, wider crosses are being employed to produce F1 populations with more favourable characteristics. The mechanics of F1 seed production are described in Chapter 7. In any case, hybrids are useful for the following reasons.
1. Hybrids permit the protection of the parental inbreds by trade-secret laws, although it is still possible for the female line to appear occasionally in the hybrid population. Companies may seek additional protection with intellectual property rights (utility patent, plant variety protection, breeder’s rights).
2. Hybrids can be quickly produced that have interesting combinations of the parental traits, such as intermediate fruit length from a long-fruited crossed with a short-fruited inbred.
3. Hybrids combine the dominant alleles from the female parent with the dominant alleles from the male parent to produce a cultivar with all of the alleles expressed together.
4. Hybrids can combine cytoplasmic traits such as chilling tolerance from the female inbred with dominant alleles such as chilling tolerance from the male inbred into a more tolerant progeny.
5. Seedless hybrids can be produced from certain interspecific combinations or, in the case of watermelon, seedless triploid hybrids can be produced by crossing tetraploid and diploid parents.
Interspecific hybridization
Augustin Sagaret and Charles Naudin tried to cross melon with cucumber in the mid-19th century, but without success. All later investigators trying to cross these distantly related species have also failed, but interspecific hybrids have been produced for other species of Cucumis (Fig. 3.2), as well as for Cucurbita (Fig. 3.3), Citrullus, Luffa, Momordica, Trichosanthes and other cucurbit genera. Cucurbita is an example of where interspecific gene transfer has been utilized successfully for crop improvement. However, Cucumis sativus has been improved using genes from C. sativus var. hardwickii. Also, C. sativus was crossed with wild Cucumis hystrix and the chromosomes doubled to create a new allotetraploid, C. hytivus. Citrullus lanatus has been improved using Citrullus mucosospermus, C. amarus and C. colocynthis.
Fig. 3.2. Crossability polygon of Cucumis species. Arrows point in the direction of the female parent. Moderately to strongly self-fertile and cross-fertile hybrids (thick solid line); sparingly self-fertile and moderately cross-fertile hybrids (thin solid line); self-sterile, usually not cross-fertile hybrids (dash-and-dot line); inviable seeds or seedlings (dashed line). Absence of a line indicates that seeded fruit were not obtained. (Nijs and Visser, 1985. Reprinted courtesy of Kluwer Academic Publishers.)
Fig. 3.3. Crossability polygon of Cucurbita species. ‘Digitata Complex’ includes C. digitata, C. palmata, C. cylindrata and C. cordata, which are considered subspecies of C. digitata by some scientists. All crossing combinations have been tried in at least one direction, except for C. pedatifolia with C. maxima, and C. radicans with C. pedatifolia, C. ficifolia, C. ecuadorensis, C. okeechobeensis, and C. digitata sensu lato. Early works describing hybridization attempts with C. radicans were in error as described in Merrick (1990); the material was misidentified. Other published sources, as well as the unpublished work of Tom Andres (New York, 1996, personal communication), were used to create this diagram. Solid lines indicate an F1 hybrid that is at least partially fertile; dashed lines indicate a viable but sterile F1 plant.
Although the production of interspecific hybrids is only the first step in a rather long process, F1 hybrids of Cucurbita maxima × C. moschata are used directly to produce elite cultivars. Both parental species are monoecious, having many more male than female flowers, but the interspecific hybrid is gynoecious or predominantly gynoecious in sex expression. The interspecific hybrid is usually productive if conditions for pollination are favourable (e.g. a monoecious cultivar is grown nearby to provide pollen, and bees are working the field). The unusual case of a gynoecious hybrid being produced by crossing two monoecious species also occurs in the cross of C. pepo × C. ecuadorensis.
The cross C. maxima × C. moschata may be difficult or easy to make depending on parental combinations (Castetter, 1930; Yongan et al., 2002; Karaağaç and Balkaya, 2013). For difficult crosses, many pollinations may be needed to set each fruit, and only a few seeds per fruit are produced. Breeders in Japan have been able to make this cross so successfully that they market interspecific F1 seed commercially. They have found C. maxima and C. moschata parents that cross well and are more compatible than most members of these species. ‘Tetsakabuto’, the first popular interspecific hybrid squash cultivar, is a cross of ‘Delicious’ (C. maxima) × ‘Kurokawa No. 2’ (C. moschata). Seed production for the interspecific hybrid is most prolific when C. maxima is the maternal parent.
The cross C. maxima × C. pepo is difficult but not impossible to make; the F1 is highly sterile. C. pepo and C. moschata can be crossed, but compatibility is influenced greatly by the choice of parents, with some cultivars crossing more readily than others. Wall and York (1960) reported that the cross was more easily made when one of the parents was an F1 hybrid, which increased gametic diversity. The bush gene of C. pepo has been introgressed into C. moschata to incorporate compact plant habit into that species. Zucchini yellow mosaic virus (ZYMV) resistance, derived from ‘Nigerian Local’ (C. moschata), has been moved