Cucurbits. James R. Myers
cultivars such as ‘Tigress’ and ‘Jaguar’.
Of all the Cucurbita species, C. moschata and C. argyrosperma are the most closely related and hybridization may occur in nature, especially where these two species are sympatric in Central America (Wessel-Beaver, 2000). The interspecific cross is easy to make, although only with C. moschata as the male parent (Wessel-Beaver et al., 2004). F1 hybrids with commercial potential are under development in Mexico (Ortiz-Alamillo et al., 2007).
Wild Cucurbita species are being used to develop disease-resistant squash. In disease studies, C. ecuadorensis and C. foetidissima (buffalo gourd) were found to be resistant to a greater number of viruses than other species of Cucurbita tested (Provvidenti, 1990). Buffalo gourd is difficult to use in squash breeding because of its distant relationship and incompatibility with the cultivated species (Fig. 3.3). However, virus resistance alleles have been successfully introgressed from C. ecuadorensis to C. maxima. Multiple virus-resistant germplasm derived from C. maxima × C. ecuadorensis was developed at Cornell University, USA, and provided to breeders in 1985. ‘Redlands Trailblazer’, a winter squash resistant to ZYMV, watermelon mosaic virus (WMV) and papaya ringspot virus (PRSV), was bred in Australia from the same interspecific cross. Cultivars of C. pepo vary considerably in their compatibility with C. ecuadorensis (Robinson and Shail, 1987). Distant interspecific crosses of Cucurbita can be made more successful if embryo culture is used.
Resistances to cucumber mosaic virus (CMV) and powdery mildew have been transferred from Cucurbita okeechobeensis to C. pepo and C. moschata. The cross C. okeechobeensis × C. pepo is difficult to make, even with embryo culture, and sterility is a serious problem with the interspecific hybrid. These difficulties are overcome by the use of a bridging species, for example crossing C. pepo to the F1 of C. okeechobeensis × C. moschata (Whitaker and Robinson, 1986).
Because undesirable traits (e.g. bitterness) of a wild parent are often dominant, unacceptable horticultural types predominate in the F2 generation of these interspecific crosses. However, only a single backcross to ‘Butternut’ (C. moschata) of the F1 of ‘Butternut’ × C. okeechobeensis ssp. martinezii was needed to produce disease-resistant (powdery mildew and CMV) plants with fairly good fruit characteristics. In general, the number of backcrosses to the domesticated parent that are needed depends on the number of genes that are different between the two parents. Once acceptable progeny are produced, self-pollination is performed to obtain uniformity.
Wild species of Cucurbita could probably be used further in squash breeding programmes to provide resistance to other pathogens and insects. Other desirable traits, such as the drought tolerance and gynoecious sex expression of C. foetidissima, could be transferred as well.
Chromosome numbers have been determined for most of the important cultivated species of the Cucurbitaceae and a number of other cucurbits since the 1990s (Table 3.1). The most common haploid numbers are 11 and 12, with 12 considered to be the ancestral karyotype number for the family. Squash species have 20 pairs of chromosomes, more than any other cultivated species in the family, and cucumber has the fewest, with only seven pairs. The hypothesis that cucumber evolved by fusion of the chromosomes of melon was rejected in the 1980s, but later was proved to be true through genome sequencing (Huang et al., 2009). Studies of synteny show that the 11 chromosomes of watermelon became the 12 chromosomes of melon, and finally became the seven chromosomes of cucumber (Huang et al., 2009).
Table 3.1. Chromosome number for some species of the Cucurbitaceae.
| Species | Haploid number of chromosomes | Genome size (Mb) |
| Benincasa hispida | 12 | 800 |
| Citrullus lanatus | 11 | 425 |
| Coccinia grandis | 12 | 719/904* |
| Cucumis anguria | 12 | – |
| Cucumis dipsaceus | 12 | – |
| Cucumis melo | 12 | 454 |
| Cucumis metuliferus | 12 | – |
| Cucumis sativus | 7 | 367 |
| Cucurbita species (e.g. C. pepo) | 20 | 283 |
| Cyclanthera explodens | 16 | – |
| Cyclanthera pedata | 16 | – |
| Lagenaria siceraria | 11 | 313 |
| Luffa species (e.g. L. cylindrica) | 13 | 790 |
| Momordica balsamina | 11 | – |
| Momordica charantia | 11 | 339 |
| Momordica cochinchinensis | 14 | – |
| Momordica dioica | 14 | – |
| Praecitrullus fistulosus | 12 | – |
| Sechium edule | 12 | – |
| Trichosanthes cucumerina |
|