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 | 11 | – |
| Trichosanthes dioica | 11 | – |
* indicates that estimated genome size for female/male plants
Chromosome morphology of cucurbits is difficult to study, because the chromosomes are small and not easily differentiated from the cytoplasm by cytological procedures. Fortunately, this can now be done using molecular marker technology. The cytology and phylogenetic relationships of many cucurbit crop species was studied in the 1980s and 1990s (Singh, 1990). Variation was observed for length of chromosomes, position of centromere, occurrence of satellites and karyotypes for cucumber, melon, Momordica species, Luffa cylindrica and Lagenaria siceraria (Ramachandran and Seshadri, 1986; Bharathi et al., 2011; Waminal and Kim, 2012). Few karyotyping studies have been conducted in Cucurbita. Waminal et al. (2011) used fluorescence in situ hybridization (FISH) based on the 5S and 45S rDNA to karyotype C. moschata, but similar studies have not been conducted for other Cucurbita spp.
DNA content using flow cytometry (C-value) distinguishes cucurbit species and sexes within species (Achigan-Dako, 2008). In bottle gourd species, inter- and intra-variation has been used to differentiate seed type and genome size (Achigan-Dako et al., 2008). Heteromorphic sex chromosomes have been identified in dioecious species of Coccinia and Trichosanthes,
