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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, but are lacking in other dioecious cucurbits (Roy and Saran, 1990). In ivy gourd (Coccinia grandis), plants with two X chromosomes are female and XY plants are male. The Y chromosome is larger and more heterochromatic than the X chromosome and is dominant in sex determination. Polyploid plants with three X and only one Y chromosome are androecious. In some dioecious species (e.g. Coccinia), C-values have shown that the Y chromosome can increase genome content up to 10% (Sousa et al., 2016).

Cucurbita is believed to be an ancient tetraploid genus derived from an ancestor with a base chromosome number of 12 (Wu et al., 2017). Isozymic and sequencing evidence have implied a polyploid nature of this genus (Weeden, 1984). That polyploidy occurred long ago is indicated by all species of the genus having 20 pairs of chromosomes and by disomic 3:1 gene ratios in segregating generations. Recent genomic sequencing efforts provide evidence of genome duplication shortly after Cucurbita diverged from other cucurbit species ~30 million years ago (Montero-Pau et al., 2017).

Aside from Cucurbita and possibly other genera of the tribe Cucurbiteae, polyploidy does not seem to have played an important role in the evolution of cucurbit tribes or genera. However, isolated cases of polyploid species are known in some genera with mostly diploid species (e.g. Cucumis and Trichosanthes) and polyploid cytotypes occasionally arise in melon and a few other species (e.g. kaksa).

Autotetraploids have been induced in cucumber, melon, squash, watermelon, luffa and bottle gourd by treatment with colchicine. Other compounds such as Surflan (oryzalin) herbicide can be used to produce tetraploids. Tetraploids have not been used directly in horticulture so far, except in the production of seedless triploid watermelons.

Within-species triploids have been created in cucumber, melon, squash and watermelon by crossing tetraploids with diploids. Triploids are highly sterile, whereas tetraploids are more fertile than triploids but less fertile than diploids. The sterility of triploids, due to embryo abortion, has been used to produce seedless watermelon (see Chapter 4 for more details). Although the use of triploidy in breeding other cucurbits has been investigated, it has not been adopted in cultivar development. Naturally occurring triploids have been found in pointed gourd (Trichosanthes dioica) and ivy gourd (C. grandis).

Haploids, which are highly sterile, occasionally occur spontaneously in cucumber and melon. In cucumber, they can be recognized by their reduced seed weight. Haploids can also be produced by interspecific hybridization, or pollination with irradiated pollen followed by embryo culture. Haploids treated with colchicine produce homozygous diploid lines more quickly than inbred lines can be developed by self-pollination. Doubled haploids are used extensively now to produce inbred lines quickly following a cross. These enable plant breeders to develop cultivars faster than before. Doubled haploids, or dihaploids, are considered one of the tools of speed breeding.

Polysomaty, in which the chromosome number of some somatic cells of a plant are multiples of the typical chromosome number for that plant, has been detected in melon, squash and other cucurbits. It is believed that triploid plants of pointed gourd were propagated vegetatively from triploid shoots on diploid plants (Singh, 1990).

In squash, sterile interspecific F₁ hybrids have been treated with colchicine to produce amphidiploids (allotetraploids). Self-fertile amphidiploid lines with the parentage of Cucurbita moschata × C. maxima have been produced that segregated for some horticulturally favourable characteristics. However, amphidiploids have not been important in the development of new cultivars.

Triploid interspecific hybrids of Cucurbita have been produced, some combining the genomes of three different species. Interspecific triploids have also been backcrossed with one of the diploid parents to create fertile interspecific trisomics. Interspecific trisomics of C. moschata and C. palmata, combining 20 chromosomes of the former and one of the latter, were synthesized and used to relate genes to specific chromosomes (Graham and Bemis, 1990). Recently, the use of trisomics to map genes on to chromosomes has been replaced with the use of molecular markers and genome sequencing.