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.
Genetic knowledge of cucurbits is behind that of maize, tomato and pea, despite the considerable natural genetic variation in many species of the Cucurbitaceae. The use of winter greenhouses, trellises and cages has reduced the space and labour requirements for crossing cucurbits for genetic studies. The establishment of the Cucurbit Genetics Cooperative, along with the publication of the Cucurbit Genetics Cooperative Report annually since 1978, has fostered communication among cucurbit researchers and stimulated more cytogenetic investigations.
Dominance relationships of genes in melon were first investigated by Sagaret in the mid-19th century. Mendelian inheritance in cucumber was reported in 1913, and many genes for each of the major cucurbit crops have since been identified. In 1976, a total of 170 individual genes were known for the Cucurbitaceae. Of these, 68 were for cucumber, 37 for melon, 30 for squash species, 25 for watermelon and ten for other genera. Numerous other genetic factors were known for cultivated cucurbits and used in breeding programmes, but were not included in the gene list because their inheritance was complex or unknown.
Many additional cucurbit genes and alleles have been identified in the intervening decades and, with the application of genomics to the various cucurbit species, the pace is accelerating. The Cucurbit Genetics Cooperative publishes gene lists for the major cucurbit crops and the earlier gene lists included 146 loci for cucumber (Wehner, 1993), 100 for melon (Pitrat, 1994a) and 81 for watermelon (Rhodes and Zhang, 1995). As of 2014, the number of genes published was 509, with 167 for cucumber, 160 for melon, 93 for squash species, 62 for watermelon and 27 for other genera. Thus, the number of known genes for the cucurbit crops has tripled in the past 40 years. Some of those genes code for isozymes. In addition, multiple alleles have been identified, e.g. YScr > YCrl > yO > y and G > gW > gM > gN > g loci in watermelon. It should also be mentioned that identification of a gene does not necessarily indicate specific knowledge about the location, structure, or sequence of that gene. In most cases, many of these genes do not yet have molecular markers associated with them.
Regarding gene nomenclature, it should be pointed out that most genes (though not those of enzymes) are recognized and named according to the discovery of an atypical expression of that gene, which is itself caused by a newly found allele at a previously unrecognized locus. The gene and atypical, or ‘mutant’, allele are given the same designation (e.g. gl is an allelic form of gene gl). The first letter of the symbol is in lower case if the atypical allele is recessive and uppercase if dominant. The designation for the normal allele of that gene is given as +, the gene name with a superscripted +, or the gene name with the first letter in the case (lower or uppercase) opposite that for the atypical allele. Continuing the above example for the glabrous (gl) gene, the common or normal allele can be designated as +, gl+ or Gl. In this volume, we use the superscript system for referring to common alleles.
When additional alleles are found and assigned to a gene, they are designated with different superscripts appended to the gene name. However, allelism testing runs far behind the discovery of genetic anomalies. When several alleles affect the same heritable trait, they are often treated as belonging to different genes until proven
