Cucurbits. James R. Myers
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* 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.
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