Cucurbits. James R. Myers
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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 otherwise, although allelism tests are recommended before proposing another locus. This may be a problem for many alleles relating to disease resistance in cucurbits. More testing is needed to assign various alleles to their proper gene locus and to determine gene linkages.
Cucumber
Many alleles conferring disease resistance have been found and incorporated into cucumber cultivars. Dominant alleles have been reported for resistance to bacterial wilt (Bw), scab (Ccu), target leaf spot (Cca), Fusarium wilt (Foc) and watermelon mosaic virus (Wmv and wmv-1-1). Linkage of scab resistance with Fusarium wilt resistance has resulted in many cultivars having both, even though they were only selected for scab resistance. A dominant allele (Cmv) at a single locus has also been postulated for resistance to cucumber mosaic virus, but alleles of additional genes are needed for a high level of resistance. Recessive alleles provide resistance to angular leaf spot (psl), zucchini yellow fleck virus (zyf) and zucchini yellow mosaic virus (zymv). Alleles Ar and cla reportedly confer resistance to different races of anthracnose. Resistance to papaya ringspot virus is provided by prsv or Prsv-2. Several genes have been proposed to govern resistance to downy mildew and powdery mildew, with the main ones being the closely linked dm and pm.
The bitterfree loci (bi and bi-2) inhibit biosynthesis of cucurbitacin, which is an attractant for cucumber beetles but a repellant for spider mites, aphids and various other insects. The bi genes are epistatic to the bitter genes (Bt and Bt-2) for increased cucurbitacin content.
Salinity tolerance is influenced by many genes, in addition to a single major gene (sa). A single gene (Sd) has also been reported to control resistance to sulfur dioxide air pollution.
Dwarf plant habit, due to short internodes, is produced by alleles bu, by, cp, cp-2 and dw. Although these alleles are currently assigned to distinct loci, allelism testing is needed to confirm that none represent allelic variants of the same gene. Allele dw also retards the development of oversized, and hence unmarketable, fruit. Vine size is also reduced by the determinate habit allele (de), which is modified by the intensifier gene In-de.
The interaction of two major genes, m and F, influences sex expression. F is modified by In-F and alleles at other genes. Some cultivars heterozygous for F have only female flowers, due to the right combination of modifying genes, but others can be monoecious under some growing conditions. Cultivars homozygous for F have been developed because they are more dependably gynoecious than heterozygous cultivars. The use of these genetic backgrounds in the production of gynoecious hybrid seed is described in Chapter 7. Additional genes have been reported to influence androecious (a), andromonoecious (m-2) and gynoecious (gy) sex expression.
Multipistillate alleles mp and Mp-2 and their modifiers at other loci increase the number of pistillate flowers per node. The gene for twin fused fruit (tf) results in two fruit at one node fusing into a single unit. The development of parthenocarpic fruit is governed by Pc and modifying genes.
Fruit spine colour is governed by the putative genes B, B-2, B-3 and B-4, with black spines being dominant to white. Spine number and size is influenced by genes ns, ss, s-1, s-2 and s-3. Spines and warts are absent on fruit of plants with the gl allele for glabrous foliage and more pronounced when possessing the tuberculate fruit allele, Tu.
European glasshouse cultivars have glossy fruit with a tender skin and uniform dark green colour, without light green stippling. These are monogenic traits, governed by the D gene for dull versus glossy fruit and the tender skin (te) and uniform colour (u) genes.
Green immature fruit colour is dominant to white (w) and yellow green (yg). The interaction of alleles at two genes, wf and yf, reportedly determines white versus yellow or orange flesh colour.
Using qualitative genetics, cis linkage groups were proposed for cucumber genes (Pierce and Wehner, 1990) based on recombination frequencies. Using molecular genetics, three accessions of cucumber (‘Chinese Long’, Gy 14, and PI 183967) have been sequenced and the data made available on the Cucurbit Genomics Database. Chromosome assignments of the linkage groups have been made, and comparison with other species for shared synteny has shown how the cucumber chromosomes evolved (Huang et al., 2009).
Melon
Many loci governing disease resistance of melon have been investigated. A single gene, Ac, governs Alternaria leaf blight resistance in melon line MR-1. Resistance to races 0 and 2 of Fusarium wilt is provided by Fom-1 or Fom-3, and there is an allele of another gene (Fom-2) for resistance to races 0 and 1. Allele Mc confers a high level of resistance to gummy stem blight, and Mc-2 a moderate degree of resistance to that disease. Five independent dominant genes, Gsb-1 through Gsb-5, have also been reported to confer high levels of gummy stem blight resistance, from PIs 140471, 157082, 511890, 482398 and 482399, respectively. Eight loci have been postulated to influence resistance to the powdery mildew disease caused by Podosphaera xanthii and three more to the powdery mildew incited by Erysiphe cichoracearum. Four genes have been reported for downy mildew resistance. Two alleles (Prv1 and Prv2) of a single gene provide resistance to papaya ringspot virus from PI 180280, but they differ in reaction to some strains of that virus. Prv2 is recessive to Prv1 but dominant to Prv+. Resistance to pathotype O of zucchini yellow mosaic virus is provided by dominant allele Zym from PI 424723. A single dominant gene, Wmr, confers partial resistance to watermelon mosaic virus. Although polygenic resistance to cucumber mosaic virus has been known, recently a single dominant gene, Creb-2, has been reported to confer resistance to CMV-B2 strain. Two complementary recessives genes, cab-1 and cab-2, confer resistance to cucurbit aphid-borne yellows virus (CABYV). Resistance to cucurbit yellow stunting disorder virus (CYSDV) has been attributed to a single dominant gene (Cys) from TGR-1551. Subsequent studies have suggested that a single recessive gene may provide stronger resistance. Another dominant gene, Liy from PI 313970, confers resistance to the related Crinivirus, lettuce infectious yellows virus. The same PI has