genes, ms-1, ms-2 and ms-3, have been reported, with ms-1 and ms-3 present in C. maxima and ms-2 in C. pepo. Sterile flowers conditioned by s-1 and s-2 have been observed in C. maxima and C. pepo, respectively. Cr and I act in concert to modify flower colour with Cr-I- conditioning intense yellow, Cr-ii pale yellow, crcrI- cream and crcrii white in colour. These genes were discovered in a C. moschata × C. okeechobeensis cross.
Fruit colour of different squash cultivars is quite diverse, and a number of fruit pigment genes have been investigated. The incompletely dominant allele of the B gene, (termed precocious yellow) which was found in an ornamental gourd, has been used to breed C. pepo cultivars with golden fruit colour and the B gene usually elevates the carotenoid content of mesocarp tissue. This allele affects many other horticultural traits, such as reduced storability and increased sensitivity to chilling injury. The fruit colour expression of the B allele is influenced by pigmentation extender genes Ep-1 and Ep-2, suppressor gene Ses-B and other modifying genes. Fruits have a yellow and green bicolour appearance when B is heterozygous. This trait has been transferred to C. moschata but naturally occurring precocious yellow types occur within this species, and that gene is also incompletely dominant. Yellow to orange fruit colour of C. maxima cultivars is controlled by the Bmax gene and modifiers. Unlike B in C. pepo and C. moschata, Bmax is completely dominant. Two genes of C. pepo, l-1 and l-2, govern light pigmentation of fruit. Allele l-1St (striped fruit) is recessive to allele 1-1+ but dominant to l-1. The dominant gene D confers dark stem colour, especially the peduncle, versus light green stems in plants homozygous for d/d, and is epistatic to l-1 and l-2. The allele Ds darkens stems but not the fruit rind. Two genes, Wf for white flesh and W for weak pigmentation, together with l-1 and l-2, govern white rind colour in C. pepo. The W gene is also epistatic to the effects of D on rind colour of fruit but not stem pigmentation. The Y allele confers development of yellow fruit shortly after anthesis in C. pepo. Green fruit colour in C. moschata is governed by gene Gr. The Mldg gene of this species determines whether immature fruits are mottled light and dark green or coloured uniformly dark green. In C. maxima, the bl allele produces blue-grey fruit skin colour in combination with genes for green skin, but pink fruit in plants carrying the Bmax allele.
The use of molecular markers for linkage map development in squash is catching up with other cucurbit crops. Early maps were based on interspecific combinations but as marker systems and genome coverage has improved, more recent maps have been based on biparental intraspecific crosses. One of the first linkage maps for Cucurbita that used markers other than morphological ones was based on the cross of C. maxima × C. ecuadorensis, where five linkage groups of isozyme genes were proposed by Weeden and Robinson (1986). An interspecific backcross population of C. pepo × C. moschata was used to map 148 RAPD markers in 28 linkage groups. Loci for three qualitative genes were mapped along with two QTL (Brown and Myers, 2002). Zraidi et al. (2007) developed a consensus map from two C. pepo recombinant inbred populations using a set of RAPD, AFLP and SCAR markers. Eight QTL associated with seed coat lignification were also identified through bulked segregant analysis. Gong et al. (2008) created separate intraspecific maps for C. pepo and C. moschata using SSR markers. The use of microsatellite markers allowed the comparison of linkage maps of the two species to determine levels of macrosynteny. The first intraspecific C. maxima map was created by Ge et al. (2015) using a combination of SSR, AFLP and RAPD markers. A more complete interspecific C. maxima × C. ecuadorensis using RAPDs was developed (Singh et al., 2011). Genotype by sequencing was applied to two species to develop high-density maps based on SNPs. Zhang et al. (2015) created an SNP map for C. maxima and used QTL analysis to identify a candidate gene for vine length. For C. pepo, the genome was sequenced and an SNP-based linkage map was generated in a summer squash background (Montero-Pau et al., 2017).
Watermelon
Two genes of watermelon have been reported to govern anthracnose resistance, Ar-1 for race 1 and Ar-2-1 for race 2 resistance. Alleles db and Fo-1 provide resistance to gummy stem blight and race 1 of Fusarium wilt, respectively. However, gummy stem blight resistance appears to be due to more than just a single gene. Susceptibility to powdery mildew is governed by pm. Resistance to race 2 powdery mildew from PI 270545 is controlled by at least two genes. Resistance to the watermelon strain of papaya ringspot virus (PRSV-W) is controlled by a single recessive gene, prv. A moderate level of resistance to zucchini yellow mosaic virus was conferred by a single recessive gene zym-FL. A high level of resistance to the Florida strain of zucchini yellow mosaic virus was controlled by a single recessive gene, zym-FL-2; not the same as zym-FL. Resistance to the China strain of zucchini yellow mosaic virus was controlled by a single recessive gene zym-CH. A single dominant allele, Zym, confers resistance to zucchini yellow mosaic virus. Insect-resistance genes for watermelon include Af (red pumpkin beetle resistance) and Fwr (fruit fly resistance).
Watermelon plants with short vines can be bred if they are homozygous recessive at the dw-2 locus or for one of the two alleles for dwarf plant habit at the dw-1 locus. Both genes make a super dwarf that has been used to develop cultivars for patio and container production. Branching at the lower nodes of the main stem is reduced by allele bl.
Andromonoecy is recessive to monoecy and conditioned by gene a. Two alleles are known that produce male sterility, ms and gms, the latter associated with glabrous foliage.
When plants are homozygous recessive for the e (explosive rind) allele, the fruit is tender (not tough rind, but can be thin or thick rind), bursting when cut. The f gene determines furrowed fruit surface. The incompletely dominant allele O governs elongate versus spherical fruit shape. The su allele suppresses fruit bitterness.
Dark green skin colour (G-1 and G-2) is dominant and other alleles at the g-1 locus determine the degree of colour and striping: G (medium or dark solid green); gW (wide stripe); gM (medium stripe); gN (narrow stripe); and g (solid light green or grey). The dominance series is G > gW > gM > gN > g. Greenish mottling of the exocarp is produced by the m allele, and pencilled lines by p. Golden mature fruit colour and chlorosis of older leaves is governed by gene go.
A gene with a dominant allele for white flesh (Wf) is epistatic to a gene for yellow flesh; the double recessive is red-fleshed. Allele C produces canary yellow flesh colour. The darkness of red colour in the flesh (and the amount of lycopene) is controlled by multiple alleles at the y locus. Scarlet red flesh (YScr) is dominant to coral red flesh (YCrl), orange flesh (yO) and salmon yellow flesh (y). The dominance series is YScr > YCrl > yO > y. The allele YScr is from ‘Dixielee’ and ‘Red-N-Sweet’; the allele YCrl is from ‘Angeleno’ (black-seeded); the allele yO is from ‘Tendersweet Orange Flesh’; and the allele y is from ‘Golden Honey’.
The interaction of alleles at several genes, including d (dotted seed coat), r (red), t (tan) and w (white seed coat), determine seed coat colour and pattern. Clump is RR TT ww; tan is RR tt WW; white is
