Читать книгу Cucurbits - James R. Myers - Страница 37

Most descriptions of genes with links to the primary literature can be found in Paris and Padley (2014). In the most recent update, a number of new disease resistance genes have been added for squash. Powdery mildew resistance of C. okeechobeensis (Small) L. H. Bailey and C. lundelliana L. H. Bailey is controlled by dominant alleles of the Pm-0 and Pm loci, respectively, and modifier genes may influence expression of these alleles. Two recessive resistance genes (pm-1 and pm-2) have been characterized in C. moschata. Three dominant complementary genes (Crr-1, Crr-2 and Crr-3) for resistance to crown rot (caused by Phytophthora capsici) were introgressed from C. lundelliana and C. okeechobeensis ssp. okeechobeensis into C. moschata. Six genes plus modifiers originating from C. moschata have been reported to affect resistance to zucchini yellow mosaic virus. For four loci, the resistant allele is dominant, whereas for the other two loci, alleles are recessive. Zym-0 and zym-6 appear to act independently, whereas Zym-1, Zym-2 and Zym-3, and Zym-4 and zym-5 form complementary sets. A modifier of zym-6 (m-zym-6) alters recessive expression to dominant expression. Recessive resistance (zymecu) has also been found in C. ecuadorensis to ZYMV. Watermelon mosaic virus resistance has been described from two different species with Wmv from C. moschata and Wmvecu identified in a C. maxima × C. ecuadorensis interspecific cross. A pair of complementary genes (prv-1 and Prv-2) identified in C. moschata confer resistance to papaya ringspot virus. Two genes (dominant Slc-1 and recessive slc-2) independently confer resistance to squash leaf curl virus in C. pepo. A single gene for cucumber mosaic virus resistance (Cmv) was identified in C. moschata. In some cases, such as for Zym-1 and Wmv, resistance to different viruses may be conferred by genes at the same locus, or genes that are tightly linked in a complex locus. Homology among resistance genes to the same pathogen from different species has yet to be resolved.

Resistance to the melon fruit fly is provided by allele Fr in C. maxima. C. pepo plants homozygous for cu, which reduces foliar cucurbitacin content, are not preferred by cucumber beetles since these insects are attracted by cucurbitacins. Resistance to silver leaf disorder caused by whitefly (Bemisia tabaci) is found in both C. pepo and C. moschata with resistance conferred by recessive sl.

A number of genes are associated with domestication traits in Cucurbita. Bitterness caused by cucurbitacin is under the control of several genes. In addition to cu which conditions bitter cotyledons, Bi-0 conditions bitter fruit in C. pepo. Three bitter fruit loci (Bi-1, Bi-2 and Bi-3) were found in an interspecific cross of C. pepo × C. argyrosperma. Bi-0 and either Bi-1 or Bi-2 may be allelic. In C. maxima and C. maxima × C. ecuadorensis crosses, Bimax conditions bitterness. Wild Cucurbita species and some summer squash cultivars have a hard fruit rind. Rind hardness is influenced by the Hr (hard rind) gene of C. pepo and Hi (hard rind inhibitor) of C. maxima. In the presence of Hr, Wt confers wartiness in C. pepo. Wartiness is also found in C. maxima but has not been characterized genetically. Bush habit due to short internodes is conditioned by allele Bu in C. pepo; expression of Bu is dominant in early stages of plant development and recessive as plants vine out. Bush habit of C. moschata is also under monogenic control, whereas two to three incompletely dominant genes have been implicated in different bush phenotypes of C. maxima. There is extreme variation in bush growth habit among species, but only one gene has been definitively described and additional ones may await discovery. While most genes affecting fruit size and shape are quantitative, two qualitative genes have been described that affect shape. Disc fruit shape is governed by the Di gene of C. pepo. Bn conditions butternut fruit shape and is dominant to crookneck fruit shape in C. moschata. The characteristic post-cooking stringiness of the flesh of ‘Vegetable Spaghetti’ (C. pepo) is reportedly governed by a single gene, fl. A recessive allele of a major gene (n) and alleles at modifying genes for naked seed reduce deposition of lignin and cellulose in the schlerenchyma and subepidermal tissue of the seed coat in C. pepo. Resistance in C. moschata to the herbicide trifluralin is governed by gene T, which is modified by an inhibitor gene, I-T.

Several genes affect flower colour, morphology and sex expression. In C. moschata, de conditions determinate plants that terminate in pistillate flowers. A single gene (G) governing gynoecious sex expression occurs in C. foetidissima, but the dominant allele has not been transferred to the cultivated squash species. All staminate flowers are conditioned by recessive a in C. pepo while ae increases the expression of staminate flowers in the same species. Cucurbita inflorescences are typically solitary, but plants that are homozygous recessive for mf in C. pepo will produce multiple flowers in an inflorescence. Three male-sterile 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-1^St (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).