Читать книгу Cucurbits - James R. Myers - Страница 35

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).