Читать книгу Cucurbits - James R. Myers - Страница 31

Progress in cucurbit breeding has been improved through the use of multivariate statistical analyses (e.g. to increase selection efficiency), research on gene mapping and linkage, the induction of mutations with chemicals and gamma radiation, marker assisted selection, the use of wider crosses within species and gene transfer among species, and the collection and screening of germplasm in genebanks for desirable characteristics.

Although genetic engineering and backcrossing usually focus on single alleles, more attention is being given to multiple gene selections for single or multiple characteristics. For example, in Saudi Arabia, the melon cultivars ‘Najd I’ and ‘Najd II’ were bred for tolerance of both high temperature and salinity. In some cases where multiple insect resistance has been found, it is possible to breed for resistance to several insects from a single cross. ‘Butternut’ squash (Cucurbita moschata), for example, is resistant to squash vine borer, cucumber beetles, pickleworm, melonworm and leafminer. Squash bugs prefer Cucurbita pepo and C. maxima, but if these species are not present, then they will feed on C. moschata. Sources of combined insect and disease resistance are also known for cucumber and melon. Often, quantitative resistance to a disease provides a lower level of resistance, but resistance to multiple isolates of the pathogen. Genetically modified cultivars of squash for virus resistance have been combined with conventionally bred resistance to powdery mildew as well as with fusarium wilt in melon (Quemada and Groff, 1995).

Selection for many morphological characteristics is standard practice in plant breeding. Progeny are visually inspected for the desired character and further breeding proceeds according to objectives. The study of Mendelian genetics in families can help reveal relationships (e.g. dominant, co-dominant, recessive, complementary) among alleles at multiple loci.

Although biochemical qualities can be evaluated with the help of laboratory equipment and analysis (e.g. using a refractometer to select for high sugar content in fruit), linkage or pleiotropy for morphological characters has also been used to breed for these traits. For example, selection for fruit flesh carotenes was initially based on fruit colour, i.e. orange-fleshed fruit were presumed to have higher concentrations of these organic compounds. However, subsequent research in squash (C. pepo) revealed that fruit colour is controlled by the complex interaction of alleles at several genes, and although total carotenoid content is affected by these genes, there are additional genes influencing the percentage of carotenes in the total carotenoid content (Paris, 1994). Also, the major allele (B) for high carotenoid content has various pleiotropic, and not always favourable, effects on fruit and foliage. Direct selection for high carotene content has proved to be more effective. The B gene is not a factor in C. maxima and C. moschata, and selection for high carotenoid content and sometimes specifically for carotenes has been more successful in these species.

Gene linkage, and also pleiotropy, has been a problem in breeding monoecious melons. The use of monoecious melons in F₁ hybrid seed production is desirable because emasculation of the female parent is easier with the pistillate flowers of monoecious inbred lines than with the perfect flowers of andromonoecious inbred lines. However, selection for monoecy has been limited by its genetic association with elongated fruit shape. Elongated shape is dominant and F₁ hybrids with a monoecious parent generally have undesirably long fruit. Other genes can influence the shape of melons with the monoecious gene, and H.M. Munger of Cornell University was successful in breeding monoecious selections with nearly-round fruit.

Many objectives in cucurbit breeding involve the improvement of traits having complex inheritance. Earliness, yield, adaptation to certain environmental conditions and fruit quality are quantitative traits. Consequently, large populations, efficient experiment designs and multivariate analyses are useful for evaluation of breeding material for future selections. Recurrent selection and pedigree selection have been used to improve cucurbits for quantitative traits. Backcrossing has been used especially for improvement of sex expression and disease resistance (Munger et al., 1993).

Studies with cucumber, melon, watermelon and squash (Whitaker and Davis, 1962; Wehner, 1999) indicate that, in general, there is little or no inbreeding depression, but there is some heterosis for certain morphological traits. Increasingly, wider crosses are being employed to produce F₁ populations with more favourable characteristics. The mechanics of F₁ seed production are described in Chapter 7. In any case, hybrids are useful for the following reasons.

1. Hybrids permit the protection of the parental inbreds by trade-secret laws, although it is still possible for the female line to appear occasionally in the hybrid population. Companies may seek additional protection with intellectual property rights (utility patent, plant variety protection, breeder’s rights).

2. Hybrids can be quickly produced that have interesting combinations of the parental traits, such as intermediate fruit length from a long-fruited crossed with a short-fruited inbred.

3. Hybrids combine the dominant alleles from the female parent with the dominant alleles from the male parent to produce a cultivar with all of the alleles expressed together.

4. Hybrids can combine cytoplasmic traits such as chilling tolerance from the female inbred with dominant alleles such as chilling tolerance from the male inbred into a more tolerant progeny.

5. Seedless hybrids can be produced from certain interspecific combinations or, in the case of watermelon, seedless triploid hybrids can be produced by crossing tetraploid and diploid parents.