Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 27

Plant breeding has come a long way from the cynical view of “crossing the best with best and hoping for the best” to carefully planned and thought‐out strategies to develop high performance cultivars. Plant breeding methods and tools keep changing as technology advances. Consequently, plant breeding approaches may be categorized into two general types: conventional and unconventional. This categorization is only for convenience.

 Conventional approachConventional breeding is also referred to as traditional or classical breeding. This approach entails the use of tried, proven, and older tools. Crossing two plants (hybridization) is the primary technique for creating variability in flowering species. Various breeding methods are then used to discriminate among the variability (selection) to identify the most desirable recombinant. The selected genotype is increased and evaluated for performance before release to producers. Plant traits controlled by many genes (quantitative traits) are more difficult to breed. Age notwithstanding, the conventional approach remains the workhorse of the plant breeding industry. It is readily accessible to the average breeder and is relatively easy to conduct, compared to the unconventional approach.

 Unconventional approachThe unconventional approach to breeding entails the use of cutting‐edge technologies for creating new variability that is sometimes impossible to achieve with conventional methods. However, this approach is more involved, requiring special technical skills and knowledge. It is also expensive to conduct. The advent of recombinant DNA (rDNA) technology gave breeders a new set of powerful tools for genetic analysis and manipulation. Gene transfer can now be made across natural biological barriers, circumventing the sexual process (e.g. the Bt products that consist of bacterial genes transferred into crops to confer resistance to the European corn borer). Molecular markers are available for aiding the selection process to make the process more efficient and effective.

Even though two basic breeding approaches have been described, it should be pointed out that they are best considered as complementary rather than independent approaches. Usually, the molecular tools are used to generate variability for selection, or to facilitate the selection process. After genetically modifying plants using molecular tools, it may be used as a parent in subsequent crosses to transfer the desirable genes into adapted and commercially desirable genetic backgrounds, using conventional tools. Whether developed by conventional or molecular approaches, the genotypes are evaluated in the field by conventional methods, and then advanced through the standard seed certification process before the farmer can have access to it for planting a crop. The unconventional approach to breeding tends to receive more attention from funding agencies than the conventional approach, partly because of its novelty and advertised potential, as well as the glamour of the technologies involved.

Regardless of the approach, a breeder follows certain general steps in conducting a breeding project. A breeder should have a comprehensive plan for a breeding project that addresses the following:

 ObjectivesThe breeder should first define a clear objective (or set of objectives) for initiating the breeding program. In selecting breeding objectives, breeders need to consider:The producer (grower) from the point of view of growing the cultivar profitably (e.g. need for high yield, disease resistance, early maturity, lodging resistance).The processor (industrial user) as it relates to efficiently and economically using the cultivar as raw materials for producing new product (e.g. canning qualities, fiber strength).The consumer (household user) preference (e.g. taste, high nutritional quality, shelf life).The tomato will be used to show how different breeding objectives can be formulated for a single crop. Tomato is a very popular fruit with a wide array of uses, each calling for certain qualities. For salads, tomato is used whole, and hence the small size is preferred; for hamburgers, tomato is sliced, round large fruits being preferred. Tomato for canning (e.g. puree) requires certain pulp qualities. Being a popular garden species, gardeners prefer a tomato cultivar that ripens over time so harvesting can be spaced. However, for industrial use as in the case of canning, the fruits on the commercial cultivar must ripen together, so the field can be mechanically harvested. Further, whereas appearance of the fruit is not top priority for a processor who will be making tomato juice, the appearance of fruits is critical in marketing the fruit for table use.

 GermplasmIt is impossible to improve plants or develop new cultivars without genetic variability. Once the objectives have been determined, the breeder then assembles the germplasm to be used to initiate the breeding program. Sometimes, new variability is created through crossing of selected parents, inducing mutations, or using biotechnological techniques. Whether used as such or recombined through crossing, the base population used to initiate a breeding program must of necessity include the gene(s) of interest. That is, you cannot breed for disease resistance if the gene conferring resistance to the disease of interest does not occur in the base population.

 SelectionAfter creating or assembling variability, the next task is to discriminate among the variability to identify and select individuals with the desirable genotype to advance and increase to develop potential new cultivars. This calls for using standard selection or breeding methods suitable for the species and the breeding objective(s).

 EvaluationEven though breeders follow basic steps in their work, the product reaches the consumer only after it has been evaluated. Agronomists may participate in this stage of plant breeding. In a way, evaluation is also a selection process, for it entails comparing a set of superior candidate genotypes to select one for release as a cultivar. The potential cultivars are evaluated in the field, sometimes at different locations and over several years, to identify the most promising one for release as a commercial cultivar.

 Certification and cultivar releaseBefore a cultivar is released, it is processed through a series of steps, called the seed certification process, to increase the experimental seed, and to obtain approval for release from the designated crop certifying agency in the state or country. These steps in plant breeding are discussed in detail in this book.