germplasm bank, and unless breeders attempt to utilize these materials in future breeding efforts valuable traits present in this germplasm may never be exploited.
BREEDING PROCEDURES AND PRACTICES
Breeding procedures − hybridization
Most breeding programs follow a similar procedure, with major variations to the procedure (described below) occurring during the selection process in later generations of inbreeding. The process starts with the need to introduce new traits through manual cross pollination. Most breeders in North America make crosses in the greenhouse during the winter season while breeders in the tropics make crosses in the field. Beans are manually cross pollinated in the bud stage before the flowers open (Figure 2.4). Once a flower is fully open, it has already been self‐pollinated. The flower structure consists of a central female style that ends in a stigma, which is coiled and surrounded by a filament that consists of 10 stamens, which suspend the anthers that contain the pollen. Detailed drawings of dissected bean flowers are illustrated in Freytag and Debouck (2002). In the past, breeders would open the immature bud and physically remove a portion of the filament with all 10 anthers. Since the stigma is receptive before the pollen is released from the anthers, the breeder then transfers pollen from male parent to that receptive stigma. Pollen is transferred by breaking off the pollinated stigma (carries excess pollen) from the male donor and hooking that excised stigma with the receptive stigma of the female parent. The flower bud is closed and tagged to indicate the pollen donor. This entire process severely damages the flower, resulting in a low success rate (<50%) with few hybrid seeds produced per pod. Forage legume flowers such as clovers and alfalfa have a natural tripping mechanism to facilitate outcrossing by insects. Beans have a similar mechanism, which breeders manipulate to trip flower buds by depressing downward on the base of two petals to force the stigma out of the filament and away from the immature anthers. At this point, pollen using excised stigma from male parent can be transferred to the tripped stigma with minimum damage resulting in a significantly higher success rate (>90%), with more hybrid seeds in the pod. Since developing hybrid pods are less competitive in development to normally self‐ pollinated pods, many of the latter are removed from the female parent by breeders to favor the production of F1 hybrid seed. Breeders mostly work with populations developed from a single cross of two parents, but in some instances, crosses between F1 hybrids are used to produce a four‐way cross as a means of introducing more traits into the progeny, from four parents rather than two.
Fig. 2.4. Open bean flower is already self‐pollinated. The immature bud in the background is used to make a cross pollination with pollen from another open flower. (For color detail, please see color plate section.)
Source: Original image by author, J.D. Kelly.
Once the tagged pods mature, the hybrid seed is harvested as F1 generation seed. The inbreeding process is initiated at this stage with self‐pollination occurring about 10 times to the F10 generation before the final new inbred variety is released to farmers. Inbreeding procedures vary as different breeding methods are used to fix traits that are inherited differently. Some highly heritable traits such as seed color are under single gene control and are referred to as qualitative traits. Other traits such are yield are controlled by many genes that are influenced by environmental conditions and are under more complex quantitative genetic control. The methods used to fix these different traits vary and will be discussed in more detail in later sections under specific traits. Bean breeders rarely practice selection in the F1 generation, as there is no segregation in typical biparental crosses. In those instances where four‐way crosses are made, selection may be initiated for single dominant gene traits segregating in that cross, assuming breeders choose to deploy a system of gamete selection (Singh 1994). Aside from this one variation in methodology, breeders allow the F1 generation plants to self‐pollinate to produce the next‐generation F2 seed. Breeders will check the F1 plants to ensure that they result from a cross and are not the result of an accidental self‐pollination. Seed production at this step is conducted in greenhouses or other favorable environments to maximize quantities of disease‐free seed. Most programs will bulk F1 hybrid seed coming from different crosses (pods) between the same parents once they confirm they are a genuine cross. The F2 is the generation where most breeders will initiate field selection as segregation is first observed in this generation.
BREEDING METHODS
For a complete description of plant breeding methods, the reader is referred to text by Fehr (1987). The pedigree method is the most widely used breeding system in beans because it provides opportunities to select for a broad array of traits with different genetic inheritance patterns – selection for qualitative traits in the early F2–F4 generations and selection in later F5–F7 generations for complex quantitative traits such as yield and quality. Early generation selection starts with selection based on single plants in the F2 and is usually continued to the F4 generation when plants are around 88% homozygous. At this stage, selection changes to a plant‐row or family basis and is continued over the next few generations with the use of increased replication to measure expression of complex traits such as yield and canning quality that require testing and confirmation over locations and years. Traits commonly selected in early generations are largely phenological such as flowering and maturity times, agronomic such as plant growth habit and plant architecture including plant height and resistance to lodging, disease resistance, and seed size, shape, and color. Although many of these traits are controlled by more than single genes, they are highly heritable and can easily be visually selected in the field and fixed genetically, producing essentially pure breeding lines. To speed up the process, breeders use greenhouses in the winter season to artificially screen for major gene disease resistances that may be difficult to evaluate in the field. Other programs use winter nurseries overseas to advance populations/lines one or two generations and in some instances select for the expression of valuable agronomic traits, such as tolerance to abiotic and biotic stresses that occur naturally in these foreign locations.
Selection for yield and quality traits require that testing be conducted in the targeted production area, so continued later generation selection is usually restricted to those regions where the variety is intended to be grown. Extensive annual yield testing may be conducted by agronomists in addition to the breeder to determine adaptation within the production region. When these advanced F7 lines begin to show potential as future varieties they are referred to as elite lines and made available to colleagues in other states where that seed type is grown. Information generated in other production areas may be used to support the release of new varieties. Fellow breeders are also interested in evaluating these elite lines not only for use in future local bean production but to use as parents in their breeding programs as these elite lines represent a valuable genetic resource. Most new varieties are the product of crosses between elite lines/varieties so future success is built on past success.
The final steps in the release of future new varieties are orchestrated by the breeder who collects and assembles data on the elite lines for agronomic, disease resistance, quality, and performance traits. A minimum of three to four years of yield testing over 20–30 location‐years is needed to support the release of a new variety. In addition to agronomic and disease resistance data, a battery of information on visual canning quality, texture, processed color is also needed. The complied information is reviewed by a committee(s) of breeders, agronomists, pathologists, food scientists, extensionists, and industry personnel to determine if the new elite line has
