Principles of Plant Genetics and Breeding. George Acquaah
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the sexual process (i.e. without crossing). More significantly, gene transfer can transcend natural reproductive or genetic barriers. Transfers can occur between unrelated plants and even between plants and animals (by genetic transformation, see Chapter 24).
6.2 Applications of crossing in plant breeding
Sometimes, crossing is done for specific purposes, within the general framework of generating variability. Hybridization precedes certain methods of selection in plant breeding to generate general variability.
Gene transferSometimes, only a specific gene (or a few) needs to be incorporated into an adapted cultivar. Crossing is used for the gene transfer process, followed by additional strategic crossing to retrieve the desirable genes of the adapted cultivar (see backcrossing in Chapter 15).
RecombinationGenetically diverse parents may be crossed in order to recombine their desirable traits. The goal of recombination, which is a key basis of plant breeding, is to forge desirable linkage blocks.
Break undesirable linkagesWhereas forging desirable linkage blocks is a primary goal of plant breeding, sometimes crossing is applied to provide opportunities for undesirable linkages to be broken.
For heterosisHybrid vigor (heterosis) is the basis of hybrid seed development. Specially developed parents are crossed in a predetermined fashion to capitalize on the phenomenon of heterosis for cultivar development.
For maintenance of parental linesIn hybrid seed development programs, crossing is needed to maintain special parents used in the breeding program (e.g. cytoplasmic‐genetic male sterility [CMS] lines, maintainer lines).
For maintenance of diversity in a gene poolPlant breeders may use a strategy of introgression (crossing and backcrossing selected entries with desired traits into adapted stocks) and incorporation to develop dynamic gene pools from which they can draw materials for crop improvement.
For evaluation of parental linesInbred lines for hybrid seed development are evaluated by conducting planned crosses to estimate combining abilities, in order to select appropriate parents for used in hybrid seed development.
For genetic analysisGeneticists make planned crosses to study the inheritance and genetic behavior of traits of interest.
6.3 Artificial hybridization
Artificial hybridization is the deliberate crossing of selected parents (controlled pollination) (Figure 6.1). There are specific methods for crossing that depend on the species in which the cross is being made, which differ according to factors including floral morphology, floral biology, possible genetic barriers, and environmental factors. Methods for selected species are described later in this book. However, there are certain basic factors to consider in preparation for hybridization:
Parents should belong to the same or closely related plant species. If they belong to different (related) plant species, all kinds of techniques may be required to obtain hybrid progeny.
The parents, obviously, should together supply the critical genes needed to accomplish the breeding objective.
One parent is usually designated as female. Whereas some breeding methods may not require this designation, breeders usually select one parent to be a female and the other a male (pollen source). This is especially so when hybridizing self‐pollinated species. Whenever genetic markers are available (e.g. white flowers, white seeds), the female exhibits the recessive morphological trait. In some cases, selected parents of cross‐pollinated species may be isolated and allowed to randomly cross‐pollinate each other.
The female parent usually needs some special preparation. In complete flowers (having both male and female organs), the flowers of the parent selected to be female is prepared for hybridization by removing the anthers, a tedious procedure called emasculation (discussed next). Emasculation is eliminated in some crossing programs by taking advantage of male sterility (renders pollen sterile) when it occurs in the species.
Pollen is often physically or manually transferred. Artificial hybridization often includes artificial pollination, whereby the breeder physically deposits pollen from the male parent unto the female stigma. However, when hybridization is conducted on large scale (e.g. commercial hybrid seed development), hand pollination is rarely a feasible option.
Figure 6.1 Crossing in yam. The use of magnifying glasses helps with the crossing process.
Source: Photo is courtesy of Asrat Asefaw, International Institute of Tropical Agriculture, Ibadan, Nigeria.
6.4 Artificial pollination control techniques
As previously indicated, crossing is a major procedure employed in the transfer of genes from one parent to another in the breeding of sexual species. A critical aspect of crossing is pollination control to ensure that only the desired pollen is involved in the cross. In hybrid seed production, success depends on the presence of an efficient, reliable, practical, and economic pollination control system for large‐scale pollination. Pollination control may be accomplished in three general ways:
1 Mechanical controlThis approach entails manually removing anthers from bisexual flowers to prevent pollination, a technique called emasculation, removing one sexual part (e.g. detasseling in corn), or excluding unwanted pollen by covering the female part. These methods are time consuming, expensive, and tedious, limiting the number of plants that can be crossed. It should be mentioned that in crops such as corn, mechanical detasseling is widely used in the industry to produce hybrid seed.
2 Chemical controlA variety of chemicals called chemical hybridizing agents (or by other names, e.g. male gametocides, male sterilants, pollenocides, androcides) are used to temporally induce male sterility in some species. Examples of such chemicals include Dalapon®, Estrone®, Ethephon®, Hybrex®, and Generis®. The application of these agents induces male sterility in plants, thereby enforcing cross pollination. The effectiveness is variable among products.
3 Genetical controlCertain genes are known to impose constraints on sexual biology by incapacitating the sexual organ (as in male sterility), or inhibiting the union of normal gametes (as in self‐incompatibility). These genetic mechanisms are discussed further in Chapter 5.
6.5 Flower and flowering issues in hybridization
The flower has a central role in hybridization. The success of a crossing program depends on the condition of the flower regarding its overall health, readiness or receptiveness to pollination, maturity, and other factors. The actual technique of crossing depends on floral biology (time of pollen shedding, complete or incomplete flower, self‐ or cross‐pollinated, size and shape of individual flowers and of the inflorescence).
6.5.1 Flower health and induction
It is important that plants in a crossing block (or to be crossed) be in excellent health and be properly developed. This is especially so when flowers are to be manually emasculated. Once successfully crossed, an adequate amount of seed should be obtained for planting the first generation. The parents to be mated should receive proper lighting, moisture supply, temperature, nutrition, and protection from pests. Parents should be fertilized with the proper amounts of nitrogen, phosphorus, and potassium for vigorous plant growth to develop an adequate number of healthy