George Acquaah - Principles of Plant Genetics and Breeding

For genetic analysisGeneticists make planned crosses to study the inheritance and genetic behavior of traits of interest.

Artificial hybridizationis 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 6.1Crossing 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.

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.

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).

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 flowers.

Plants growing in the greenhouse should be provided the proper intensity and duration of light. If the species is photoperiod sensitive, the lighting should be adjusted accordingly. Proper temperature is required for proper plant growth and development. In some species, a special temperature treatment (vernalization, usually some period of low temperature) is required for flower induction. Furthermore, temperature affects pollen shed in flowers. Consequently, extreme temperatures may cause inadequate amounts of pollen to be shed for successful artificial pollination. Pollen quantity and quality are influenced by the relative humidity of the growing environment. Extreme moisture conditions should be avoided.

In artificial pollination, the breeder should be familiar with the species to know its flowering habits regarding time from planting to flowering, duration of flowering, mechanisms and timing of natural anther dehiscence and fertilization, and time of peak pollen production, in order to take advantage of the window of opportunity of anthesis (pollen shed) for best crossing outcomes. To ensure that parents in a crossing program will have flowers at the same time, the practice of staggered planting is recommended: to plant sets of parents at different times. This way, a late‐planted early flowering genotype may be pollinated by an early‐planted late flowering genotype. When depending on natural pollination, interspersed planting on different dates will favor even pollen distribution.

Photoperiod may be manipulated in photoperiod‐sensitive species to delay or advance flowering as appropriate, in order to synchronize flowering of the parents in a cross. Other techniques that have been used in specific cases include manipulation of temperature and planting density, removal of older flowers to induce new flushes of flowers, and pinching (e.g. removal of plant apex to induce tillering or branching for additional flowers). In corn, the silk of an early flowering inbred parent may be cut back to delay the time to readiness for pollination.

After selecting lines to be parents in a cross, it is necessary in artificial crosses to designate one parent as female (as previously stated), as well as identify which type of flowers on the parent would be most desirable to cross. In crossing programs in which the CMS system is being used, it is critical to know which plants to use as females (these would be the male sterile genotypes, or A and B lines; see Chapter 19). Because the pollen or male gamete is practically without cytoplasm, and because certain genes occur in the extranuclear genome (such as CMS), it is critical that parents selected as female plants be selected judiciously.