George Acquaah - Principles of Plant Genetics and Breeding

With every advance in generation, the heterozygosity in the segregating population decreases by 50%. The chance of finding a plant that combines all the desirable alleles decreases as the generations advance, making it practically impossible to find such a plant in advanced generations. Some calculations by J. Sneep will help clarify this point. Assuming 21 independent gene pairs in wheat, he calculated that the chance of having a plant with all desirable alleles (either homozygous or heterozygous) are 1 in 421 in the F 2, 1 in 49 343 in the F 3, and 1 in 176 778 in the F 4, and so on. However, to be certain of finding such a plant, he recommended that the breeder grow four times as many plants.

Another genetic consequence of hybridization is the issue of linkage drag. As previously noted, genes that occur in the same chromosome constitute a linkage block. However, the phenomenon of crossing over provides an opportunity for linked genes to be separated and not inherited together. Sometimes, a number of genes are so tightly linked they are resistant to the effect of recombination. Gene transfer by hybridization is subject to the phenomenon of linkage drag, the unplanned transfer of other genes associated with those targeted. If a desired gene is strongly linked with other undesirable genes, a cross to transfer the desired gene will invariably be accompanied by the linked undesirable genes.

A breeding program starts with an initial population that is obtained from previous programs, and existing variable populations (e.g. landraces), or is created through a planned cross. Hybridization may be used to generate a wide variety of populations in plant breeding, ranging from the very basic two‐parent cross (single cross) to very complex populations in which hundreds of parents could be involved. Single crosses are the most widely used in breeding. Commercial hybrids are mostly produced by single crosses. Complex crosses are important in breeding programs where the goal is population improvement. Hybridization may be used to introgress new alleles from wild relatives into breeding lines. Because the initial population is critical to the success of the breeding program, it cannot be emphasized enough that it be generated with much planning and thoughtfulness.

Various mating designs and arrangements are used by breeders and geneticists to generate plant populations. These designs require some type of cross to be made. Factors that affect the choice of a mating design include: (i) the predominant type of pollination (self‐ or cross‐pollinated); (ii) type of crossing used (artificial or natural); (iii) type of pollen dissemination (wind or insect); (iv) presence of male sterility system; (v) purpose of the project (for breeding or genetic studies); and (vi) size of the population required. In addition, the breeder should be familiar with how to analyze and interpret or use the data to be generated from the mating.

The primary purpose of crossing is to expand genetic variability by combining genes from the parents involved in the cross to produce offspring that contain genes they never had before. Sometimes, multiple crosses are conducted to generate the variability in the base population to begin the selection process in the program. Based on how the crosses are made and their effects on the genetic structure of the plants or the population, methods of crossing may be described as either divergent or convergent.

Genetically divergent parents are crossed for recombination of their desirable genes. To optimize results, parents should be carefully selected to have a maximum number of positive traits and a minimum number of negative traits with no negative traits in common (i.e. elite × elite cross). This way, recombinants that possess both sets of desirable traits will occur in significant numbers in the F 2. The F 1contains the maximum number of desirable genes from both parents. There are several ways to conduct divergent crosses ( Figure 6.2):

Single crossIf two elite lines are available that together possess all desired traits at adequate levels, one cross (single cross [A × B]) may be all that is needed in the breeding program.

Three‐way crossSometimes, for combining all desirable traits several cultivars or elite germplasm are required, since each pair may have some negative traits in common. In this case, multiple crosses may be required in order to have the opportunity of obtaining recombinants that combine all the desirable traits. The method of 3‐way crosses ([A × B] × C) may be used. If a 3‐way cross product will be the cultivar, it is important that especially the third parent (C) be adapted to the region of intended use, since it contributes more genes than each of the A and B parents.

Double crossA double cross is a cross of two single crosses ([A × B] × [C × D]). The method of successive crosses is time consuming. Further, the complex crosses such as double cross have a low frequency of yielding recombinants in the F2 that possess a significant number of desirable parental genes. When this method is selected, the number of targeted desirable traits should be small (at most about 10). The double‐cross hybrid is genetically more broad‐based than the single‐cross hybrid but is more time consuming to make.

Diallel crossA diallel cross is one in which each parent is crossed with every other parent in the set (complete diallel), yielding n − (n−1)/2 different combinations (where n = number of entries). This method entails making a large number of crosses. Sometimes, the partial diallel is used in which only certain parent combinations are made. The method is tedious to apply to self‐pollinated species. Generally, it is a crossing method for genetic studies, and less for the purpose of creating populations for breeding.

Figure 6.2The basic types of crosses used by plant breeders. Some crosses are divergent (a) while others are convergent (b).

These are conservative methods of crossing plants. The primary goal of convergent crossing is to incorporate a specific trait into an existing cultivar without losing any of the existing desirable traits. Hence, one (or several) parents serve as a donorof specific genes and is usually involved in the cross only once. Subsequent crosses entail crossing the desirable parent (recurrent parent) repeatedly to the F 1, in order to retrieve all the desirable traits. A commonly used convergent cross is the backcross(see Chapter 17).

The first choice of parents for use in a breeding program is cultivars and experimental materials with desirable traits of interest. Most of the time, plant breeders make elite × elite crosses (they use adapted and improved materials). Even though genetic gains from such crosses may not always be dramatic, they are nonetheless significant enough to warrant the practice. After exhausting the variability in the elite germplasm as well as in the cultivated species, the breeder may look elsewhere, following the recommendation by Harlan and de Wet. These researchers proposed that the search for desired genes should start from among materials in the primary gene pool (related species), then proceed to the secondary gene pool, and if necessary, the tertiary gene pool. Crossing involving materials outside the cultivated species is collectively described as wide crosses. When the wide cross involves another species, it is called an interspecific cross (e.g., kale). When it involves a plant from another genus, it is called an intergeneric cross(e.g. wheat) (see Box 6.1). Crosses between crops with their wild progenitor species should not be considered wide crosses, despite the sometimes‐used different scientific names (barley, Hordeum vulgare, was derived from H. spontaneum ; lettuce, Lactuca sativa, was derived from L. serriola ). Genetically such “species” are fully compatible and behave genetically as an intraspecificcross (i.e. cross within the same species).