George Acquaah - Principles of Plant Genetics and Breeding

There are practical factors to consider in deciding on the number of crosses to make for a breeding project. These include the ease of making the crosses from the standpoint of floral biology, and the constraints of resources (labor, equipment, facilities, and funds). It will be easier to make more crosses in species in which emasculation is not needed (e.g. monoecious and dioecious species) than in bisexual species. Some breeders make a small number of carefully planned crosses, while others make even thousands of cross combinations.

Generally, a few hundred cross combinations per crop per year would be adequate for most purposes, for species in which the F 1is not the commercial product. More crosses may be needed for species in which hybrids are commonly produced for the purpose of discovering heterotic combinations. As will be discussed next, breeding programs that go beyond the F 1usually require very large F 2populations. Regarding the number of flowers per cross combination, there is variation according to fecundity. Species such as tomato may need only one or two crosses, since each fruit contains over 100 seeds. Plants that tiller also produce large numbers of seed. Each crop species has its own reproduction rate, which may be huge (e.g. tobacco: 1000s of seeds produced per plant, 100s per bowl) or relatively small (e.g. pea: about 100 per plant, about 2–5 per pod).

Because hybridization involves combining two sets of genes in a new genetic matrix through the meiotic process, it is accompanied by a variety of genetic‐based effects.

The immediate effect of hybridization is the assembly of two different genomes into a newly created individual. Several genetic consequences may result from such union of diverse genomes, some of which may be desirable, whereas others may not be desirable. The key ones are as follows:

Expression of recessive lethal geneCrossing may bring together recessive lethal genes (that were in the heterozygous state) into the expressible homozygous state. The resulting hybrid may die or lose vigor. By the same token, hybridization can also mask the expression of a recessive allele by creating a heterozygous locus. Individuals carry a certain genetic load (or genetic burden), representing the average number of recessive lethal genes carried in the heterozygous condition by an individual in a population. Selfing or inbreeding predisposes an individual to having deleterious recessive alleles that were protected in the heterozygous state to becoming expressed in the homozygous recessive form.

Hybrid necrosisEspecially the crossing of parents that are somewhat distantly related (but still the same crop species), may result in the phenomenon of hybrid necrosis. Interactions between pairs of genes in both parents may work out unfavorably to the physiology of the plant. This phenomenon has been reported in wheat and rye, but also in Arabidopsis.

HeterosisGenes in the newly constituted hybrid may complement each other to enhance the vigor of the hybrid. The phenomenon of hybrid vigor (heterosis) is exploited in hybrid seed development (see Chapter 19).

Transgressive segregationHybrids have features that may represent an average of the parental features, or a bias toward the features of one parent, or even new features that are unlike either parent (transgressive segregates). When the parents “nick” in a cross, transgressive segregates with performance superseding either parent is likely to occur in the segregating population.

Genome‐plastome incompatibilityPlastomes (the genetic material found in plastids such as in chloroplasts) and genomes in most genera function to form normal plants, regardless of the taxonomical distances between the plastid and nuclear genomes. However, in some genera, plastomes and genomes, having co‐evolved to a significant degree, are only compatible within a specific combination.

The subsequent effect of hybridization, which is often the reason for hybridizing parents by breeders, occurs in the F 2and later generations. By selfing the F 1hybrid, the parental genes are reorganized into new genetic matrices in the offspring. This occurs through the process of meiosis, a nuclear division process that occurs in flowering plants. Contrasting alleles segregate and subsequently recombine in the next generation to generate new variability. Furthermore, the phenomenon of crossing over that leads to the physical exchange of parts of chromatids from homologous chromosomes provides an opportunity for recombination of linked genes, also leading to the generation of new variation.

The goal of crossing for generating variability for selection is to produce a large number of gene recombinations from the parents used in the cross. In hybrid seed programs, the F 1is the end product for commercial use. However, in other crosses, the F 2and subsequent generations are evaluated to select genotypes that represent the most desirable recombination of parental genes. The F 2generation has the largest number of different gene combinations of any generation following a cross. The critical question in plant breeding is the size of F 2population to generate in order to have the chance of including that ideal recombinant this is homozygous for all the desirable genes in the parent. Three factors determine the number of gene recombination that would be observed in an F 2population:

1 The number of gene loci for which the parents in a cross differ;

2 The number of alleles at each locus;

3 The linkage of the gene loci.

Plant breeders are often said to play the numbers game. Table 6.1summarizes the challenges of breeding in terms of size of the F 2population to grow. If the parents differ by only one pair of allelic genes, the breeder needs to grow at least 16 plants in the F 2to have the chance to observe all the possible gene combinations (according to Mendel's laws). On the other hand, if the parents differ in 10 allelic pairs, the F 2population size needed is 59 049 (obtained by the formula 3 n, where n = the number of loci). The frequencies illustrate how daunting a task it is to select for quantitative traits.

Table 6.1The variability in an F 2population as affected by the number of genes that are different between the two parents.

The total possible genotypes in the F 2based on the number of alleles per locus is given by the relationship [k (k + 1)/2] nwhere k = number of alleles at each locus, and n = number of heterozygous loci. With 1 heterozygote and 2 alleles, there will be only 3 kinds of genotypes in the F 2, while with 1 heterozygote and 4 alleles, there will be 10. The effect on gene recombination by linkage is more important than for the number of alleles. Linkage may be desirable or undesirable. Linkage reduces the frequency of gene recombination (it increases parental types). The magnitude of reduction depends on the phase ( coupling phase– with both dominant gene loci in one parent, e.g. AB/ab , and repulsion phase– with one dominant and one recessive loci in one parent, e.g. Ab/aB ). The effect of linkage in the F 2may be calculated as ¼ (1‐P) 2× 100 for the coupling phase, and ¼ P 2× 100 for the repulsion phase, for the proportion of AB/AB or ab/ab genotypes in the F 2from a cross between AB/ab × Ab/aB . Given, for example, a crossing over value of 0.10, the percentage of the homozygotes will be 20.25% in the coupling versus only 0.25% in the repulsion phase. If two genes were independent (crossing over value = 0.50), only 6.25% homozygotes would occur. The message here is that the F 2population should be as large as possible.