Principles of Plant Genetics and Breeding. George Acquaah
Читать онлайн книгу.
in the population in heterozygote state for many generations.
As population size decreases, the effect of random drift increases. This effect is of importance in germplasm collection and maintenance. The original collection can be genetically changed if a small sample is taken for growing to maintain the accession.
3.6 Modes of selection
There are three basic forms of selection – stabilizing, disruptive, and directional – the last form being the one of most concern to plant breeders. These forms of selection operate to varying degrees under both natural and artificial selection. A key difference lies in the goal. In natural selection, the goal is to increase the fitness of the species, whereas in plant breeding, breeders impose artificial selection usually to direct the population toward a specific goal (not necessarily the fittest).
3.6.1 Stabilizing selection
Selection as a process is ongoing in nature. Regarding characters that directly affect the fitness of a plant (i.e. viability, fertility), selection will always be directionally toward optimal phenotype for a given habitat. However, for other characters, once optimal phenotype has been attained, selection will act to perpetuate it as long as the habitat remains stable. Selection will be for the population mean and against either extreme expression of the phenotype. This mode of selection is called stabilizing selection (or also called balancing or optimum selection). Taking flowering for example; stabilizing selection will favor neither early flowering nor late flowering. In terms of genetic architecture, dominance will be low or absent or ambidirectional, whereas epistasis will not generally be present. Stabilizing selection promotes additive variation.
3.6.2 Disruptive selection
Natural habitats are generally not homogeneous but consist of a number of “ecological niches” that are distinguishable in time (seasonal or long‐term cycles), space (microniches), or function. These diverse ecological conditions favor diverse phenotypic optima in form and function. Disruptive selection is a mode of selection in which extreme variants have higher adaptive value than those around the average mean value. Hence, it promotes diversity (polymorphism). The question then is how the different optima relate (dependent or independent) for maintenance and functioning. Also, at what rate does gene exchange occur between the differentially selected genotypes? These two factors (functional relationship and rate of gene exchange) determine the effect of genetic structure of a population. In humans, for example, a polymorphism that occurs is sex (female and male). The two sexes are 100% interdependent in reproduction (gene exchange is 100%). In plants, self‐incompatibility is an example of such genetically controlled polymorphism. The rarer the self‐incompatibility allele at a locus, the higher the chance of compatible mating (and vice versa). Such frequency‐dependent selection is capable of building up a large number of self‐incompatibility alleles in a population. As previously indicated, several hundreds of alleles have been found in some species.
3.6.3 Directional selection
Plant breeders, as previously stated, impose directional selection to change existing populations or varieties (or other genotypes) in a predetermined way. Artificial selection is imposed on the targeted character(s) to achieve maximal or optimal expression. To achieve this, the breeder employs techniques (crossing) to reorganize the genes form the parents in a new genetic matrix (by recombination), assembling “co‐adapted” gene complexes to produce a fully balanced phenotype, which is then protected from further change by genetic linkage. The breeding system will determine whether the newly constituted gene combinations will be maintained. Whereas inbreeding (e.g. selfing) would produce a homozygous population that will resist further change (until crossed), outbreeding tends to produce heterozygous combinations. In heterozygous populations, alleles that exhibit dominance in the direction of expression targeted by the breeder will be favored over other alleles. Hence, directional selection leads to the establishment of dominance and/or genic interaction (epistasis).
3.7 Effect of mating system on selection
Four mating systems are generally recognized. They may be grouped into two broad categories as random mating and non‐random mating (comprising genetic assortative mating, phenotypic assortative mating, and disassortative mating).
3.7.1 Random mating
In plants, random mating occurs when each female gamete has an equal chance of being fertilized by any male gamete of the same plant, or with any other plant of the population, and further, there is an equal chance for seed production. As can be seen from the previous statement, it is not possible to achieve true random mating in plant breeding since selection is involved. Consequently, it is more realistic to describe the system of mating as random mating with selection. Whereas true random mating does not change gene frequencies, existing variability in the population or genetic correlation between close relatives, random mating with selection changes gene frequencies and the mean of the population, with little or no effect on homozygosity, population variance, or genetic correlation between close relatives in a large population. Small populations are prone to random fluctuation in gene frequency (genetic drift) and inbreeding, factors that reduce heterozygosity in a population. Random mating does not fix genes, with or without selection. If the goal of the breeder is to preserve desirable alleles (e.g. in germplasm composites), random mating will be an effective method of breeding.
3.7.2 Non‐random mating
Non‐random mating has two basic forms: (i) mating occurs between individuals that are related to each other by ancestral descent (promotes an increase in homozygosity at all loci), and (ii) individuals mate preferentially with respect to their genotypes at any particular locus of interest. If mating occurs such that the mating pair has the same phenotype more often than would occur by chance, it is said to be assortative mating. The reverse is true in disassortative mating, which occurs in species with self‐incompatibility or sterility problems, promoting heterozygosity.
Genetic assortative mating
Genetic assortative mating or inbreeding entails mating individuals that are closely related by ancestry, the closest being selfing (self‐fertilization). A genetic consequence of genetic assortative mating is the exposure of cryptic genetic variability that was inaccessible to selection and was being protected by heterozygosity (i.e. heterozygous advantage). Also, repeated selfing results in homozygosity and brings about fixation of types. This mating system is effective if the goal of the breeder is to develop homozygous lines (e.g. developing inbred lines for hybrid seed breeding or development of synthetics).
Pheotypic assortative mating
Mating may also be done on the basis of phenotypic resemblance. Called phenotypic assortative mating, the breeder selects and mates individuals on the basis of their resemblance to each other compared to the rest of the population. The effect of this action is the development of two extreme phenotypes. A breeder may choose this mating system if the goal is to develop an extreme phenotype.
Disassortative mating
Disassortative mating may also be genetic or phenotypic. Genetic disassortative mating entails mating individuals that are less closely related than they would under random mating. A breeder may use this system to cross different strains. In phenotypic disassortative mating, the breeder may select individuals with contrasting phenotypes for mating. Phenotypic disassortative mating is a conservative mating system that may be used to maintain genetic diversity in the germplasm from which the breeder may obtain desirable genes for breeding as needed. It maintains heterozygosity