Principles of Plant Genetics and Breeding. George Acquaah
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Plant breeders often handle very large numbers of plants in a segregating population using limited resources (time, space, labor, money, etc.). Along with the large number of individuals are the many breeding characters often considered in a breeding program. The sooner they can reduce the numbers of plants to the barest minimum, but more importantly, to the most desirable and promising individuals, the better. Highly heritable and readily scorable traits are easier to select for in the initial stages of a breeding program.
Tandem selection
In this mode of selection, the breeder focuses on one trait at a time (serial improvement). One trait is selected for several generations, then another trait is focused on for the next period. The question of how long each trait is selected before a switch and at what selection intensity, are major considerations for the breeder. It is effective when genetic correlation does not exist between the traits of interest, or when the relative importance of each trait changes throughout the years.
Independent curling
Also called truncation selection, independent curling entails selecting for multiple traits in one generation. For example, for three traits, A, B, and C, the breeder may select 50% plants per family for A on phenotypic basis, and from that select 40% plants per family based on trait B; finally, from that subset, 50% plants per family is selected for trait C, giving a total of 10% selection intensity (0.5 × 0.4 × 0.5).
Index selection
A breeder has a specific objective for conducting a breeding project. However, selection is seldom made on the basis of one trait alone. For example, if the breeding project is for disease resistance, the objective will be to select a genotype that combines disease resistance with the qualities of the elite adapted cultivar. Invariably, breeders usually practice selection on several traits simultaneously. The problem with this approach is that as more traits are selected for, the less the selection pressure that can be exerted on any one trait. Therefore, the breeder should select on the basis of two or three traits of the highest economic value. It is conceivable that a trait of high merit may be associated with other traits of less economic value. Hence, using the concept of selection on total merit, the breeder would make certain compromises, selecting individuals that may not have been selected, were it based on a single trait.
In selecting on a multivariate phenotype, the breeder explicitly or implicitly assigns a weighting scheme to each trait, resulting in the creation of a univariate trait (an index) that is then selected. The index is the best linear prediction of an individual's breeding value. It takes the form of a multiple regression of breeding values on all the sources of information available for the population.
The methods used for constructing an index usually include heritability estimates, relative economic importance of each trait, and genetic and phenotypic correlation between the traits. The most common index is a linear combination that is mathematically expressed as follows:
where z is the vector of phenotypic values in an individual, and b is a vector of weights. For three traits, the form may be
where a, b, and c are coefficients correcting for relative heritability and the relative economic importance of traits A, B, and C, respectively, and A1, B1, and C1 are the numerical values of traits A, B, and C expressed in standardized form. A standardized variable (X1) is calculated as
where, X is the record of performance made by an individual; X is the average performance of the population; and σx is the standard deviation of the trait.
The classical selection index has the following form:
where x1, x2, x3, xn are the phenotypic performance of the traits of interest, and b1, b2, and b3 are the relative weights attached to the respective traits. The weights could be simply the respective relative economic importance of each trait, the resulting index called the basic index, and may be used in cultivar assessment in official registration trials.
An index by itself is meaningless, unless it is used in comparing several individuals on a relative basis. Further, in comparing different traits, the breeder is faced with the fact that the mean and variability of each trait is different, and frequently, the traits are measured in different units. Standardization of variables resolves this problem.
4.2.14 Concept of intuitive index
Plant breeding was described in Chapter 2 as both a science and an art. Experience (with the crop, the methods of breeding, breeding issues concerning the crop) is advantageous in having success in solving plant breeding problems. Plant breeders, as previously indicated, often must evaluate many plant characters in a breeding program. Whereas one or a few would be identified as key characters and focused on in a breeding program, breeders are concerned about the overall performance of the cultivar. During selection, breeders formulate a mental picture of the product desired from the project, and balance good qualities against moderate defects as they make final judgments in the selection process.
Explicit indices are laborious, requiring the breeder to commit to extensive record‐keeping and statistical analysis. Most breeders use a combination of truncation selection and intuitive selection index in their programs.
4.2.15 The concept of general worth
For each crop, there is a number of characters, which considered together, define the overall desirability of the cultivar from the combined perspectives of the producer and the consumer. These characters may range between about a dozen to several dozens, depending on the crop, and constitute the primary pool of characters that the breeder may target for improvement. These characters differ in importance (economic and agronomic) as well as ease with which they can be manipulated through breeding. Plant breeders typically target one or few of these traits for direct improvement in a breeding program. That is, the breeder draws up a working list of characters to address the needs embodied in the stated objectives. Yield of the economic product is almost universally the top priority in a plant breeding program. Disease resistance is more of a local issue, since what may be economically important in one region may not be important in another area. Even though a plant breeder may focus on one or a few traits at a time, the ultimate objective is the improvement of the totality of the key traits that impact the overall desirability or general worth of the crop. In other words, breeders ultimately have a holistic approach to selection in a breeding program. The final judgments are made on a balanced view of the essential trait of the crop.
4.2.16 Nature of breeding characteristics and their levels of expression
Apart from relative importance, the traits the plant breeder targets vary in other ways. Some are readily evaluated by visual examination (e.g. shape color, size), whereas others require a laboratory assay (e.g. oil content) or mechanical measurement (e.g. fiber characteristics of cotton). Special provisions (e.g. greenhouse, isolation block) may be required in disease breeding, whereas yield evaluations are most reliable when conducted over seasons and locations in the field.
In addition to choosing the target traits, the breeder will have to decide on the level of expression of each one, below which a plant material would be declared worthless. The acceptability level of expression of a trait may be narrowly defined (stringent selection) or broadly defined (loose selection). In industrial crops (e.g. cotton), the product quality may be