Selection for General and Specific Combining Ability

Table of Contents
1. Introduction
2. Combining Abilities and Mean Performance
3. Measurement of Combining Ability
4. Selection Strategies
5. Approaches to Estimating General and Specific Combining Ability in Plant Breeding
6. Diallele and Fractional Diallele Crosses in Breeding Programs
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Introduction

The variance between crosses in plant breeding is traditionally calculated under the assumption that a large number of lines were crossed at random. This assumption implies that each line was used solely in one cross. However, when each line is crossed with several others, the resulting variance between crosses takes on greater significance. This partitioning of variance is crucial for comprehending the utilization of crossbreeding in plant improvement.

Combining Abilities and Mean Performance

General Combining Ability (GCA):

• The mean performance of a line, expressed as a deviation from the overall cross mean.
• Obtained when a line is crossed with several others.
• Represents the average value of all crosses involving the line as one parent.

Specific Combining Ability (SCA):

• The deviation from the expected value in a particular cross.
• Represents the interaction between the two parental lines.
• Increases more rapidly than GCA with higher levels of inbreeding.

Mathematical Expression of Cross Mean:

• The true mean of a cross between lines P and Q expressed as X = X̄ + GCA + SCA.
• Where X̄ is the mean of all crosses, GCA is the general combining ability, and SCA is the specific combining ability.
• Includes a term for sampling error (E) in estimating X.

Measurement of Combining Ability

Crossbreeding Method:

• Lines crossed in various ways to measure GCA and SCA.
• SCA becomes significant as inbreeding levels increase.

Testcrossing Method:

• Individuals from the line to be tested are crossed with individuals from the base population.
• Identifies lines with good general combining ability.

Selection Strategies

Selection for General Combining Ability:

• Some lines may be discarded based on their own performance before crosses are made.
• Testing at relatively low coefficients of inbreeding.
• Recurrent selection involves repeated cycles of selection for GCA.

Selection for Specific Combining Ability:

• Requires mating and testing specific crosses.
• Intensity of selection increases with a large number of cases made and tested.
• Development of many inbred lines, main crosses, and selection of lines with favorable cross abilities.

Approaches to Estimating General and Specific Combining Ability in Plant Breeding

The estimation of general combining ability (GCA) and specific combining ability (SCA) is a critical aspect of plant breeding. A convenient method for this estimation, particularly suitable for plants, is known as the polycross method. This method involves growing several plants from different lines together, allowing natural pollination while preventing self-pollination through either natural mechanisms or careful arrangement.

Polycross Method

Implementation:

• Plants from all lines to be tested are grown together.
• Natural pollination occurs, preventing self-pollination.
• Seeds collected from each line represent a mixture of random crosses with other lines.

Evaluation of General Combining Ability:

• The performance of these seeds when grown tests the general combining ability of each line.
• General combining abilities are estimated for lines used as female parents.

Variance Estimation:

• Assuming equal variances of general combining ability for male and female parents allows estimation of both general and specific combining ability.

Testcross Method

Procedure:

• General combining ability of a line is estimated by crossing it with individuals from the base population.
• Equivalent to crossing with a random set of inbred lines from the base population without selection.

Diallel Cross Experimental Design

Diallel Cross Implementation:

• Involves crossing each line with every other line in the experiment.
• Commonly used for crossing inbred lines.

Complexity of Analysis:

• The analysis of a diallel cross for estimating variances of general and specific combining ability is intricate.
• Depends on whether reciprocal crosses are included and assumptions about the population's genetic components.

Estimating general and specific combining abilities in plant breeding involves practical methods like the polycross and testcross approaches, as well as the utilization of experimental designs like diallel crosses. Understanding the intricacies of these methods is crucial for effective selection and improvement in plant breeding programs.

Diallele and Fractional Diallele Crosses in Breeding Programs

Diallele and fractional diallele crosses are techniques used to assess the breeding value of multiple males by breeding them at different times to the same groups of females and comparing the resulting averages. This method helps eliminate differences between males and minimizes environmental variations influenced by dam effects.

Diallele Crosses

Practical Application:

• Breeding value is determined by breeding males multiple times to different groups of females.
• Averages of each set of offspring are compared for evaluation.

Example Scenario:

• Consider breeding ten boars (labeled 1 to 10) to two different sows (A and B) in a progeny test.
• Breeding arrangements ensure each boar is bred to each sow during the year, eliminating differences between litters.
• Environmental differences due to dam effects are also minimized.

Suitability in Swine and Poultry:

• Effective in predicting traits in swine and poultry where each female can produce multiple offspring multiple times per year.
• Less practical when only one offspring is produced per year.

Calculation of General Combining Ability

Large Number of Lines:

• General combining ability of each line is calculated as a deviation from its mean (TA-1) in relation to the overall mean (X).

Limitations with Small Number of Lines:

• With a small number of lines, this approach is not valid as each line contributes a fraction (1/n-1) of the general combining ability to the mean of the line.
• For instance, the mean (A) of Line A in all its crosses is affected by a fraction contributed by the other lines.

where the Ge are the general combining abilities of the lines A to N as indicated by the subscripts.

It is more convenient to work with the total than with the mean subscribing.

The expected value of each cross can now be calculated. The difference between observed and expected values estimates the specific combining abilities of the two lines in combination.