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UPSC MainsANI-HUSB-VETER-SCIENCE-PAPER-I202510 Marks150 Words
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Q20.

Write short notes on/Answer the following in about 150 words each : (a) Recurrent selection and Reciprocal recurrent selection.

How to Approach

The question asks for short notes on Recurrent Selection and Reciprocal Recurrent Selection. The approach should involve defining each method, outlining their procedures, stating their primary objectives, and highlighting their key differences. It's crucial to mention the type of gene action each method primarily exploits (additive vs. non-additive) and their applications in animal breeding.

Model Answer

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Introduction

Animal breeding techniques are crucial for enhancing desirable traits like growth rate, milk yield, or disease resistance in livestock populations. Quantitative traits, influenced by multiple genes and environmental factors, often require sophisticated selection strategies. Recurrent selection and Reciprocal Recurrent Selection are two such methodologies, involving repeated cycles of selection and intermating to improve the genetic makeup of populations. These methods systematically accumulate favorable alleles, contributing significantly to genetic gain and improved productivity in animal husbandry.

Recurrent Selection

Recurrent selection (RS) is a systematic breeding technique designed to improve quantitative traits within a single population over successive generations. Coined by Hull in 1945, it involves repeated cycles of selecting individuals with superior phenotypes or genotypes, intermating them to allow for genetic recombination, and then selecting again from their progeny. This cyclical process aims to increase the frequency of desirable alleles (genes) in the population, thereby enhancing the overall performance for the target trait.

  • Objective: To improve the mean performance of a single population, primarily by exploiting additive gene action. It can also be used for general combining ability (GCA) by using a broad-based tester.
  • Procedure:
    1. Selection: Identify superior individuals or families based on their performance for the trait of interest.
    2. Intermating: Selected individuals are intercrossed to generate the next breeding population, allowing for genetic recombination.
    3. Evaluation: Progeny are evaluated, and the cycle is repeated.
  • Application: Effective for traits controlled by additive gene action with moderate to high heritability.

Reciprocal Recurrent Selection (RRS)

Reciprocal Recurrent Selection (RRS) is a specialized form of recurrent selection proposed by Comstock et al. in 1949, specifically designed to improve the performance of crosses between two distinct populations (Population A and Population B) while maximizing hybrid vigor (heterosis). It simultaneously improves the general combining ability (GCA) of both populations and exploits specific combining ability (SCA), which is often associated with non-additive gene action like dominance and epistasis.

  • Objective: To improve the general combining ability of two different populations simultaneously and to exploit heterosis in their crossbred progeny.
  • Procedure:
    1. Test-Mating: Individuals from Population A are test-mated with a sample from Population B, and vice versa.
    2. Selection: Selection within each population (A and B) is based on the performance of their crossbred progeny.
    3. Intermating: Superior purebred individuals (from A and B) identified through progeny testing are intermated within their respective populations to produce the next generation.
    4. Cycle Repetition: This cycle of test-mating, selection, and intermating is repeated across generations.
  • Application: Highly effective in commercial poultry and swine breeding programs where hybrid vigor is crucial for traits like growth rate and reproductive efficiency.

Comparison of Recurrent Selection and Reciprocal Recurrent Selection

Feature Recurrent Selection (RS) Reciprocal Recurrent Selection (RRS)
Number of Populations One Two distinct populations (A and B)
Primary Goal Improvement of the base population itself Improvement of hybrid performance between two populations
Gene Action Exploited Mainly additive gene action (GCA) Both additive (GCA) and non-additive (SCA) gene action
Selection Basis Phenotypic performance or progeny test within the population Progeny test of crossbred individuals between populations
Product Improved pure lines or synthetic varieties Superior crossbred progeny (hybrids)
Complexity Simpler, shorter generation intervals More complex, often longer generation intervals due to progeny testing

Conclusion

Both recurrent selection and reciprocal recurrent selection are powerful tools in animal breeding, each tailored to specific objectives. Recurrent selection is ideal for incrementally improving traits within a single population by leveraging additive genetic variance. In contrast, reciprocal recurrent selection is crucial for maximizing heterosis in hybrid breeding programs, effectively exploiting both additive and non-additive gene actions by improving the combining ability of two distinct populations. The choice between these methods depends on the breeding goal, the genetic architecture of the trait, and the desired final product, underscoring their strategic importance in modern animal genetics.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Additive Gene Action
Refers to the cumulative effect of individual genes on a quantitative trait, where each gene contributes a small, independent increment to the phenotype. It forms the basis of general combining ability and is highly heritable.
Heterosis (Hybrid Vigor)
The phenomenon where the progeny of a cross between two different breeds or lines shows superior performance compared to the average of their parents, often attributed to the masking of deleterious recessive alleles and beneficial gene interactions (dominance and epistasis).

Key Statistics

In poultry breeding, RRS has been instrumental. For instance, in broiler chickens, RRS programs have demonstrated a progressive increase in body weight, with selected strains being over 11% heavier at 10 weeks of age compared to unselected controls in certain experimental settings (Griesbach, L., 1962).

Source: Canadian Journal of Animal Science, L. Griesbach, 1962

Simulations in oil palm breeding (Cros et al., 2015) suggest that integrating genomic selection with RRS could potentially reduce the generation time for an RRS breeding cycle from approximately 20 years to 6 years, highlighting the impact of modern genomic tools.

Source: Frontiers in Plant Science, Cros et al., 2015

Examples

Maize Breeding Success

Recurrent selection has been widely successful in maize breeding, significantly increasing grain yield. Over decades, cycles of recurrent selection have led to substantial genetic gains in various maize populations, adapting them to diverse environmental conditions and improving traits like disease resistance and nutrient use efficiency.

Frequently Asked Questions

▶What is the main limitation of Reciprocal Recurrent Selection?

A primary limitation of RRS is the elongation of breeding cycles due to the necessity of producing and evaluating sufficient progeny from crossbred matings before selecting parents for the next purebred generation. This can make the process more time-consuming and resource-intensive compared to simpler selection methods.

Topics Covered

Animal BreedingGeneticsQuantitative GeneticsSelection MethodsGenetic ImprovementBreeding Strategies
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