Introduction
Recurrent selection is a cyclic breeding method used to improve the average genetic merit of a population for quantitative traits while conserving genetic variability. It is particularly valuable in cross-pollinated species (e.g., maize, sorghum, many forages) where heterozygosity and continuous recombination permit sustained improvement. The method does not aim to produce a finished cultivar in a single cycle; instead, it produces successive, improved base populations that can later be used to extract superior lines, create synthetics, or develop hybrids.
Concept and Historical Background
The phrase "recurrent selection" was popularized by Hull (1945) and later expanded by Comstock, Robinson & Harvey (1949) who described reciprocal recurrent selection. Fundamentally, the method consists of repeated rounds of: (1) selecting superior individuals, and (2) intermating these individuals to recombine favorable alleles. Over cycles, the frequency of favorable alleles increases and the population mean improves.
Key Features
- Cyclic nature: Repeated selection–intermating cycles.
- Selection + recombination: Intermating is essential to combine favorable alleles from different plants.
- Gradual but cumulative gain: Improvement is slow per cycle but accumulates over many cycles.
- Focus on polygenic traits: Suited for yield, quality, tolerance—traits controlled by many genes.
Objectives
- Increase frequency of favourable alleles controlling quantitative traits.
- Improve population mean for complex traits such as yield, disease tolerance, drought resilience and quality.
- Maintain genetic variability and avoid rapid fixation.
- Produce improved base populations for hybrid or synthetic development.
- Exploit additive and (when planned) non-additive genetic variance.
Detailed Stepwise Procedure
1. Choice of Base Population
Select a broad, variable starting population: an adapted landrace, a synthetic/composite, or a diverse breeding population assembled from crosses among elite lines. The base should contain abundant genetic variability to allow sustained genetic gain and to reduce risk of inbreeding depression.
2. Selection of Superior Plants
Selection strategies vary by trait heritability and breeding objective:
- Phenotypic (simple) selection: Select the best-looking individuals based on observed performance — suitable when heritability is high.
- Half-sib or full-sib progeny testing: Evaluate progeny of selected plants to improve selection accuracy for low-heritability traits.
- Test-cross based selection: Evaluate general or specific combining ability by crossing plants with a tester and assessing progeny.
- Marker-assisted selection: Use molecular markers linked to QTL to select parents faster and more precisely (MARS).
3. Intermating of Selected Plants
After selection, selected plants must be intermated to allow recombination of favorable alleles. Intermating approaches include:
- Random open pollination of selected plants in an isolation block.
- Controlled hand crosses among selected plants.
- Synthetic mating designs (e.g., circular mating) to ensure equal contribution.
4. Formation of New Population & Repetition
Seeds produced by intermating are bulked (or maintained as family bulks) to create the improved population for the next cycle. The cycle of selection and recombination is repeated for several cycles until satisfactory gain is achieved or gain per cycle plateaus.
Types of Recurrent Selection (with examples)
Simple Recurrent Selection
Definition: Selection based only on the phenotype of individual plants; selected individuals are intermated without progeny testing.
When to use: For traits with high heritability and when resources limit progeny testing.
Recurrent Selection for General Combining Ability (GCA)
Definition: Selection based on the average performance of a plant's progeny when crossed with a broad-based tester population. Targets additive genetic variance.
Use: To improve populations that will serve as general seed sources or as parents for inbred extraction.
Recurrent Selection for Specific Combining Ability (SCA)
Definition: Selection based on superior performance of specific crosses (non-additive effects). Plants are evaluated by crossing with a narrow or single inbred tester and selecting based on cross performance.
Use: To develop heterotic pools and populations destined for hybrid production where dominance and epistasis are exploited.
Reciprocal Recurrent Selection (RRS)
Definition: Two populations (A and B) are improved simultaneously. Plants from A are test-crossed with B and vice versa; the best individuals are selected and intermated within their original population. This captures both GCA and SCA and increases hybrid performance between A and B.
Classic example: Development of heterotic groups in maize where two distinct populations are reciprocally improved to maximise hybrid vigour.
Modern Modifications
Contemporary breeders often incorporate molecular tools: Marker-Assisted Recurrent Selection (MARS) and Genomic Selection (GS) are widely used to speed up cycles and increase selection accuracy. Genomic prediction models can estimate breeding values without extensive phenotyping in each cycle.
Genetic Principles and Expected Responses
Recurrent selection changes allele frequencies by repeatedly selecting and recombining favorable alleles. Key genetic considerations:
- Additive variance: Primary driver of cumulative response under recurrent selection.
- Dominance & epistasis: Exploitable under SCA and RRS strategies.
- Selection intensity & population size: Stronger selection increases gain per cycle but risks reducing variability; thus, large base populations are recommended to reduce genetic drift.
- Heritability: Higher heritability improves response to simple phenotypic recurrent selection; lower heritability favors progeny-testing or marker-based evaluations.
Practical Applications & Case Studies
Maize: The most notable success—recurrent selection produced improved open-pollinated varieties and heterotic pools that underlie modern hybrids.
Forages: Alfalfa, ryegrass and clovers have been improved for yield, persistence and fodder quality using recurrent selection cycles.
Sorghum & Pearl millet: Population improvement for drought tolerance, disease resistance and grain yield using recurrent selection strategies.
Advantages
- Maintains and uses genetic variation for long-term gain.
- Accumulates favorable alleles for complex quantitative traits.
- Directly produces improved populations for synthetics and hybrids.
- Flexible: can adopt marker technologies to improve accuracy and speed.
Limitations and Challenges
- Time-consuming—multiple cycles (each 1–3 years depending on crop and generation time).
- Resource-intensive—requires isolation blocks, controlled crosses or large field layouts and careful record-keeping.
- Requires large effective population sizes to prevent genetic drift and inbreeding.
- Less effective for traits with extremely low heritability unless supported by progeny testing or genomic tools.
Integrating Molecular Tools
Marker-assisted methods (MARS) and genomic selection (GS) have transformed recurrent selection:
- MARS: Use of a few markers linked to major QTL to guide parental selection.
- GS: Genome-wide marker profiles and prediction models estimate genomic estimated breeding values (GEBVs), allowing rapid selection without full progeny testing.
- Benefits: Faster cycles, higher accuracy under low heritability, and effective targeting of many small-effect loci.
Recommendations for Breeders (Practical Tips)
- Start with a broad, variable base population to ensure long-term gain.
- Choose selection method based on trait heritability: simple phenotypic selection for high-heritability traits; progeny testing or genomic selection for low-heritability traits.
- Keep effective population sizes large (several hundred plants) and avoid intense bottlenecks to minimise drift.
- Use balanced mating designs (circular or random mating) to ensure fair contribution of selected parents.
- Monitor genetic diversity (molecular markers) periodically to prevent unwanted erosion of variability.
- Combine recurrent selection with doubled-haploid or inbred extraction schemes when the goal is to create elite inbreds for hybrid production.
Diagram Prompt: Recurrent Selection Cycle
Use the following detailed prompt to create a clear diagram (for illustration or figure creation):
Create a labelled diagram showing one full cycle of recurrent selection with the following elements:
Conclusion
Recurrent selection is a cornerstone method for population improvement in cross-pollinated crops. By balancing selection with recombination, breeders can continuously enrich favourable alleles while maintaining variability necessary for future progress. Modern genomic tools have accelerated the process, increasing precision and shortening cycle times. When implemented with careful population management and adequate resources, recurrent selection offers sustainable, long-term genetic improvement for complex quantitative traits.
