Handling of Segregating Populations

Introduction

In plant breeding, a segregating population is the progeny derived from a cross between genetically different parents where alleles segregate according to Mendelian laws. These populations (typically F₂ and later generations) are the main source of genetic variability for selection. Proper handling of segregating populations is essential to recover favourable recombinants and to fix desirable gene combinations into stable lines.

Nature of Segregating Populations

• Source: F₂ and subsequent generations after a cross.
• Genetic composition: Heterogeneous and heterozygous in early generations; becomes more homozygous with selfing/inbreeding.
• Variation present: Qualitative traits (discrete) and quantitative traits (continuous polygenic variation).

Objectives

• Fix desirable gene combinations into homozygous lines.
• Eliminate inferior segregants and retain superior recombinants.
• Balance selection intensity with maintenance of useful genetic diversity.
• Efficiently use land, labour, and time resources.
• Develop stable, uniform varieties for release.

General Principles

• Early vs. late generation selection: Early selection works for high-heritability traits; delay selection for low-heritability quantitative traits.
• Population size: Larger populations increase probability of recovering desirable recombinants.
• Crop breeding system: Self-pollinated crops commonly use pedigree, bulk or SSD; cross-pollinated crops use recurrent selection or family selection.
• Resource consideration: Choice of method depends on available land, labour and time.

Methods for Handling Segregating Populations

1. Pedigree Method

Select superior plants each generation and maintain pedigree records. It is effective in self-pollinated crops for both qualitative traits and some quantitative traits. Advantage: systematic tracking of lines. Limitation: labour- and record-intensive.

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2. Bulk Method

Mix and grow all segregants together without selection during early generations; harvest as bulk and apply selection in later generations (commonly from F₅ onward). Useful for quantitative traits where natural selection will remove weak genotypes. Risk: desirable recombinants may be lost before selection.

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3. Modified Bulk Method

Similar to bulk but with mild selection in early generations to retain desirable plants while still allowing population-level buffering.

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4. Single Seed Descent (SSD)

Advance generations rapidly by taking a single seed per plant each generation without selection until near homozygosity (F₆–F₇), then select. Advantage: rapid fixation and minimal field space early. Limitation: demands controlled generation advancement (off-season/greenhouse) and may lose rare good recombinants if population size is too small.

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5. Backcross Method

Used to transfer a specific gene (e.g., resistance) from a donor into a well-adapted recurrent parent. Segregating populations in each backcross generation are handled by selection for the target trait and recurrent parent phenotype. Very effective for single-gene traits.

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6. Recurrent Selection

Select superior individuals and intermating among them followed by repeated cycles. Best suited for cross-pollinated crops; maintains variability while improving population mean. Requires larger populations and controlled pollination.

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7. Doubled Haploid (DH) Technique

Use haploid induction (anther/microspore culture or wide crosses) followed by chromosome-doubling to obtain instant homozygous lines. Advantage: produces pure lines in 1–2 years. Limitation: requires tissue-culture facilities and expertise.

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When to Select: Early vs Late Generation

• Early-generation selection: For traits with high heritability or simple inheritance (disease resistance, seed colour).
• Late-generation selection: For complex quantitative traits (yield, quality) where environmental variance masks genetic differences.
• Progeny testing: Use to verify that selected plants transmit superior genetics to offspring.

Practical Strategies

• Start with a sufficiently large F₂ to capture recombination (size depends on trait complexity and resources).
• Use mixed strategies: e.g., SSD to F₅ then pedigree selection, or modified bulk followed by progeny testing.
• Apply marker-assisted selection (MAS) for target loci to speed up selection, especially for traits with low heritability.
• Integrate speed breeding or off-season nurseries to advance generations quickly.
• Use replicated and multi-location trials for advanced lines to assess stability and adaptability before release.

Challenges

• Separating genetic effects from environmental noise in early segregating generations.
• Maintaining sufficiently large populations within limited land and labour resources.
• Fixing superior recombinants while avoiding loss of useful diversity.
• Cost and time for evaluation of large numbers of lines.

Crop-wise Examples

• Wheat & Rice: Pedigree and bulk methods are commonly used; DH increasingly used for rapid line fixation.
• Maize: Recurrent selection and family selection are standard due to cross-pollination.
• Soybean & Barley: SSD and DH methods used to obtain homozygous lines quickly.

Workflow Example (A Suggested Practical Scheme)

1. Make cross and produce a large F₁ → self to produce a large F₂ population.
2. Grow a large F₂; perform mild selection for gross defects and tag superior plants (if trait permits).
3. Use SSD or advance as bulk to F₅–F₆ to obtain lines approaching homozygosity.
4. Conduct preliminary yield trials (2–3 reps) of F₆/F₇ lines; select best lines.
5. Advance selected lines to multi-location trials and conduct progeny tests if required.
6. Select stable, high-performing lines for national/state trials and eventual release.

Conclusion

Handling segregating populations well is central to successful plant breeding. The choice of method must consider trait inheritance, crop mating system, available resources, and breeding goals. Traditional methods (pedigree, bulk, backcross) remain valuable, while modern techniques (Doubled Haploids, MAS, genomic selection, speed breeding) vastly improve efficiency and shorten breeding cycles.