Pureline Selection in Plant Breeding

Introduction

Pureline selection is a classical and foundational breeding method for self-pollinated crops. Introduced by Wilhelm Johannsen (early 1900s) from his experiments with beans, the pureline concept demonstrates that progeny derived by repeated selfing from a single superior plant become genetically uniform and true-breeding. A pureline is the progeny of one homozygous individual and hence is stable, uniform and predictable.

Objectives of Pureline Selection

• To isolate the best homozygous line from a heterogeneous landrace or farmer population.
• To improve yield, quality and other agronomic characters in self-pollinated crops.
• To develop varieties with specific desired traits—disease resistance, earliness, drought tolerance, etc.
• To stabilise and fix the beneficial variability present in traditional cultivars.

Genetic Basis

Self-pollination reduces heterozygosity rapidly; after several generations of selfing (commonly by F₆–F₈) individuals become nearly homozygous. Within a pureline, genetic variation is negligible, so selection within a pureline yields little or no genetic gain. The effective selection is done initially from the heterogeneous population to capture desirable genotypes which are then fixed by selfing.

Steps in Pureline Selection

1. Collection of material

Choose a genetically variable source: landrace, farmer’s cultivar, or heterogeneous population showing useful variability.

2. Selection of superior plants (Year 1)

From the base heterogeneous population, visually identify and tag 200–500 superior plants (depending on crop and resources). Harvest each plant separately keeping identity of individual plants.

3. Progeny testing (Year 2)

Grow progeny rows (from each selected plant) and evaluate each progeny for uniformity and genetic merit. Poor or non-uniform progenies are discarded; promising progenies are advanced.

4. Preliminary and comparative yield trials (Years 3–5)

Conduct replicated preliminary yield trials with selected purelines and compare against local checks. Advance the best lines to more extensive multi-location trials.

5. Multilocation testing and release (Years 6–8)

Best purelines undergo coordinated multi-location testing for yield stability, adaptability, disease resistance and quality. Superior, stable purelines may be released as new varieties for cultivation.

Year-wise Process (Concise Timeline)

1. Year 1 (Base population): Select 200–500 superior plants from heterogeneous source; harvest individually.
2. Year 2: Grow progeny rows for each selected plant; evaluate uniformity and discard poor lines.
3. Year 3: Preliminary small replicated yield trials with promising purelines; select best performers.
4. Years 4–5: Advanced yield trials across multiple sites/seasons; evaluate stability & resistance.
5. Years 6–7: Large-scale multilocation testing, coordinated or national/state trials; farmer participatory trials possible.
6. Year 8: Release the best pureline as a variety (if superior and stable).

Merits (Advantages)

• Produces genetically uniform, stable and true-breeding varieties.
• Method is simple, inexpensive and well-suited to self-pollinated crops.
• Beneficial for improving local landraces and conserving desirable traits.
• Provides stable parental material for hybridization or further breeding.

Demerits (Limitations)

• Narrow genetic base — resulting variety may have limited adaptation across diverse environments.
• Uniformity increases vulnerability to pests/pathogens — resistance may break down quickly.
• Genetic gain restricted to the variability present in the original population.
• Time-consuming: typically requires multiple years (6–8 years) to fix and test lines thoroughly.

Crops Commonly Improved by Pureline Selection

Wheat, barley, rice, pulses (mungbean, lentil, chickpea), soybean and other predominantly self-pollinated crops.

Applications & Practical Notes

• Excellent for converting heterogeneous farmer varieties into improved, uniform cultivars while preserving local adaptation.
• Often serves as a first step: selected purelines can be used as parents in crosses to combine desirable genes.
• Effective progeny-testing and careful field records are essential; maintain identity of each line during early generations.
• Use of checks, replications and standard statistical analysis (ANOVA, LSD) in yield trials is necessary to confirm superiority.

Example: Practical Recommendations for a Small Breeding Program

1. Start with 300–500 tagged plants (if feasible). Keep accurate tags and plot maps.
2. Grow progeny rows in a single block with row identity maintained; select only those rows showing high mean performance and within-row uniformity.
3. Advance 20–30 promising purelines to preliminary trials; use 2–3 replications and a standard local check.
4. Conduct multi-location trials for top 5–10 lines over 2–3 years before recommending release.

Comparison (Quick reference)

Feature	Pureline Selection
Best suited for	Self-pollinated crops
Genetic uniformity	High (true-breeding)
Genetic base	Narrow (depends on source variability)
Time required	Long (6–8 years typical)
Risk	High vulnerability if uniform resistance is broken

Concluding Remarks

Pureline selection remains a simple, robust and historically important method for improving self-pollinated crops and converting local heterogeneous populations into uniform varieties. While limited by a narrow genetic base, careful use of progeny testing, multilocation trials and integration with other breeding strategies can produce durable and high-performing varieties.