1. Introduction
Bulk selection is a classical and widely used method in plant breeding, particularly effective for self-pollinated crops. The method emphasizes handling segregating populations as a whole (a bulk) during early generations so that natural environmental forces eliminate poorly adapted genotypes. Artificial selection is delayed until later generations when populations have become more homozygous and stable.
This approach balances simplicity and efficiency, allowing breeders to manage large populations with modest resources while encouraging adaptation to local agro-climatic conditions.
2. Definition
Bulk selection is a breeding method in which seeds from an entire segregating generation (F2 and subsequent generations) are harvested together (bulked) and sown as a composite population across successive generations. Natural selection acts during these early generations; deliberate (artificial) selection is introduced later when the population attains relative genetic stability.
3. Historical Background
The bulk method was first systematized by Nilsson-Ehle (1908) for cereals such as wheat and barley. Its adoption grew across Europe and later worldwide because of its suitability for selfing crops and its low operational cost. In many breeding programs, bulk selection became the default early-generation strategy before switching to more detailed selection methods.
4. Objectives
- Simplify management of large segregating populations.
- Allow natural selection to improve adaptation and eliminate weak genotypes.
- Preserve genetic variability across early generations.
- Ultimately develop stable, superior pure lines for release as varieties.
- Reduce cost and labour compared with pedigree-based systems.
5. Underlying Principles
5.1 Genetic segregation and recombination
Crossing two parents produces F1 hybrids that are largely heterozygous. The F2 generation shows maximum segregation and recombination — a rich source of variation. Bulk selection postpones intensive selection until this variation has been reshuffled and allowed to stabilize.
5.2 Natural selection
Growing the bulk population under target environmental conditions allows climatic and biotic stresses to remove unfit genotypes naturally, thereby enriching the population for adaptation to those conditions.
5.3 Increasing homozygosity
Selfing over generations increases homozygosity and reduces heterozygosity. Artificial selection in later generations (typically F7–F8 onward) is therefore more reliable because phenotypes reflect genotype more closely.
6. Detailed Year-by-Year Procedure
Year 1 — Cross and F1 production
Perform deliberate crosses between selected parents (Parent A × Parent B). Harvest F1 seed; F1 plants are generally uniform.
Year 2 — F2: Establish a large segregating population (Bulk starts)
- Sow a very large F2 population to capture the full spectrum of genetic recombinants.
- Do not perform selection; grow under representative field conditions to allow natural pressures to act.
- Harvest seed from all plants together — this is the first bulk.
Years 3–7 — F3 to F7: Bulk propagation and natural selection
Continue sowing the bulked seed each year and harvesting seed in bulk. Over these generations:
- Natural selection (disease, drought, competition) eliminates weaker lines.
- Genetic drift and chance events may alter allele frequencies — maintain large population sizes to minimize unwanted drift.
- Observe and record major changes (e.g., uniformity, maturity) but avoid heavy selection that might prematurely reduce variability.
Year 8 — F8: Begin artificial selection
By F8 (or when sufficient homozygosity is assumed), begin selecting individual plants based on agronomic merit: yield components, disease resistance, grain quality, maturity, and plant type. Mark, harvest, and keep seed of selected plants separately for progeny testing.
Years 9–10 — Early yield trials and progeny testing
Grow progenies from selected plants under replicated conditions to assess true performance; discard inferior lines and retain promising ones.
Years 11–13 — Advanced multi-location testing
Evaluate the best lines across different environments and seasons for stability and broad adaptation. Compare to standard checks.
Year 14+ — Release and seed multiplication
Lines that consistently outperform checks and satisfy quality requirements are proposed for official release. Multiply breeder, foundation, and certified seed for dissemination.
7. Advantages
- Operational simplicity — few records required in early generations.
- Cost-effective — well-suited for programs with limited resources.
- Improves adaptation via natural selection — beneficial for marginal environments.
- Preserves genetic variation across several generations.
- Works well for quantitative traits influenced by environment (yield, drought tolerance).
8. Limitations and Risks
- Slow progression to release — many years before stable lines emerge.
- Natural selection may inadvertently remove rare but valuable genotypes.
- Less precise for monogenic traits or when immediate fixation of a trait is desired.
- Risk of genetic drift — especially if population size falls below recommended levels.
- Requires large land area for bulk generations under representative conditions.
9. Modifications and Hybrid Strategies
9.1 Modified Bulk Method
Bulking in early generations but applying obvious selection (e.g., remove disease-symptomatic plants) to enrich the population for critical traits without losing diversity for other traits.
9.2 Mass-pedigree Method
Combine bulk handling in F2–F4 with pedigree-based advancement from F5 onward. This saves labour initially and increases selection precision later.
9.3 Early-bulk, late-select
Maintain bulk until F6–F7 and then apply stringent selection and pedigree recording in F8+. This leverages maximum homozygosity before selection.
9.4 Stress-enforced bulk
Grow bulk populations under specific stress (drought, salinity, heavy disease pressure) so that only tolerant genotypes survive. Useful for breeding climate-resilient varieties.
9.5 Head-row and head-to-row modifications
Grow heads (or spikes) of selected F2 plants in separate head-rows for observation, then bulk seed from chosen rows. This helps to retain more diversity while allowing identification of promising families.
10. Practical Applications and Examples
Wheat: Bulk and modified bulk methods have produced several regionally adapted wheat varieties worldwide. Nilsson-Ehle’s early work is a prime example.
Rice: Bulk selection used to develop landraces and locally adapted lines, often combined with on-farm selection by farmers.
Barley and Pulses: Improved lines for drought tolerance and disease resistance were developed using bulk and modified bulk strategies.
11. Best Practices
- Maintain large effective population sizes (preferably several thousand plants).
- Use representative target environments for bulk generations to ensure adaptation.
- Record basic field observations (disease outbreaks, abnormal weather) across generations.
- Combine bulk selection with progeny testing to validate that phenotype is heritable.
- Consider participatory selection with farmers during the late-bulk phase to capture farmer-preferred traits.
