Notes on Handling of Segregating Generations in Plant Breeding

Handling of Segregating Generations in Plant Breeding

General Overview
  • Mass selection and pure line selection are not applicable in segregating populations (e.g., F2, F3).
  • Three primary methods for handling segregating generations:
    • Pedigree Method
    • Bulk Method
    • Back Cross Method
  • Objectives: Develop pure line varieties.
Pedigree Method
  • Definition: Involves individual plant selection from segregating generations with pedigree records maintained.
  • Process:
    1. Hybridization: Selected parent plants are crossed to produce F1 seeds.
    2. F1 Generation: Seeds from hybridization attempt are planted; seeds from 20-30 plants are harvested for F2 generation.
    3. F2 Generation: Grow about 10,000 plants. Select and harvest about 500.
    4. F3 Generation: Raise 30 or more progenies from F2 selected plants, selecting 100-400 superior plants.
    5. F4 Generation: Space planting of F3 seeds; selection of fewer desirable plants than in F3.
    6. F5 Generation: Individual progenies are planted in 3 or more row plots for comparison.
    7. F6 Generation: Similar to F5; segregation progenies get eliminated.
    8. F7 Generation: Preliminary yield trials with checks; quality tests conducted.
    9. F8 to F12 Generations: Conduct multi-location yield trials; tests for quality and disease resistance.
    10. F10 or F13 Generation: Seed multiplication for distribution.
  • Merits:
    • Excellent for high heritability characters.
    • Traceability through pedigree records.
    • Progeny testing based on genotypic rather than phenotypic value.
    • Efficient breeding with early identification of superior populations.
  • Demerits:
    • Costly, labor-intensive, skilled staff required.
    • Time-consuming record maintenance.
    • Ineffective yield selection in F2 and F3 due to decreased genetic variation.
Application of Pedigree Method
  1. Selection of desirable plants from self-pollinated crops.
  2. Correcting specific weaknesses in established varieties.
  3. Useful in developing superior recombinant types (transgressive breeding).
  4. Suitable for improving characteristics like disease resistance and plant height.
Bulk Population Method
  • Definition: Also known as mass method; involves growing a species in bulk from F1 to F5.
  • History: Introduced by Nilsson Ehle in 1908.
  • Process:
    1. Hybridization: Select and cross parents for breeding objectives.
    2. F1 Generation: Space plant and harvest in bulk.
    3. F2-F6 Generations: Grown with commercial seed rate, harvested in bulk.
    4. F7 Generation: Space planting of 30-50,000 plants; select 1000-5000 with superior phenotypes.
    5. F8 Generation: Grow individual plant progenies from selections.
    6. F9 Generation: Conduct preliminary yield trial and quality tests.
    7. F10-F12 Generations: Replicate yield trials across locations; evaluate against standard checks.
    8. F13 Generation: Seed multiplication for distribution.
  • Merits:
    • Simple, convenient, and inexpensive.
    • Less attention required in earlier generations.
    • No need for pedigree records.
    • Eliminates undesirable types while increasing desirable frequency.
    • Capable of isolating transgressive segregates.
  • Demerits:
    • Longer time to develop new varieties compared to pedigree method.
    • Limited breeder input in selection process.
    • Information on inheritance may be lost.
Single Seed Descent Method
  • Definition: Variant of the bulk method focusing on rapid advancement of generations.
  • Process:
    1. Select a single seed from each of 1000 F2 plants to bulk for the F3 generation.
    2. Repeat for subsequent generations until F5 or F6 to achieve near homozygosity.
    3. Select and grow individual progenies for evaluation in later generations.
  • Features:
    • No selection during F2-F6 until homozygosity is reached.
    • Uses off-season nurseries and greenhouses to speed up generation advancement.
  • Advantages:
    1. Rapid advancement in breeding cycles.
    2. Minimal space, effort, and labor required.
    3. Ensures random sample representation from the original population.
  • Challenges:
    1. No selection enforced in early generations.
    2. Risks associated with reducing population size over generations.
Summary of Key Points
  • All methods aim to create superior plant varieties but differ in techniques, timing, and flexibility.
  • Each method has its specific application, advantages, and disadvantages, impacting the breeding strategy selected by plant breeders.