PB Ch 19. Single Seed Decent Method
Definition and Origin
|
Objective of SSD:
- Rapidly advance generations with minimal space, effort, and labour.
- At the end, a RANDOM SAMPLE of near-homozygous genotypes representing full F2 diversity is obtained.
- This is different from bulk (where natural selection changes genotype frequencies) and pedigree (where breeder selection changes frequencies from F2).
6.2 Procedure
|
Generation |
Population size |
Action |
|
F1 |
Normal |
Grown normally; seeds harvested in bulk |
|
F2 |
1,000-2,000 plants |
ONE seed taken from each F2 plant and all seeds mixed (bulked) — this ensures each F2 plant represented in all subsequent generations |
|
F3 to F5/F6 |
Equal to F2 size |
ONE random seed from every plant planted in bulk to raise next generation; grow at VERY HIGH density; use greenhouse/off-season nurseries for 2-3 generations per year |
|
F5/F6 |
~97% homozygous |
Plants now near-homozygous; space-plant 1,000 to 5,000 individuals |
|
F6/F7 |
100-500 progenies |
Individual plant selections; individual plant progenies grown; selection mainly among progenies |
|
F7/F8 |
20-50 progenies |
Preliminary yield trials and quality tests begin |
|
F8/F9+ |
Few lines |
Coordinated yield trials; variety release if superior to checks |
Key Features of SSD
- NO selection (natural or artificial) till F5/F6 — each F2 plant equally represented in final population
- Final population = RANDOM SAMPLE from F2 — every F2 genotype has equal probability of surviving
- Generations from F2 to F5/F6 raised from a BULK of one seed per plant — maximum retention of F2 diversity
Merits and Demerits
|
Merits |
Demerits |
|
Fastest method to reach homozygosity — 2-3 generations per year using greenhouse; combined with speed breeding (Watson et al., 2018) achieves 4-6 generations/year |
No selection during inbreeding — many inferior genotypes reach F5/F6 stage and must be screened there |
|
Minimal space requirement — very high density during inbreeding phase; space only needed for F1 and final selection stage |
Progressive reduction in population size each generation due to plant deaths from disease, insects, accidents |
|
Best use of greenhouse and off-season nursery facilities |
Cannot select for environmentally sensitive traits (lodging, tillering) in greenhouse |
|
Retains full F2 genetic diversity — no selection pressure alters genotype frequencies |
In some crops (pulses: chickpea, lentil), high plant mortality is a serious problem |
|
No pedigree records needed during inbreeding |
Random elimination may occasionally lose useful genotypes |
All Three Methods Compared — Pedigree vs Bulk vs SSD
|
Feature |
Pedigree |
Bulk |
SSD |
|
Selection during inbreeding |
Yes — artificial from F2 |
Natural selection mainly |
NO selection at all |
|
Plant density |
Space-planted each generation |
Commercial density during bulk |
Very high density |
|
Genotype frequency change from F2 |
Yes — breeder selection changes frequencies |
Yes — natural selection changes frequencies |
No — equal representation maintained |
|
Genetic diversity retained from F2 |
Partial — breeder may miss useful types |
Changes over time |
Fully retained — random sample |
|
Records needed |
Extensive pedigree records |
None during bulk |
None during inbreeding |
|
Generations per year |
1-2 (field + off-season) |
1 normally |
2-3 (greenhouse + speed breeding) |
|
Time to variety release |
12-13 years |
15+ years |
8-10 years (with greenhouse) |
|
Best for |
Qualitative and quantitative traits; most common method |
Traits favoured by natural selection; barley USA |
Rapid inbreeding; large segregating populations; soybean, wheat CIMMYT |
|
PYQ: CSE 2016 (Q7, 20M) — Describe pedigree, bulk and SSD methods. |