PB Ch 14. Selection in Self Pollinated Crops
1.1 What is Selection?
|
Selection: The isolation of desirable plant types from a population and allowing them to contribute to the next generation. It is one of the two fundamental steps of any breeding programme: (1) creation of variation and (2) selection. |
Two basic characteristics of selection:
(1) Selection is effective for heritable differences only.
(2) Selection does not create new variation — it only utilises variation already present in a population.
Three Basic Principles of Selection
1. Selection operates on existing variability: Selection can only discriminate between individuals when sufficient variation is present. It cannot create new variation.
2. Selection acts only through heritable differences: Environmental (non-heritable) variation is useless for selection. The effectiveness of selection primarily depends upon the degree to which phenotype reflects genotype.
3. Selection works because some individuals are favoured in reproduction: Superior individuals are retained for reproduction while others are discarded.
Genetic Basis — Effect of Self-Pollination on Homozygosity
- Self-pollination rapidly increases homozygosity. Every generation of selfing reduces heterozygotes to 50% of that in the previous generation.
- Formula for proportion of completely homozygous plants: [(2^m - 1) / 2^m]^n, where m = generations of self-pollination, n = number of genes segregating.
|
Selfing generations |
AA (%) |
Aa (%) |
aa (%) |
% Homozygosity |
|
F1 (0) |
0 |
100 |
0 |
0% |
|
F2 (1) |
25 |
50 |
25 |
50% |
|
F3 (2) |
37.5 |
25 |
37.5 |
75% |
|
F4 (3) |
43.75 |
12.5 |
43.75 |
87.5% |
|
F5 (4) |
46.875 |
6.25 |
46.875 |
93.75% |
|
F6 (5) |
48.44 |
3.13 |
48.44 |
96.87% |
|
F7 (6) |
49.22 |
1.56 |
49.22 |
98.44% |
|
F8 (7) |
49.61 |
0.78 |
49.61 |
99.22% |
|
F9 (8) |
49.80 |
0.39 |
49.80 |
99.61% |
|
F10 (9) |
49.90 |
0.20 |
49.90 |
99.80% |
- Key insight : number of genes segregating does NOT affect the rate of homozygosity increase per gene. Similarly, linkage does not affect this rate.
- Variety types in self-pollinated crops: Almost all varieties are pure lines — homozygous, homogeneous populations.
- Exception: multiline varieties (homozygous but heterogeneous — mixture of near-isogenic lines).
Classification of breeding methods for self-pollinated crops:
- Without hybridization: Plant introduction, pure line selection, mass selection, progeny selection
- With hybridization: Pedigree method, bulk method, backcross method, single seed descent (SSD), distant hybridization