PB Ch 15. Pure Line Selection

Johannsen's Pure Line Theory (1903)

Johannsen's Experiment — Three Lines of Confirmatory Evidence

• Evidence 1: In Line 13 (mean seed weight 450 mg), seeds divided into 200, 300, 400, 500 mg classes. Progenies of all classes gave 458-475 mg — entirely environmental variation, no heritable basis.
• Evidence 2: From a pure line, selected for large and small seeds for 6 generations. Both selections converged at 680-690 mg — selection within pure line was completely ineffective.
• Evidence 3: Parent-offspring regression within Line 13 = -0.018 ± 0.038 (statistically zero). In original commercial lot = 0.336 ± 0.008 (highly significant). Zero correlation within pure line = non-heritable variation.

Two main conclusions of the pure line theory:

• A self-fertilised population (commercial seed lot) = mixture of several genetically different homozygous genotypes; variation has a genetic component; selection between pure lines is effective.
• Within a pure line = homozygous plants of identical genotype; variation purely environmental; selection within a pure line is ineffective.

Characters of Pure Lines 

• All plants within a pure line have the same genotype
• Variation within a pure line is environmental and non-heritable
• Pure lines are stable; become variable over time due to: mechanical mixture, natural hybridization, natural mutation, chromosomal aberrations 

Applications of Pure Line Selection

Used to improve: (1) local varieties/landraces; (2) old pure line varieties; (3) introduced varieties. 

Procedure — General Steps

The pure line selection has three steps:

Year 1 (Individual Plant Selection): 

• Select 200-3000 superior plants from a local variety or mixed population. 
• Harvest each plant individually. 
• Select for easily observable characters: flowering date, maturity, plant height, disease reaction, grain type.

Year 2 (Progeny Evaluation — Head-to-Row): 

• Progenies of individual plants grown separately (plant-to-row). 
• Elaborate data taken on plant height, duration, grain type, ear characters, disease reaction.
• Disease epiphytotics created artificially. 
• Inferior progenies (diseased, segregating, off-type) rejected. 
• Superior progenies harvested separately. If needed, repeat for another year.

Year 3 (Preliminary Yield Trial): 

• Selected progenies ('cultures') grown in replicated trial with best local variety as check (grown every 15-20 cultures). 
• Superior cultures selected; number drastically reduced.

Years 4-5 (Initial Yield Trials): 

• Testing against local checks; detailed statistical analysis; 4-5 promising cultures identified.

Years 6-8 (Multi-location Trials / AICRP): 

• Evaluation at several locations through All India Coordinated Research Programmes. 1-2 outstanding lines identified.

Year 9 (Release): 

• Best line multiplied, named, released through Variety Release Committee (CVRC). DUS testing under PPV&FR Act 2001.

Merits and Demerits of Pure Line Selection

Achievements of Pure Line Selection in India