INTRODUCTION TO PLANT BREEDING

AGRONOMY 815 / COURSE NOTES

P. STEPHEN BAENZIGER, 338 Keim Hall, 472-1538

DEPARTMENT OF AGRONOMY / UNIVERSITY OF NEBRASKA

PURE LINE SELECTION

Ref. Briggs and Knowles, Chapt. 9.


Johannsen's experiments (reported in 1903) dealing with selection for seed weight in beans showed that a land variety in a self pollinated crop is composed of a mixture of pure lines. Diversity, or variation, is between lines and not within lines.

Pure line selection is used to exploit land varieties where desirable types exist. The best genotype already present in the mixed population is isolated through careful testing procedures.

Method: 1. A large number of selections from the original population are made.
2. Progeny rows are grown for observation.

Poor lines are eliminated.

This step is repeated for several generations.

3. Remaining lines are evaluated in replicated yield trials and the best entered into regional trials.
4. New variety (= progeny of a single pure line) if released if data warrants it.
Advantages: 1. Pure line is very uniform in appearance and performance.
Disadvantages: 1. New genotypes are not 'created' -- limited to the genotypes already present in the original population. No increase in population variation in this method.
2. Pure line may be limited in adaptation.

This method was used as a logical first step in developing a uniform variety in a new crop/region. ex., Cheyenne selected out of Crimea, other selections out of Turkey.

Variability and recombination is limited. Sources of variability -- gene mutation and limited outcrossing.

This method has limited practical application today. Can be used where heterogeneous varieties or inbred lines are released, ex., Scout 66 from Scout, Centurk 78 from Centurk, and Redland from Brule.

"It is now easily understood that the action of selection cannot go beyond the known limits -- it must stop when the purification, or practically speaking, the isolation of the most strongly divergent line is complete." CAN MIX MASS SELECTION AND PURELINE BREEDING METHODS BY SELECTING PURELINES AND THEN COMPOSITE THEN TO FORM A NEW VARIETY. Johannsen (1903)

NON TRADITIONAL BREEDING PROCEDURES

Traditionally, breeding self pollinated crops involved . . .

Cross between 2 parents

selfing several segregating generations to obtain a high level of homozygosity.

Yield testing

When the 'cycle' was complete, superior segregates were crossed to start a new cycle.

Pedigree and bulk methods were developed during the early years of this century. These methods were followed by the backcross procedure and single seed descent. Disadvantages of these procedures . . . Opportunities for gene recombination are limited by narrow input into the gene pool -- only 2 - 4 parents -- plus restricted opportunities to break up linkage blocks.

Modifications to increase contributions into the gene pool and to increase recombination:

  1. Large numbers of crosses;
  2. Early yield testing;
  3. Population improvement . . . Recurrent selection.