INTRODUCTION TO PLANT BREEDING

AGRONOMY 815 / COURSE NOTES

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

DEPARTMENT OF AGRONOMY / UNIVERSITY OF NEBRASKA

BREEDING METHODS IN SELF POLLINATED CROPS


File SSEL
Fehr, Chapt. 22,23,24,25,27,28
Briggs and Knowles, Chapt. 9, 11, 12
Allard, Chapt. 11, 12, 13
Poehlman, Chapt. 8
Simmonds, p. 128-135.


Several basic methods are available with numerous modifications of each. All the methods have strong and weak points. The method chosen depends upon the nature of the trait(s) of interest, its inheritance and the amount of variability present or available. Economic factors and resources available will also be important in influencing the approach taken.

Additive and epistatic gene action are probably the most important types of genetic effects influencing quantitative traits in self-pollinated crops.

Land Varieties (endemic mixture of types well adapted to a particular region) were the early 'populations' from which new cultivars were derived.


PEDIGREE SELECTION: Fehr, Chapt. 25.


METHOD:


     P1  x  P2  [called single cross]
or  (P1  x  P2) x P3 [called three-way cross]
or  (P1  x  P2) x (P3 x P4) [called double cross] etc.

                                               About 25 - 50 F1 plants are grown
     x x x x x x                               and allowed to self to produce a
     x x x x x x               F1              large F2 population.
     x x x x x x                               Number will depend upon parents
                                               and type of cross.

  x x x x x x x x x                            1000-5000 F2 plants are space
  x x x x x x x x x                            planted. Selections are made based
  x x x x x x x x x                            on individual x plant phenotype.
  x x x x x x x x x            F2              (5-10% selected). Begin keeping
  x x x x x x x x x                            careful records. culling.
  x x x x x x x x x

  x  x  x  x  x  x                             Seed of selected plants is space
  x  x  x  x  x  x             F3              planted to give a progeny row of
  x  x  x  x  x  x                             each selection. 10-30 plants per row.

   x  x  x  x  x                               Superior plants are selected within
   x  x  x  x  x               F4              in and among progeny rows.
   x  x  x  x  x                               Number of selections less than number of
                                               F4 progeny rows.

    x  x  x  x                                 Previous step is repeated.
    x  x  x  x                 F5              Numbers advanced is reduced.
    x  x  x  x                                 May select best of related lines
                                               at this stage.

                                 Selections are evaluated as a plot.
                 F6              commercial seeding rate)
                                 Numbers advanced greatly reduced.

                                 Replicated test.
                 F7              Further selections are made.

  F8    F10                                    Replicated tests continued.

    F11                                        Best one or two lines entered
                                               in regional trials.

Comments:

In the F1 need to produce enough seed for a large F2 population plus reserve in case of crop failure. Comment on PIPELINE THEORY.

F2 -- practical and theoretical considerations such as qual. vs quant. traits plus number of F3 families it is feasible to handle. Ratio F2 : F3 may range from 10:1 to 100:1(e.g., for a wide cross). Warning vigor of many F2 plants may be dependent upon heterozygosity. This will be detected in the F3. QUALITATIVE GENES.

In the F3 need enough plants per F3 family to give an indication of the general features of that family. Selection on single plant performance but emphasis is placed on superior family performance as well.

F4 and F5 -- As in the F3, selection is on single plant basis. It is important to start reducing numbers. Only the best appearing and uniform lines are usually advanced. F4 is usually considered a key selection generation. AMONG VS. WITHIN LINE SELECTION.

F6 -- Families with a common ancestor 1 or 2 generations removed are likely to be very similar to one another. The pedigree record is very important at this point one may want to eliminate all except one member of such closely related families. Assuming there are other equally good families being advanced.

F7 to F11 evaluations permit observation of and elimination of weaknesses, quality testing, and precise yield trial testing.

Many modifications exist based on the pedigree method. Yield testing may be done as early as the F3 or F4 generation. Often the residual seed for a progeny row is used for the yield test after the plant selections have been made. Individual plant selection may be terminated earlier if lines appear uniform. Early generation testing may involve disease/insect resistance screening, cold temperature tests, quality evaluation using micro tests. Note these are culling tests easy to use and very precise.

Note: Many quantitative traits are not easily evaluated visually and this can be a problem with single plant selection.


PEDIGREE METHOD

Advantages:
  1. Early elimination of much material.
  2. Know a great deal about materials collect data and evaluate over a period of time. If resources were limitless, most breeders would use this method.
    3. 3. Rapid approach to homozygosity.
  3. Can push outstanding lines early in the program.
  4. Each selection generation is in a different environment, hence will have the opportunity to have many genetic characters expressed.
  5. The pedigree system is based upon sequential progeny tests, hence will know the genetic structure of the population.
Disadvantages:
  1. Very time consuming extensive data taking/bookkeeping.
  2. Can generally work on only a few crosses.
  3. Total population size is limited.
  4. Very resource intensive (need more land than most inbreeding systems).