INTRODUCTION TO PLANT BREEDING

AGRONOMY 815 / COURSE NOTES

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

DEPARTMENT OF AGRONOMY / UNIVERSITY OF NEBRASKA

SINGLE SEED DESCENT METHOD

Ref. Poehlman, p.124-126.
Brim (soybeans) 1966.Crop Sci.6:220.
Fehr, Chapt. 23
First described by Goulden in 1939.

Objective of this method is to maintain descendants from a maximum number of F₂ plants in the population during the segregating generations in order to reduce loss of superior genotypes for characters which have low heritability such as yield (i.e., maintain genetic variation). This method is now being used widely by breeders of annual, self-pollinated crops. CAPTURING THE GAMETIC ARRAY.

        P1  x  P2                 etc.



     x x x x x x x x
     x x x x x x x x              F1         50 - 100 plants.
     x x x x x x x x


  o.o.o.o.o.o.o.o.o.o.o
  o.o.o.o.o.o.o.o.o.o.o                      2000 - 3000 plants (often less).
  o.o.o.o.o.o.o.o.o.o.o           F2         Single seed is harvested from each
  o.o.o.o.o.o.o.o.o.o.o                      plant.  Note: Identity of F2 plant
  o.o.o.o.o.o.o.o.o.o.o                      is not maintained.


  o.o.o.o.o.o.o.o.o.o.o           F3         Seeds harvested in the previous
                                             generation are grown and a single


  o.o.o.o.o.o.o.o.o.o.o           F4         seed from each plant is harvested.
                                             Again, identity is not maintained.


  o.o.o.o.o.o.o.o.o.o.o           F5


   x x x x x x x x x x
   x x x x x x x x x x                       Plants or progeny rows superior
   x x x x x x x x x x            F6         for the desired characteristics
   x x x x x x x x x x                       are harvested.
   x x x x x x x x x x


   x  x  x  x  x  x  x                       Progeny rows are planted from seed
   x  x  x  x  x  x  x                       of selected F6 plants.
   x  x  x  x  x  x  x            F7         Note:  Each row will have originated
   x  x  x  x  x  x  x                       from a different F6 plant. Harvest rows
                                             superior for the desired characteristics.

            F8         Preliminary yield trials.






      F9  -  F11                              Yield trials.
                                              Best lines(s) entered in regional testing
                                              Seed increase and release of new variety
                                              if data warrants it.

SINGLE SEED DESCENT METHOD

ADVANTAGES:

Simple and inexpensive. Maximize genetic variability within a population (equal representation of genotypes -- no competition or selection). Since only one seed per plant is required optimum growing conditions are not needed. Can grow plants in the greenhouse in the winter, thus increasing the number of generations per year. It is very effective for selecting traits that have very low heritabilities or are recessive (ex., aa is 25% of an F₂, but almost 50% of an F₇), where there is little genetic diversity (difficult to select), and where there is short growing season. See Brim for additional comments.

DISADVANTAGES:

While maintaining genetic variation is an advantage, undesirable plants are maintained in the population. For many traits, it is advantageous to select or cull early to rid obvious off-types. Some plants may be lost from the population and may have to correct for poor germination. Also may be more time consuming than other inbreeding systems.