INTRODUCTION TO PLANT BREEDING

AGRONOMY 815 / COURSE NOTES

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

DEPARTMENT OF AGRONOMY / UNIVERSITY OF NEBRASKA

A fundamental equation for plant breeders is:

PHENOTYPE = GENOTYPE + ENVIRONMENT + GENOTYPE X ENVIRONMENT

or P = G + E + G x E.

Fehr, Chapt. 18
Allard, Chapt. 9
Briggs and Knowles, Chapt. 8
Simmonds, Pages 91-99.

Measuring the environmental variation is critical for plant breeders because their varieties or hybrids will be grown in the environment. Testing environments should adequately represent the environments that the genotype will eventually be grown in or identify environments that are unsuitable for the genotype. This and resources will determine the number of environments that are used for testing. While environments are usually considered a random effect, it should be understood that parts of the environmental variance can be controlled (e.g., fertility, weed control, tillage system, irrigation, etc.). Hence E and G x E can have a fixed variation component.

Because environmental and genotype x environmental variation lessens heritability, breeders want to decrease these aspects of the variation or at least explain the variation. Methods of controlling environmental variation include:

1. Cultural practices:
   
   
   1. Using fertilizers to remove deficiencies and maximize yields;
      
      
   2. Irrigation to remove the effects of drought and soil variation;
      
      
   3. Plant population and row spacing to provide environmental buffering (e.g., semi-dwarf wheat and narrow row spacing with higher populations); and
      
      
   4. Date of planting or harvest to monitor possible environments.
      
      
   2. Use of experimental designs.
      
      
   3. Use of well characterized check varieties (e.g., Scout 66 for mill and bake comparisons; best competitive varieties; and differential checks to identify unique circumstances -- powdery mildew isolines).
      
      
   4. Use of controlled environment chambers.
      
      
   5. Note: It is very difficult to get uniformly good testing environments. It is even more difficult to et uniformly poor testing environments (e.g., salinity or aluminum toxicity testing sites).
      
      

   A KEY QUESTION THAT BREEDERS MUST CONSIDER IS HOW MANY ENVIRONMENTS ARE NEEDED TO ADEQUATELY REPRESENT THE ENVIRONMENTS THAT THE GENOTYPE WILL BE GROWN IN AND HOW TO MAXIMIZE THE TESTING EFFORTS RELATIVE TO THE VALUE OF THE LINE. ADVANCED LINES ARE TESTED IN MORE ENVIRONMENTS THAN EARLY GENERATION LINES. GENERALLY EARLY GENERATION MATERIAL EXHIBIT LARGER DIFFERENCES (EASIER SELECTION) AND CAN BE READILY CULLED. THIS RAISES THE CONCEPT OF SELECTION ENVIRONMENTS RATHER THAN TESTING ENVIRONMENTS.

   A standard ANOVA will contain models such as :

   Trait = mean + Years + Locations + Y x L + Reps (Y, L) + Genotypes + G x Y + G x L + G x Y x L + Error.

   Y and L are often pooled as environmental variance. IN general for plant breeders G x E is the biggest problem. Only when G x E = 0 do the genotypes respond similarly in all environments.

   Now returning to heritability:

   Just as the phenotypic variance can be partitioned in genetic and environmental variance, the genetic variance can be partitioned.

   Breeding Value: the value of an individual judged by the mean value of its progeny.

   Additive genetic variance -- the variance of the breeding value which is associated with the average effect of substituting gene A for gene a. The variance of average effects of a gene. Can be fixed by inbreeding.

   Dominance variance -- variance associated with the heterozygote deviating from the midparental value (the additive value). The variance of gene interactions within a locus. It cannot be fixed with inbreeding.

   Interaction variance -- variance associated with interloci gene interaction (epistasis). The portions that do not involve dominance deviation interactions can be fixed by inbreeding.

   "There are, however, two distinctly different meanings of 'heredity' and heritability, according to whether they refer to genotypic values or to breeding values. A character can be hereditary in the sense of being determined by the genotype or in the sense of being transmitted from the parents to offspring, and the extent to which it is hereditary in the two senses may not be the same. The ratio of Vg/Vp expresses the extent to which individuals' phenotypes are determined by their genotypes. This is called heritability in the broad sense, or the degree of genetic determination. The ratio of Va/Vp expresses the extent to which phenotypes are determined by the genes transmitted from the parents. This is called heritability in the narrow sense, or simply heritability."

   Falconer p.113.

   Remember that heritability, and additive, dominance, and interaction genetic variances are specific to the populations they are derived from:

   (s_a)² = 2pq [a + d(q - p)]², (s_d)² = (2pqd)².

   Hence, each value will change with p and q.