INTRODUCTION TO PLANT BREEDING

AGRONOMY 815 / COURSE NOTES

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

DEPARTMENT OF AGRONOMY / UNIVERSITY OF NEBRASKA

HYBRIDS IN SELF POLLINATED CROPS

Fehr, Chapt. 12, 29, 35
Briggs and Knowles, Chapt. 15, 17, 18.

Motivation is protecting a variety (have to purchase new seed) or heterosis. Plant variety protection laws are not working well and the amount of certified seed bought by growers depends upon where you live (big regional differences).

Utilization of hybrid vigor -- pioneered in corn (1930's), a cross pollinated crop. However, heterosis for yield and other characteristics have been observed in self-pollinated crops e.g., sorghum, wheat, barley, oats, rice, etc. Heterosis is generally higher in diploids than in polyploids.

Secret of success in the hybrid industry lies in the ability to find the right combination of inbreds to produce competitive hybrids for the marketplace.

HETEROSIS -- definitions vary, but we shall consider it the improvement of the F₁ over the best parent (1). Classically defined as improvement over mid-parent (2). Commercially, heterosis is defined improvement over the best commercial check (3).

GENETIC CAUSES OF HETEROSIS . . . Thought to be one or more of the following:

1. The accumulated action of favorable dominant or partially dominant genes dispersed amongst the two parents, i.e., dominance. AA=Aa > aa



2. Complementary interaction of additive, dominant or recessive genes at different loci, i.e., non-allelic interaction or epistasis. A_B_ > A_ or B_ or A_ + B_



3. Favorable interaction between two alleles at the same locus, i.e., intralocus or inter-allelic interaction . . . overdominance. Note: this can be fixed in polyploids. Aa > AA, aa. (Hayes & Foster, 1976. In Janossy & Lupton (eds.) Heterosis in Plant Breeding . Elsevier, NY.)

Theoretically, it should be possible to fix heterosis caused by (1) and (2). Inbred lines are continually being improved genetically. However, linkage may make accumulation of all favorable dominants, for example, a difficult task -- correct recombination must take place between every dominant and recessive allelic combination. With several loci involved, the probability of this happening with effective selection of correct gametes may be extremely low. Also, breeders are constantly adding genes to the gene pool.

It is not possible to fix heterosis caused by intralocus interaction (3) in the homozygous condition. Only by the production of F₁ hybrids or the duplication of the relevant chromosome segments can heterozygosity be maintained, and this type of gene action be exploited in diploid species. Exception is polyploids.

Another possibility giving rise to heterosis is cytoplasm x nuclear interaction. We have already observed the interaction between chromosomes and the cytoplasm in male sterility systems.

There is some evidence, though limited as yet, indicating a nuclear-cytoplasm effect in heterosis. Explanation of heterosis probably includes varying proportions of all of the above.

Utilization of hybrid vigor is dependent upon a system for economically producing the F₁ seed. In most normally self-fertilized crops some type of male sterility system is mandatory if hybrid vigor is to be exploited.

CYTOPLASMIC MALE STERILITY-NUCLEAR FERTILITY RESTORER SYSTEM

This is the most widely used system in the hybrid industry, e.g., sorghum and wheat; also, field beans, cotton, flax, tobacco, carrots, petunias, red peppers, rice and many grasses.

Breeding and production involves 3 steps:

1. Development and maintenance of male sterile lines. A and B-lines;



2. Development of fertility restoring lines. R-lines;



3. Utilization of these lines in commercial production of hybrid seed.

• Can you make 4-way hybrids? Why R lines are cytoplasmically male sterile?

Difficulties associated with cms system:

• Mechanism is often environmentally sensitive.



• Modifiers may be necessary for complete restoration; e.g., in wheat.



• Genetic vulnerability. Exemplified by the Southern Corn Leaf Blight epiphytotic in 1972. T-cytoplasm particularly sensitive. Most hybrids at the time contained T-cytoplasm. Same situation could arise in self-pollinated crop where a single cytoplasm was widely used.



• Pollen longevity and movement may be limited in naturally self-pollinated species. Also, flower design -- stigma may not be designed for receiving foreign pollen.

GENETIC MALE STERILITY

Recessive alleles for male sterility have been found in almost all diploid plant species.

• ms ms sterile

• Ms ms fertile

• Ms Ms fertile

This system is very simple genetically, and can be readily used to promote cross pollination in self-pollinated species to maintain and enhance genetic diversity with minimum labor.

Problem in hybrid production . . . maintenance of male sterile lines.

The ms line must be homozygous recessive to be used in making the hybrid.

Could utilize a heterozygous Ms ms maintainer but then one must be able to identify the homozygotes vs. heterozygotes.

• Rogue fertiles at flowering . . . costly.
• Link Ms with a chemical sensitive gene and spray.
• Link Ms to aleurone color gene and sort seed via electronic color sorting.
• Tertiary trisomic system (Ramage).

MALE GAMETOCIDES

Frankel & Galun. 1977.

Pollination Mechanisms, Reproduction and Plant Breeding.

Gametocide = Pollen Suppressor = Chemical Hybridizing Agent.

Ethrel (2-chloroethylphosphonic acid) effective in some cereals. Chemical applied prior to anthesis.

DCIB (sodium 2, 3 dichloroisobutyrate) and sodium dichloroacetate have also been found to be effective in some crops.

GA (gibberellic acid) was found to be a male gametocide in onions, however, GA has also been shown to overcome nuclear male sterility, i.e., may also have application as a FERTILITY RESTORER.

2-carboxylic azetidine: very effective in wheat -- but causes cancer in dogs.

Difficulties with male gametocides:

• Appropriate developmental stage for application . . . can have adverse effects if wrongly timed.
  
  
• Effect of the environment is hard to predict. Chemicals fail.
  
  
• Difficult to have uniform (for growth stage) fields for application of gametocide.
  
  
• Weather can prevent application of gametocide at optimal treatment stage.