INTRODUCTION TO PLANT BREEDING

AGRONOMY 815 / COURSE NOTES

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

DEPARTMENT OF AGRONOMY / UNIVERSITY OF NEBRASKA

MUTATION BREEDING


Fehr, Chapt 20
Briggs and Knowles, Chapt 24


MUTATION: Broadly defined as change from one hereditary state to another.
GENE MUTATION: A gene can mutate from one allelic form to another (usually to a less expressed form).
CHROMOSOMAL MUTATION: Segments of chromosomes, whole chromosomes or even entire sets of chromosomes may be involved in genetic change. Gene mutation not necessarily involved, may be just rearrangement (example would be an inversion).

Consequences of somatic and germinal mutation:

• Somatic mutation sector of mutant cells, may only be passed on through vegetative propagation. This can be very important in fruit tree and ornamental plant breeding.

• Germinal mutation passed on to next generation.

Examples of spontaneous mutation rates in corn. (Stadler, 1942. Some observations on gene variability and spontaneous mutation. Spragg Memorial Lectures on Plant Breeding. Michigan State University.)

Forward mutation frequencies at some specific corn loci

MUTATION TRAIT MUTATION FREQUENCY PER
MILLION GAMETES
R r Red to colorless aleurone 492.0
I i Color inhibitor to noninhibitor 106.0
Pr pr Purple to red aleurone 11.0
Su su Non sugary to sugary endosperm 2.4
Sh sh Non shrunken to shrunken endosperm 1.2

Obtaining rare mutations is facilitated by use of selective recovery systems and/or mutagens. This is absolutely critical in most mutation systems. Field testing is difficult because, 1 acre of wheat has about 2,000,000 plants and 1 acre of corn has about 25,000 plants. Can be very hard to find a low occurring phenomenon in a large field.

MUTAGENS -- agents that induce mutations above the spontaneous rate.

Many factors will influence mutation frequency following exposure to a mutagenic agent . . . e.g., Plant materials treated, environment in which treatment made, mutagenic agent used, mutagen dosage. Mutagens should be handled with extreme care -- powerful carcinogen.

Mutagens are used to generate variability:

  1. Mutagenize pollen. Dominant mutations will appear in the next generation, further generations of selfing will reveal recessive genes.


  2. Mutagenize seeds. Depends whether somatic or germinal cells are affected if selection of mutants will be possible.

  3. Mutagenize plant or callus tissue. Again, effective selection of mutant types will only be possible if germinal cells are affected.


MUTAGENIC AGENTS

  1. IONIZING RADIATIONS


    2. (X-rays, neutrons, gamma rays -- less damage, radioisotopes, ultraviolet light)

    Changes that occur if ionization in/near chromosomes in seeds, plant/plant parts/pollen:

      1. Structural changes within the gene;
      2. Deletion of genes or sequences of genes;
      3. Chromosomal rearrangements;
      4. Increase/decrease in frequency of chiasmata;
      5. Misdivision of the centromere;
      6. Loss/addition of chromosomes;
      7. Alterations in number of chromosome sets due to spindle destruction.



  2. CHEMICAL MUTAGENS -- alkylating agents (ethyl methane sulfonate (EMS), ethyleneimine nitroso ethyl urethane, nitroso ethyl urea, etc.).


    3. Simpler to apply and less damaging than radiation. Are applied (usually by soaking) to seeds, buds, roots or other plant propagules.

    Chemical mutagens produce more gene mutations and fewer chromosome disruptions.

MUTAGENIC TREATMENT -- Mutagen dose . . . a very high dose: sufficient to kill about 50% of seed treated, (LD50). A lower dose will cause a growth inhibition in 50% of the treated plants without causing a reduction in germination.

Survivors are referred to as M1 plants. They have reduced vigor and are often sterile. Almost all mutants recessive. M2 (2nd generation after mutagen treatment = segregating generation for recessive traits.

VEGETATIVELY PROPAGATED PLANTS -- e.g., sugarcane, potatoes, fruit crops, ornamentals.

Can mutagenize any part of the plant which may give rise to a plant propagule -- tubers, rhizomes, cuttings, grafts or plantlets.

Generally will treat as early in the formation of the propagule as possible because there will be fewer target cells and if one is affected it will have a greater result (affect a larger proportion of the tissue which is derived from the affected cell).



RESULTS OF EARLY MUTATION EXPERIMENTS

Soon after Stadler demonstrated that mutations could be induced in barley by X-rays and UV mutation experiments were begun in barley at Svalof, Sweden.

    1. Isolated mutant types of barley with denser spikes and shorter, stiffer straw. Over 200 ert (erectoide) mutants, as they were designated, were identified at 25 or more loci on the seven chromosomes of barley.

      All ert mutations are pleiotropic, affecting more than one plant characteristic. Mutagens can be used to explain genetic phenomena. Sometimes many mutants combined will have a similar effect to a single major gene.



Mutagens have proved useful in gene substitution from alien chromosomes, e.g., Sears' work in wheat.

Mutation breeding is not an exclusive breeding procedure but rather a tool to produce new, heritable variation which may be utilized in traditional recombination breeding programs.

PRODUCTS OF MUTAGENESIS

May get . . . gross morphological/physiological changes single gene mutations -- affecting both qualitative/quantitative traits somatic mutations, breakup of linkage groups, gene(s) transfer, or pollen sterility (particularly in rice).

Mutation breeding was "in vogue" after the second world war, particularly due to the interest in atomic energy. Many experiments were done near the atomic bomb test sites near Bikini Island and near Los Alamos, New Mexico. The International Atomic Energy Agency (IAEA) still has many active mutation breeding programs. In the developed world, mutation breeding is generally now out of favor (science has a long history of doing what is trendy). However in certain specific cases, mutation breeding can be extremely effective. Some examples are inducing male sterility or dwarfing genes in rice. In triticale, mutations have lessened the effect of a major dwarfing gene so that intermediate height classes are available instead of only tall or short classes. Generally, mutations can be readily obtained that affect height, maturity, and fertility. They have also been used to develop improved nutritional quality (particularly in soybeans) and disease resistance.

It should also be recognized that many of the early experiments to understand biochemical pathways used mutants lines that blocked the pathway at different enzymatic points. Mutation has been a very powerful tool in understanding plant physiology and biochemistry.

REF. Gottschalk, W. and G. Wolff, 1983. Induced Mutations in Plant Breeding. Springer-Verlag, Berlin.