Principles of Plant Genetics and Breeding

Purpose and expected outcomes
It was previously pointed out that mutation is the ultimate source of variation. Without adequate variation, plant
breeding is impossible. To start a breeding program, the breeder must find the appropriate genotype (containing the
desired genes) from existing variation, or create the variation if it is not found in nature. Mutagenesis is the process
by which new alleles are created. The purpose of this chapter is to discuss mutagenesis as both a technique and a breeding
method. The newly created mutants may be used as parents in future breeding programs, in which case mutagenesis
is a breeding technique as a source of variation. However, an induced mutant can be systematically processed
through conventional breeding steps to be released as a cultivar, hence making it a breeding method (mutation
breeding). Mutations arise spontaneously in nature and are pivotal in natural evolution. After completing this
chapter, the student should be able to:
1 Define mutation and mutagenesis.
2 Discuss mutagenic agents.
3 Discuss the steps in a mutation breeding program.
4 Discuss the limitations of mutation breeding.
Brief historical perspective
The discovery of the mutagenic effects of X-rays on the
fruit fly (Drosophila) by H. Muller in the 1920s paved
the way for researchers to experiment with its effects on
various organisms. In 1928, H. Stubbe demonstrated
the use of mutagenesis in producing mutants in
tomato, soybean, and other crops. The first commercial
mutant was produced in tobacco in 1934. Reports by
B. Sigurbjornsson and A. Micke mentioned 77 cultivars
that were developed via mutagenesis prior to 1995. In
1995, the number was 484. This number has since been
significantly exceeded. They include food crops (e.g.,
corn, wheat, pea), ornamentals (e.g., chrysanthemum,
poinsettia, dahlia), and fruit trees (e.g., citrus, apple,
peach). Traits modified include agronomic ones such as
plant maturity, winter hardiness, lodging resistance, and
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Mutagenesis in plant
breeding
product quality (e.g., protein and lysine content), and
numerous ornamental mutants.
The perceived role of mutation in plant breeding was
initially treated with skepticism by some, as demonstrated
by L. J. Stadler, who is said to have advised his
students against using mutation breeding for commercial
crop improvement as a reaction to the overoptimism
by protagonists who saw it as a revolutionary plant
breeding method. Currently, induced mutations are
used more often in a supplementary role as a source of
new alleles. However, it is still important in breeding
vegetatively propagated species, including field crops,
ornamentals, and fruit and forest species. It is especially
useful in ornamental plant breeding where novelty is
often advantageous and can become commercially
significant. Furthermore, with the advent of genetic
engineering and its radical tools, which allow targeted