Principles of Plant Genetics and Breeding

MUTAGENESIS IN PLANT BREEDING 211
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growing tip determines the sectorial patterns. Sectorial
patterns are mostly unstable, eventually producing periclinal
structures. Because of this behavior, the breeder
needs to grow several clonal generations successively,
and select for the desired mutant as well as vegetative
stability. Vegetative stability is attained when uniformly
periclinal lines of subclones have been isolated. In a
vegetative system, the treated plant meristem is called
the M 1 V 1 , the next vegetative plant being M 1 V 2 , etc.
Mutations from tissue culture systems
Useful mutations have arisen spontaneously or are
deliberately induced in culture conditions. Such processes,
somaclonal variation and somatic selection,
were discussed in Chapter 11.
Using induced mutants
A plant may be selected for a specific mutant trait.
However, it does not mean that this plant contains only
one genetic change. It is recommended to backcross this
mutant to its original parents to reselect a pure version
of the mutant. Mutants may be directly used in cultivar
development. Like the backcross method, an adapted
line can be improved through mutation breeding to
correct a shortcoming. In sexually reproducing species,
the target mutant traits can be fixed and isolated in the
M 2 or M 3 generation (compared to F 6 or F 7 generations
in conventional breeding). The mutant lines may be
used in genetic studies as markers, among other uses.
Limitations of mutagenesis as a plant
breeding technique
There are a number of disadvantages of mutageneis
when used in plant breeding.
1 Associated side effects. Mutations induced by
mutagens are very diverse in nature. Invariably, the
mutagen kills some cells outright while surviving
plants display a wide range of deformities. Even those
plants with the desirable mutations always inherit
some undesirable side effects (akin to linkage drag,
or is it a “mutation drag”?). Therefore, it is often
necessary to transfer a new mutant into a stable
genetic background, free of some of the associated