Principles of Plant Genetics and Breeding

200 CHAPTER 12
genetic alteration (versus the random genetic alteration
produced by conventional mutagenesis), it appears that
breeders are gravitating towards this truly revolutionary
technology for creating new variability. However, no
approach should be written off as every now and then
some breeders find good reason to use a technique or
technology that has been marginalized by advances in
science and technology.
In conventional breeding of sexual plants, genetic
variability is derived from recombination. Parents must
not be identical, or else there would be no segregation
in the F 2 generation. Even when parents are dissimilar,
they often have similar “housekeeping genes” that are
common to both parents. Whereas segregation will not
occur for these common genes, mutagenesis can create
variability by altering them.
Types of mutation
In terms of origin, mutations may be spontaneous
(natural) or induced (artificial, with the aid of agents).
Spontaneous mutations arise at the very low rate of
about 10 −5 or 10 −6 per generation for most loci in most
organisms. This translates to one in 100,000 or one in
1,000,000 gametes that may carry a newly mutated
allele at any locus. They are caused by mistakes in molecular
processes associated with the replication of DNA,
recombination, and nuclear division. However, because
mutagenic agents are common in the general environment,
induced mutations, as a result of these agents
(natural radiations), are hard to distinguish from spontaneously
induced mutations due to cellular processes.
Mutations may also be classified according to the type
of structural change produced:
1 Genomic mutation: changes in chromosome number
(gain or loss in complete sets of chromosomes or
parts of a set).
2 Structural mutation: changes in chromosome structure
(e.g., duplications of segments, translocation of
segments).
3 Gene mutation: changes in the nucleotide constitution
of DNA (by deletion or substitution).
Mutation may occur in the nuclear DNA or chromosomes,
or in extranuclear (cytoplasmic) genetic systems.
A good example of the practical application of mutations
in plant breeding is the cytoplasmic-genetic malesterility
gene, which occurs in chloroplasts.
In terms of gene action, a mutation may be recessive
or dominant:
1 Recessive mutation: change of a dominant allele to a
recessive allele (A → a).
2 Dominant mutation: change of a recessive allele to a
dominant allele (a → A).
Mutations that convert the wild type (the common
phenotype) to the mutant form (the rare phenotype) are
called forward mutations, while those that change
a mutant phenotype to a wild phenotype are called
reverse mutations. Forward mutations are more common
than reverse mutations. Recessive mutations are
the most common types of mutations. However, recessive
alleles in a diploid are expressed only when they are
in the homozygous state. Consequently, an organism
may accumulate a genetic load without any consequence
because of heterozygous advantage. As previously discussed,
outcrossing species are susceptible to inbreeding
depression (loss of vigor), because of the opportunities
for expression of deleterious recessive alleles.
Induced mutations versus spontaneous mutations
Spontaneous mutations produce novel alleles for the
evolutionary process. Natural mutations have the benefit
of being subjected to the evolutionary process whereby
viable mutants become recombined with existing forms
and become adapted under the guidance of natural
selection. Mutagenesis can be used to create new alleles
that can be incorporated into existing cultivars through
recombination following hybridization and under the
guidance of artificial selection. Modern crop production
systems are capable of providing supplemental care to
enable a mutant that would not have survived natural
selection to become productive. As previously discussed,
a significant number of commercial cultivars originated
from mutation breeding techniques. Furthermore, the
rate of spontaneous mutation is low (10−6 per locus).
Artificial mutagenesis aims to increase mutation rates for
desired traits.
Cell type: gametic versus somatic mutations
Mutations may originate in the gametic or somatic cells.
Gametic mutations are heritable from one generation
to the next and are expressed in the entire plant.
However, mutations in a somatic tissue will affect only
that portion of the plant, resulting in a condition called
chimera. In species that produce tillers, it is possible
to have a tiller originate from a chimeric tissue, while
others are normal. A chimera consists of two genetically
distinct tissues and may produce two distinct flowers on