Agriculture Reference
In-Depth Information
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The frequency of desirable mutants is likely to be low,
and it is essential that there are suitable selection tech-
niques to screen the thousands of mutants generated
to identify the few that have the required mutated
trait. The most common example of mass selection
in mutation breeding relates to herbicide tolerance.
Most herbicide tolerance in crop species is qualita-
tively controlled and many thousands of mutated
breeding lines can easily be screened for tolerance
by simply spraying the mutants with the herbicide
of choice with the premise that all those that sur-
vive carry a form of the mutant tolerant gene. Other
examples would include selecting mutants which
exhibit morphological changes in plant structure
(i.e. dwarfs), maturity, flower colour, qualitative dis-
ease resistance, or enhanced end-use quality (i.e. fatty
acid profile or starch types).
and introduce them back into the plant species (or, of
course, to another) - clearly the potential to 'mutate'
these DNA sequences and reintroduce them is a reality.
One lesson, which was learned from the early efforts
of mutation breeders, is that it is necessary to have clear
objectives, which are biologically reasonable, if success
is to be achieved. However, even in cases which have
been well organized with realistic objectives, the effort
that was required to sort out the desired products in a
useable form was often greater than what would have
been required to achieve the same results from a more
traditional hybridization breeding scheme.
In plant breeding there will always be a need for new
sources of variation and mutations (natural or induced)
will feature as part of future breeding efforts. There-
fore, mutation breeding has a very real place in cultivar
development, but it would be unwise to base a complete
variety development program on mutagenesis.
In summary, a mutagenesis breeding program must
deal with large numbers of mutated lines so that the
low frequency of desirable mutations, in an acceptable
genetic background, can be selected. Similarly, a muta-
tion scheme must offer a quick and effective selection
screen to identify the few desirable mutants.
When desirable mutants are selected they are usu-
ally adversely affected by the mutagenic treatment
and it will be necessary to 'clean up' the mutant
genotype either by recurrent selfing and selection,
or more likely by recurrent back-crossing and selec-
tion. Most mutations are recessive, and identification
of recessive mutations is difficult due to dominance
effects. In diploid seed-propagated crops, recessive
mutations can be identified by selfing or inter-mating
mutated lines, albeit that the frequency of lines that
are homozygous recessive for the desirable gene muta-
tion will be very low. For obvious reasons selection
of desirable recessive mutations in clonally propa-
gated crops is considerably more difficult and breeders
would require excessive effort in selfing mutant lines
or hope for the extremely rare event where all alleles
at a given loci have the same recessive mutated gene.
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INTERSPECIFIC AND INTERGENERIC
HYBRIDIZATION
Another method of increasing the genetic diversity of a
crop species is by interspecific or intergeneric hybridiza-
tion. When sources of variation for a character of
interest (e.g. disease or pest resistance) cannot be found
within existing genotypes in a species, it seems sensi-
ble to look at related species or genera and examine the
possibility to introgress traits from them into the one of
interest.
Interspecific hybridization refers to crosses between
species within the same genus (i.e.
B. rapa
The unpredictable nature of mutagenesis raises the
question arises as to whether it is possible to 'direct'
the mutational effects towards changing only charac-
ters of interest and to affect them in rather particular
ways. The first possible 'direction' to the affects that
can be exploited is that different mutagens have differ-
ent forms of action, as noted earlier. Some induce point
mutations and so are more likely to produce a partic-
ular array of effects while others are likely to induce
grosser structural changes. An even more specific array
of possibilities is arising from the potential to induce site
specific mutagenesis. Consider our increasing ability to
identify particular genes, to clone or synthesize these
B. oleracea
)
while intergeneric hybrids are crosses between different
genera (
Triticum
×
Secale
).
The probability of developing a successful new culti-
var is related to the frequency of desirable (or undesir-
able) characteristics in the parents used in hybridization.
The most commonly used parental lines will be adapted
cultivars or highly desirable genotypes from within
breeding programmes. When a character of interest is
not available within this gene pool then, the obvious
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