Agriculture Reference
In-Depth Information
time and place, and in a selected genotype for a selected purpose is a much more
attractive option.
Mutation Induction
A physical or chemical agent that changes the genetic information (usually DNA)
of an organism and thus increasing the number of mutations above the natural back-
ground level is called a mutagen (Fig.
4.1
). DeVries (
1905
) suggested the use of
radiation to induce mutations. The discovery that X-ray induced mutations in
Dro-
sophila melanogaster
(Muller
1927
) and in
Hordeum vulgare
(Stadler
1928
) led to
the use of radiation-induced mutations for changing plant traits by plant breeders
and geneticists. Auerbach and Robson (
1946
) reported the use of chemicals such
as mustard gas to be highly mutagenic. Since then a number of agents have been
discovered that can increase the frequency of artificially induced mutations. The
main mutagens available for induction of mutations include UV radiation, electro-
magnetic waves such as X-rays, γ-rays and cosmic rays; fast moving particles such
as α-particles, β-particles and neutrons; and chemical agents such as, alkylating
agents, acridines, azides, hydroxyl amides, etc. In general, ionizing radiations such
as X-rays and γ-rays are preferred over chemical mutagens because of their ease of
application, good penetration and reproducibility, high mutation frequency and less
disposal problems. The X-rays are obtained from X-ray machines by bombarding
tungsten or molybdenum with electrons in a vacuum, whereas γ-rays are obtained
from radioisotopes like
60
Co and
137
Caesium in the γ-chamber. The UV-radiations
possess limited tissue penetrating ability [low linear energy transfer or in short
LET] and cause relatively little damage except after prolonged exposure as a result
of which their use is restricted to pollen grains (Kovacs and Keresztes
2002
).
The application of this phenomenon has come a long way to become a real
tool, not only in crop breeding but also in basic research on the plant genome, its
structure and function. Breeders were the first to recognize the potential of induced
mutations through analogy with spontaneous mutants, often selected as new plant
types in many crops, from cereals to apples, not to mention ornamental and decora-
tive plants. Many mutants with desired traits were selected in the second or third
generation after mutagenic treatment and subsequently released as new cultivars
after agronomic evaluation in regional and national trials. These or other mutants
developed with mutations in desired traits, even though not released as new cul-
tivars, have been used in cross-breeding programmes as a source of particular al-
leles, often allelic to the spontaneous ones, but in a desired genotype. Among them
were sources for characters such as short stature and lodging resistance; disease
resistance; oil quality; and increased nitrogen fixation. These mutated genes are
especially valuable as the best currently grown cultivar was usually selected for
mutagenic treatment. A desired mutation in a good genetic background is a very
attractive component in breeding programmes. This approach is much simpler and
faster than crossing with an exotic source, and it is one of the main reasons for the
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