Mutation breeding

MODERN METHODS OF PLANT BREEDING
Conventional methods of plant breeding are age-old
techniques that were developed by farming communities and
improved by refined plant breeders. On the other hand,
modern methods are developed by the plant breeders with the
help of modern scientific tools. Modern methods of plant
breeding include:
 Mutation Breeding
 Polyploidy Breeding
 Biotechnological Methods

MUTATION BREEDING
 Mutations are heritable changes in the phenotypes of
organisms. These changes are the results of chemical
changes at the level of genes.
 Such changes are capable of bringing about new and
heritable character variations in crop plants and such
variations can be selected and used for the establishment
of crop varieties with new characters.

 Mutations occur in nature in very low frequency. Such
mutations are called spontaneous mutations. However, the
frequency of mutations can be increased with the help of
certain chemical or physical agents that are called
mutagens or mutagenic agents and mutations induced in
this way are called induced mutations.
 Such agents can be used to induce mutations in crop
plants and the desirable variations produced in this way
can be selected. This approach of plant breeding in which
new variations of crops with desirable characters are
developed with the help of induced mutations is called
mutation breeding.

MUTAGENS AND THEIR MODE OF ACTION
Mutagens are the physical or chemical agents used to
enhance the frequency of mutations:
A. Physical Mutagens
B. Chemical Mutagens
A. Alkylating Agents
B. Base Analogs
C. Intercalating agents
D. Other chemical mutagens

PHYSICAL MUTAGENS
 These are physical agents that are capable of inducing
mutations. These are different types of radiations.
Radiations can be generally classified into two classes based
on their energy levels.
 Radiations with lower energy levels are capable of
causing excitations at the level of nitrogen bases of the
genetic material and they are called non-ionizing
radiations.

 UV light is a good example of non-ionizing radiations.
 Radiations with high energy level are capable of causing
both excitation and ionization at the level of nitrogen
bases. They are called ionizing radiations.
 X-rays, gamma rays, alpha particles, beta particles etc. are
the examples of ionizing radiations.

CHEMICAL MUTAGENS
 There are chemicals that are capable of enhancing the
frequency of mutations. The chemical mutagens are
mainly classified into three categories on the basis of their
nature of action.
 Alkylating Agents
 Base Analogs
 Intercalating agents

PROCEDURE OF MUTATION BREEDING
It includes:
1. Selection of the material
2. Choice of the mutagen
3. Mutagen Treatment
4. Handling of the mutated populations

CHOICE OF THE MUTAGEN
 Based on the nature of mutation to be induced and
the knowledge on the nature of action of the mutagen,
the appropriate mutagen is selected.
 Generally, chemicals are preferred for the seed
treatment and radiations are preferred for vegetative
propagules, pollen etc.

MUTAGEN TREATMENT
 In the case of chemical treatment, presoaking
materials in water or solutions of some other
chemicals enhance the effect of mutagens. This is
called pre-treatment.
 Later, the materials are transferred into the solutions
of the mutagen. A concentration close to LD50 of the
mutagen is considered optimum.

 In the case of physical mutagens, the source of the mutagen
is kept at a safe distance and the treatment is remotely
controlled. Gamma ray treatment is carried out in protected
experimental areas known as gamma gardens. The duration
of treatment is also decided based on the information
available.
 LD50 is the dose of the mutagen that causes 50 percent
mortality of the treated material (any mutagen is toxic to
biological systems and it may cause considerable death and
deformities).

HANDLING OF THE MUTATED POPULATIONS IN
THE CASE OF SEED PROPAGATED SPECIES
 All the germinated seeds are grown to produce the M1
population. Generally the mutations will be recessive and most
of them can be selected only in later generations. However,
dominant mutations and pseudo-dominant mutations can be
selected in the M1 itself.
 The M1 plants are selfed and the seeds are harvested separately.
 The M2 generation is raised from the seeds collected from the
M1 generation. Oligogenic mutations can be selected at this
level. Their seeds are grown separately and desirable mutants
isolated after necessary trials. Superior and desirable M2 plants
are selected and M3 seeds are collected.

 M3 progenies are raised from the seeds and they are evaluated
for breeding behavior. The seeds of true breeding progenies
are bulked together to conduct yield trials.
 Preliminary yield trials are conducted in the M4. Co-ordinated
yield trials are carried out from M5 onwards. By M8 or M9,
the most promising lines are selected and released.
 In the case of polygenic traits, inferior plants are rejected at
M3 and M4 levels and based on screening tests, the remaining
seeds are bulked and used for yield trials and finally released
as new varieties.

Methods for validation of mutants

Practical considerations in induced
crop mutagenesis
 A perfect understanding of the genetic makeup of the traits to be improved is
very important. For example, a trait controlled by many genes (i.e., polygenic)
has less chances of inducing modification compared to a trait that is governed
by a single gene (i.e., monogenic).
 Understanding the mode of reproduction of the target crop is also a
prerequisite, whether asexually or sexually propagated.
 If it is asexually propagated, then the method to employ is the next question:
whether it will be in vitro or in vivo.
 If the crop is seed propagated, the question will be on the type of fertilization (self
or cross-fertilization) to be used.
 The determination of the material that is to be used for the propagation prior to
treatment, i.e. gametes or seeds for sexually propagated crops; and stem
cuttings, buds, nodal segments or twigs for asexually propagated ones.

Practical considerations in induced
crop mutagenesis
 Knowledge of the number of sets of chromosomes in the nucleus of a
cell (ploidy) of the target crop.
 Selection of an appropriate mutagen (physical or chemical mutagens)
and dose (duration and concentration of mutagens). That is why a pilot
assay is advisable to be carried out prior to the large-scale treatment of
propagules.
 Identification of infrastructure (irradiation house, laboratories,
screen/glass house, fields, etc.) for successful selection of desired
mutants.
 Screening techniques for dissociation of chimeras from stable mutants.

Advantages
 Possible to achieve instant progress in elite material.
 Single trait improvements can be made to an established variety
preferred by producers, processors and/or consumers.
 Novel variation can be produced.
 Single gene mutants with no negative pleiotropic effects are
possible.
 The treated material is safe to handle.
 Specific genes/traits can be targeted.
 Possible to calculate chances of success (mutation frequency).

Limitations...
 The process is generally random and unpredictable.
 Useful mutants are rare and predominantly recessive.
 Large population sizes and effective mass screening methods are
required to select rare mutants.
 Mutants can have strong negative pleiotropic effects on other traits.
 Health risks: handling, chemical mutagens; radiations.
 Most mutants are of no use to breeding even if a large number of
mutants can be produced.
 Field trialling and germplasm storage can be expensive and require a
lot of space and careful management if large mutant populations are
handled.

Applications of Mutation
Breeding

 Mutation breeding can be used to develop improved
crop varieties, for the production of haploids, to create
additional genetic variability, and to improve the
adaptability of crops.

VARIATIONS IN CHROMOSOME NUMBER

Mutation breeding

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