Mutation
Mutation is defined as sudden heritable changes in the phenotype of an individual. In the
molecular term, mutation is defined as the permanent and relatively rare change in the number or
sequence of nucleotides. In other words, mutations arise due to change in DNA bases. The
change in the nucleotides may involve replacement of one purine by another purine (A G) or
one pyrimidine by another pyrimidine (C T) and this change is called transition. Sometimes,
change in the nucleotide involves substitution of a purine by a pyrimidine and vice versa
(A T or C) and (G T or C) and this change is called transversion. Both these changes
may occur in nuclear DNA (say gene or point mutations) as well as cytoplasmic DNA (say
cytoplasmic mutations).
History: 1901: Hugo de Vries gave the name “mutation” to sudden heritable change in
characters observed by him.
1927: H.J Muller observed mutagenic action of X-rays in drosophila.
1927: Muller produced mutations in fruit flies using x-rays
1928: Stadler produced mutations in barley
An individual showing changed in characters is known as mutant.
The allele producing the changed phenotype is called mutant allele
Types of Mutations: Mutations do occur in nature (Spontaneous mutations) as well as can be
artificially induced by various mutagenic agents (Induced mutations). Induced mutations are two
types:
(1) Macro-mutations: Mutations with distinct morphological changes in the phenotype
are referred as macro-mutations. Such mutations are found for qualitative characters and
therefore, are also called oligogenic mutations and identification of such mutations is easy.
(2) Micro-mutations: Mutations with very small and invisible morphological changes in
the phenotype are referred as micro-mutations. Such mutations are found for quantitative
characters and therefore, are also called polygenic mutations and identification of such mutations
is very difficult but has economic value in plant breeding.
Types of mutagen: (Mutagen are physical or chemical agents that induce or enhance the
mutation)
I. Ionizing radiations
a. Particulate radiations e.g. Alpha rays, Beta rays, Fast neutrons etc.
b. Non-particulate radiations e.g. X-rays, Gamma rays
II. Non-ionizing radiations e.g. Ultra violet rays
III. Alkylating agents e.g. Mustard gas, EMS, MMS etc.
IV. Base analogous e.g. 5-BU(5-Bromouracil), 2-AP(2-Aminopurine)
V. Acridine dyes e.g. Acriflavin, Proflavin etc.
VI. Others e.g. Nitrous acid, Sodium azide etc
Mutation breeding:
The genetic Improvement of crop plants for various economic characters through the use of
induced mutations is referred as Mutation breeding. Commonly used in selfed pollinated and
asexually propagated species. It is useful for improvement of oligogenic traits.
Mutation breeding procedure: Consists of four important steps.
1. Choice of material
2. Choice of mutagen
3. Mutagen treatment and
4. Handling of treated material
1. Choice of material: The best adapted variety of a crop should be chosen for mutagenesis,
because such variety has all acceptable agronomic features. Only one or two features of such
variety have to be altered through mutagenesis depending upon the objective of mutation
breeding.
2. Choice of mutagen: The commonly used mutagens include X- rays, gamma rays, ultra violet
rays, etc. Ethyl methane sulphonate ( EMS), methyl methane sulphonate (MMS), ethylene imine
(EI) and several others among chemical mutagens. Generally, chemical mutagens are more
preferred for seed treatment and while radiations for the treatment of vegetative parts, because
ionizing radiations can easily penetrate the vegetative tissues.
3. Mutagen treatment: The procedure of mutagenic treatment takes three things into account:
1. Plant species
2. Dose of mutagen
3. Duration of treatment
In vegetatively propagated species, buds, cutting or suckers are used for
mutagenic treatment. The mutagen dose varies from species to species. A mutation dose close to
LD50 is considered optimum by many workers. (LD50 refers to a dose of mutagen that kills 50%
of treated individuals). The LD50 dose of same mutagen differs from species to species. The
duration of treatment depends on the intensity of radiations or concentration of chemical
mutagens. The seeds are water soaked before treatment. After treatment the seeds or cuttings are
called M1 plants. Plants grown from the seed obtained from pollination of treated pollens are
also called M1 plants.
4. Handling of treated material:
(A) For seed propagated species
M1 generation
-A large M1 population is raised from treated seeds.
- Plants are grown using wider spacing for easy identification.
-Generally, the mutants are recessive.
-All the M1 plants are selfed to avoid contamination from cross pollination.
-Each M1 plants are harvested separately.
M2 generation
-The M2 generation is raised from the seed obtained from M1 using wider
spacings.
-The oligogenic mutants with distinct features are identified and selected.
-The seed of such mutants is harvested separately.
M3 generation
-The M3 progeny are raised from selected M2 and evaluated for homozygosity.
-The homologous M3, progeny are bulked together to conduct yield trial in M4
generation
M4 generation
The M4 progeny are raised in replicated trial using local check for comparison.
M5-M9 generation
Selected lines are tested in coordinated multilocation trials. The best performing
line is released as a variety.
(B) For vegetatively propagated species
In clonal crops, the mutations occurring in buds or somatic tissues which are used for
propagation is called bud mutation or somatic mutation. In vegetatively propagated species,
mutations are expressed in the form of chimeras. (The chimera refers to the presence of
genetically different tissues in an individual. The individual has one type of tissues in one part
and another type of tissues in another part). In apical bud, axillary bud and adventitious buds,
there are three functional layers, viz. L1, L2 and L3 layer.
L1 gives rise to epidermis, L2 produces a part of leaf mesophyll and L3 give rise to rest of plant
body.
In Woody Species:
Mutagenize plant part having normal or adventitious buds
Allow the bud to sprout (VM1)
Repeatedly prune (VM2, VM3,…etc) the growing shoot to promote the cells from
mutated sectors to regenerate (Fig)
Take cutting from the mutant branch for multiplication
Evaluate the possible release
(Here each pruning cycle represents a vegetative cycle)
In Tuber crops:
Mutagenize full tuber, half tuber or disks containing single eye
Raise VM1
Select tubers showing sectorial chimera in VM1
Raise VM2
Select solid mutation in VM2
Raise VM3 to confirm breeding bahaviour and multiplication of the mutation.
Evaluate in successive generations for the desired attributes and possible release.
Applications in crop improvement:
Induced mutations play an important role in crop improvement. The main uses of induced
mutations are:
1. Development of improved varieties: Pusa Nanha (Papaya); PKM 1, S-12 ( Tomato); MDU 1
(Brinjal); Vikram, Co 2 , TG 3 (Groundnut)
2. Induction of male sterility: Ethidium bromide in cereals, sugarbeets & cotton
3. Production of haploids: The X-rays irradiated pollen has helped in production of haploids in
many plants and chromosome doubling of these haploids results in the development of inbred or
homozygous lines for use in crossing programme.
4. Creation of genetic variability: -In sugarcane, potato, wheat etc
5. Overcoming self incompatibility: In prunus ovium and many other crops by irradiation.
6. Improvement in adaptation: In Hibiscus subdarifa mutant could successfully be grown 1600
km north of its normal habitat.
7. Contribute to crop evolution by creating new plant type and variability for various polygenic
and oligogenic characters.
Limitations:
Most of the mutations are deleterious and undesirable.
Identification of micro-mutations, which are more useful to a plant breeder, is usually
very difficult.
Since useful mutations are produced at a very low frequency (0.1%), a very large plant
population has to be screened to identify and isolate desirable mutants.
Mutation breeding has limited scope for the genetic improvement of quantitative or
polygetic characters.
