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Mutation
An alteration in the genetic material (the genome) of a cell of a living organism or of a
virus that is more or less permanent and that can be transmitted to the cell's or the virus's
descendants.
Mutant
A phenotypic variant resulting from a change in its DNA is called mutant.
Why are we interested in Mutants?
1. Source of variation.
2. To understand the workings of wild-type genes by seeing what affects it.
 Mutation occur in two main ways
o By alternation in nuclear DNA, change in DNA bases. The change may involve
replacement of one purine/ pyramiding by another purine/ pyramidine.
Sometimes it involves loss or duplication of DNA bases.
o By change in cytoplasm DNA (cytoplasmic mutations, e.g. male sterility in
plants)
Types of mutation
 Spontaneous mutation:
– Occurs in nature, as a result of errors during cell division and DNA repair. Solar
radiation plays a role in this.
– Contributes to the evolutionary process in nature.
 Induced mutation:
– It can be artificially induced by the action of mutagenic agents.
Types of Induced mutation
 Macro-mutations:
– Mutations with distinct morphological changes in phenotype
– found for qualitative characters
– Identification of such mutations is easy.
 Micro-mutations:
– Mutations with invisible phenotypic changes
– observed in quantitative characters
– Identification of such mutations is very difficult
Mutations may be
Point Mutation: A mutation resulting from a change in a single base pair in the DNA
molecule, caused by the substitution of one nucleotide for another. It basically is about
changes within the genes.
Deletion mutation: A mutation caused by the removal of one or more nucleotides from a
gene or chromosome.
Insertion mutation: A mutation caused by the insertion of at least one extra nucleotide base
in a DNA sequence.
Forward mutation: Any mutation which renders a formerly functional gene nonfunctional.
This is the opposite of a back mutation.
Back mutation: A mutation that causes a mutant gene to revert to its original wild-type base
sequence.
Lethal mutation: A mutation that disrupts a critical gene, thus killing the developing
organism.
Germinal mutation: Alteration in the genetic constitution of the reproductive cells occurring
in the cell divisions that result in sperm and eggs is called germinal mutation.
Suppressor mutation: A mutation that counteracts the effects of a prior mutation, either in
the same or in a different gene.
Somaclonal variation: The tissue cultured induced mutations are referred to as somaclonal
variations. The plants developing from callus have shown large variations in their characters.
Such somaclonal variations have been reported in some crops such as Potato, Rice, Wheat.
Mutagenesis
– Mutagenesis is the process in which we induce mutations through the use of
various forms of energy or various chemical treatments.
Mutagenesis as a plant breeding technique
 Mutagenesis is used for the production of artificial genetic variability in plants.
 Induced mutations contribute to crop evolution by creating new plant types and
variability of various traits.
 Mutation breeding is used as a tool by plant breeders, usually with a specific trait in
mind, when a desired trait is not easily available within a species.
Advantages
 Induced mutation is used for the induction of cytoplasmic male sterility.
 Mutation breeding is cheap and rapid method of developing new varieties as
compare to back cross, pedigree and bulk breeding methods.
 Mutation breeding is more effective for the improvement of oligogenic characters
such as diseases resistant than polygenic traits.
 Mutation breeding is simple, quick and best way when a new character is to be
induced in vegetatively propagated crops.
Limitations
 Most of the mutations are deleterious and undesirable.
 Identification of micro-mutations, which are more useful to plant breeders, 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 quantitative or polygenic characters.
Mutagenic agents Mutation induction
 Mutagens are various physical or chemical agents which are used to induce mutations.
 Mutagens are of two types:
1. Physical mutagens
Physical mutagens include various types of radiations:
– X-rays, gamma rays
– alpha particles
– beta particles,
– fast and (thermal) neutrons
– Ultra-violet rays.
These mutagens produce all types of mutation in nucleotides e.g. addition, deletion,
inversion, transposition, transitions and tranversions.
2. Chemical mutagens
Chemical mutagens include various type of chemicals:
– alkaline agents
– base analogues
– acridine dyes etc.
 Chemical mutagens are often preferred over radiation because they are simpler to
apply and produce less damaging effects. The most widely used chemical mutagen
is ethyle methane sulfonate (EMS). It is an alkaline agent.
 Chemical mutagens are usually less drastic in their effects than ionizing radiations,
producing more gene mutations and fewer chromosome disruptions.
Procedure of mutation induction for plant breeding
The mutation breeding process consists of four important steps. These are:
1. choice of materials
 The best adapted variety should be chosen for mutagenesis. Any plant part
including seed can be used. Seed is preferred over other plant parts.
 Only one or two features of such varieties have to be altered through mutagenesis
depending upon the objective of mutation breeding.
2. choice of mutagens
 The type of mutagen depends upon the part of plant to be treated.
 Chemical mutagens are more preferred for seed treatment.
 Radiations for treatment of vegetative parts, because ionizing radiations can easily
penetrate the vegetative tissue.
3. Mutagen treatment
The procedure of mutagenic treatment involves three things:
 Plant species
 Dose of mutagen: The mutagen dose should be sufficient to kill 50% seeds to obtain
the maximum number of mutations.)
 Duration of treatment
4. handling of treated material
– Seed propagated species:
 A large seperate M1 generation is raised from treated seeds using wider
spacing for easy identification.
 M2 generation is raised from the seed obtained from M1 using wider
spacing.
 M3 progeny are raised from selected M2 and evaluated for homozygosity.
 The M4 progeny are raised in replicated trial using local check for
comparison.
 In M5-M9 generation selected lines are tested in coordinated multilocation
trials
– Vegetatively propagated species
 In vegetatively propagated species mutations are expressed in the form of
chimeras, refers to the presence of genetically different tissue in an
individual.
 In VM1 Generation large numbers of cuttings are treated with mutagens,
then planted and raised.
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In VM2Generation plant cuttings selected from VM1 are grown in separate
rows in M2.
In VM3 Generation plant cuttings from VM2 are grown in separate rows
andIdentical progeny are bulked together .
In VM4 Generation replicated trial of selected mutants is conducted using
local variety as a check.
In VM5 –VM9 Generation the selected mutant lines are tested in
coordinated trials at multilocations.
The best line is released as new variety.
Role of mutagenesis in crop improvement
Induced mutation play an important role in crop improvement. The main uses of
induced mutation are:
– Development of improved varieties
– Induction of male sterility
– Production of haploids
– Creation of genetic variability
– Overcoming self incompatibility
– Improvement in adaptation
Practical achievements
– The first results of mutation breeding programs were commercially exploited in
the 1950's. Since then thousands of cultivars have been released from mutation
breeding programs
– In plant species, 2252 mutant varieties have been developed till December 2000
(IAEA 2000). Of these 1585 have been directly released and 667 through the
use of mutants in hybridization.
– Out of 2252 mutant varieties, 1700 have been in seed propagated crops and 552
in vegetatively propagated species.
– Among seed propagated species, the maximum mutant varieties have been
developed n rice 434, barley 269, and wheat 222. Other crops soy bean 90,
maize 68, common bean 54, pea 32, cotton 24 and mung bean 19.
– Maximum varieties have been developed through radiations, gamma rays 910,
X-rays 311, gamma chronic 61, fast neutrons 48, and thermal neutrons 22.
Study Book chapter 6 MUTATION and Try to answer following questions.
1. Why is it that mutation breeding hasn’t led to the development of more
improved cultivars?
2. How does the plant breeder induce mutations? Which method do you
prefer and why?
3. If you have isolated two mutants with similar phenotypic expressions
from two different parental sources, how would you determine if the
mutants were the result of modification at the same locus or different
loci?