The genetic improvement of crop plants for various economic characters through the use of induced mutations is referred to as mutation breeding. It is one of the special methods of crop improvement. Mutation breeding is commonly used in self pollinated and asexually propagated species.
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Significance:
1. When the desired variability is not found in the cultivated varieties or in the germplasm of cultivated species.
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2. When a high yielding variety has oligogenic defect such as susceptibility to a disease, mutation breeding is the best course of line for a breeder. Because induced mutagenesis leads to improvement of such variety without much alteration in the genetic background.
3. When there is a tight linkage between desirable and undersirable characters, mutation breeding is the best way of overcoming such problems.
4. When in fruit crops, the improvement has to be made without change in the taste and colour of the fruits, it can best be achieved through mutation breeding, because mutation breeding causes little changes in thf ^snetic background of parental material.
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5. When a particular reaction is to be blocked, mutation breeding is the only way. For example in synthesis of morphine taken place in step-wise manner. If the biochemical path way is blocked at bane level, it will block the synthesis of morphine without affecting the conversion process of the bane into useful pharmaceutical product.
6. In those crops where sexuality is absent and generation of variability through recombination process is not possible.
7. In those species where generation cycle is very long, such as plantation crops, fruit trees and forest trees, mutation breeding is the short cut way of genetic improvement.
8. When attactive flower and foliage colours have to be developed in ornamental plants which fetch high price in the market, mutation breeding is the only short cut method.
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Mutagens and their mode of action:
Mutagens refer to a various physical or chemical agents which greatly enhance the frequency of mutations. Various radiations and chemicals are used as mutagens. Mutagens are of two types, viz. (1) physical mutagens, and (2) chemical mutagens.
Table 5: Some commonly used chemical mutagens and their mode of action
| Group of mutagen | Name of chemical | Mode of action |
| i. Alkylating agents | Ethyl methane sulphonate Methyl methane Sulphonate Ethyl ethane Sulphonate Ethylene Imines | AT↔ GC Transitions Transitions GC ↔AT Transitions Transitions |
| 2. Base analogues | 5 Bromo Uracil Amino Purine | AT↔ GC Transitions AT ↔ GC Transitions |
| 3. Acridine Dyes | Acriflavin, Proflavin | Delection addition and Frameshifts |
| 4. Others | Nitrous Acid Hydroxylamine Sodium Azide | AT↔ GC Transitions GC ↔A TTransitions Transitions |
The mutation breeding process consists of four (2) choice of mutagen. (3) mutagen, treatment, and important steps. These are (1) choice of material, (4) handling of treated material.
Handling of treated material:
General procedure for handling of mutagen treated material consists of five important steps as discussed below:
1. M1 Generation:
A large M, population is raised from treated seeds. Plants are grown using wider spacing for easy identification. Generally, the mutants are recessive. If some dominant mutations are obtained, they are selected in M1; otherwise observations are recorded on chlorophyll sectoring and fertility. All the M1 plants are selfed to avoid contamination from cross pollination. Each M1 plant is harvested separately.
2. M2 Generation:
The M1 generation is raised from the seed obtained from M, using wider spacings. The oligogenic mutants with distinct features are identified and selected. The seed of such mutants is harvested separately.
3. M3 Generation:
The progeny are raised from selected M2 and evaluated for homozygosity. The homologous M3 progeny are bulked together to conduct yield trail in M4 generation.
4. M4 Generation:
The M4 progeny are raised in replicated trail using local check for comparison.
5. M5-M9 Generation:
Selected lines are tested in coordinated multilocation trials. The best performing line is released as a variety.
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, (2) induction of male sterility, (3) production of haploids, (4) creation of genetic variability, (5) overcoming self incompatibility, and (6) Improvement in adaptation.
Table 6: Achievements:
Improved character Crops and mutant varieties
Higher yield:
High yielding mutation varieties have been developed in barley (DL 253), chickpea (Pusa 408, Pusa 413, Pusa 417), Cowpea (cowpea 88, V 16, V 37, V 38, V 240), Groundnut (Co 2, TG 17), Mungbean (ML 26-10-3). Oriental mustard (RLM 514). Okra (MDU 2), Pea (Hans). Ridge gourd (PKM 1), Tomato (PKM 1). Tossa jute (Mahadev TJ 40) and sorghum (Co 21).
Short stature Earliness:
Barley (RDB 1), Rice (Hybrid mutant). Sesame (Kalika). Rice (Prabhavati), Rice (AU 1, IIT 48, IIT 60, Indira, К 84, Padmini, Sattari, CNM 20, CNM 25, CNM 6, CNM 31), Castor bean (Aruna) Cotton (MCU 7), Blackgram (Co 4), Mungbean (TAP 7), Pigeonpea (Co 5) and Sugarcane (Co 85035)
Stress resistance:
Drought resistance in cotton (MA 9, MCU 10), Salt tolerance in Rice (Mohan) and water logging tolerance in Jute (Padma)
Advantages:
Mutation breeding has several advantages. Some important advantages of mutation breeding are briefly presented below:
1. Induced mutagenesis is used for the induction of cytoplasmic male sterility. Ethidium bromide (EB) has been used for induction of cytoplasmic male sterility in pearl millet and barley.
2. Mutation breeding is a cheap and rapid method of developing new varieties as compared to backcross, pedigree and bulk breeding methods.
3. Mutation breeding is more effective for the improvement of oligogenic characters such as disease resistance than polygenic traits.
4. Mutation breeding is the simple, quick and best way when a new character is to be induced in vegetatively propagated crops.
Limitations:
Induced mutations have some limitations which are briefly presented below:
1. Most of the mutations are deleterious and undesirable.
2. Identification of micro-mutations, which are more useful to a plant breeder is usually very difficult.
3. Since useful mutations are produced at a very low frequency (01%), a very large plant population has to be screened to identify and isolate desirable mutants.
4. Mutation breeding has limited scope for the genetic improvement of quantitative or polygenic characters.