(Induced Mutations for Crop Improvement)
What is mutation?
Mutation is the “Sudden heritable change in an organism”. Mutation may be the change in gene, chromosome or plasmagene (genetic material inside mitochondria and chloroplasts. The mutation produced by change in the base sequence of gene is called point mutation or gene mutation. The gene mutation may be further classified as transition, transversion, deletion, duplication or inversion.
Chromosomal mutations are the change in chromosome structure. The change in the structure of chromosome can occur as a result of large deletion, inversion, duplication, translocation and change in chromosome number. Most of the mutations are lethal to the organism.
A very small number of mutations are beneficial to the organism. Additionally, by the use of mutation inducing agents, a breeder can induce desirable changes in the genetic constitution of plants and thereby he can improve the performance of a cultivated variety.
Definition: “The utilization of induced mutations in crop improvement is called mutation breeding”
The term mutation breeding was first coined by Freisleben and Lein in 1944 to refer to the deliberate induction and development of mutant lines for crop improvement.
Spontaneous and Induced mutations:
(1). Spontaneous mutation:
Mutation occurs in nature are called spontaneous mutation. Spontaneous mutation occurs in the organism without any treatment at low rate in the nature. The frequency of spontaneous mutation is 10-6 (one in 10 lakhs). Different genes in and organism show different mutation rate.
(2). Induced mutation:
Mutations induced in an organism by treatment with physical or chemical mutagen are called induced mutations. The agents which are used to induce mutation are called mutagens. Certain genes in an organism promote the mutation of other genes nearby in the chromosomes. For example, the gene Dt in Chromosome number 9 of maize increases mutation rate of other genes.
Characteristics of Mutation:
Ø Mutations are generally recessive; Dominant mutations do occur in nature.
Ø Mutations are generally harmful to organism; small percentage of mutation is beneficial.
Ø Mutation occurs at random in the chromosome, may occur in any gene.
Ø Some genes show higher mutation rate than others.
Ø Some mutations are recurrent, they occur again and again in the organism.
Ø Induced mutations usually show pleotropy.
Effects of Mutation:
Generally mutations are harmful to the organism. They reduce viability in the individuals. Based on the effects of mutation, they are classified as:
(1). Lethal mutation: Lethal mutations kill the individual.
(2). Sub-lethal mutation: They do not kill all individuals but reduce the viability.
(3). Vital: Vital mutations do not kill the individuals, vital mutations are important mutation program and they have been successfully used in crop improvement programmes.
Classification of mutation based on its magnitude:
Depending upon the magnitude, the mutations in the organisms can be classified into two categories (1) Macro mutation and (2) Micro mutations. Macro mutations produce large phenotypic changes whereas in micro-mutation only small phenotypic changes will be produced.
History of Mutation Breeding:
Ø The term mutation was for the first time introduced by Hugo de Vries.
Ø The mutagenic activities of X-rays were first described by Muller on Drosophila melanogaster (Nobel Prize).
Ø Stadler and Baoley described the mutagenic activity of γ- rays.
Ø Auerbach and Rohion proposed the mutagenic ability of mustard gas (Sulfur mustard).
Ø Nilsson Ehle initiated the mutation breeding programme in USSR for the first time.
What are mutagens?
Agents with cause mutation in the organism are called mutagens. There are different classes of mutagens based on their chemical or physical properties. All these mutagens induce mutations in the genome by the structural or chemical modification of the genetic material (DNA).
Mutagens are classified into two broad categories:
(1). Chemical mutagens
(2). Physical mutagens
Chemical molecules which induce mutations are called chemical mutagens.
Different class chemical mutagens are:
(1). Alkylating agents: Example: Sulphur mustard, nitrogen mustard, epoxides, Ethyl-methane sulphonate (EMS), Methyl methane sulphonate (MMS), nitroso compound (Eg. N-methyl-N-nitro-N-guanidine-MNNG)
(2). Acridine dyes: Example: Proflavine, Acridine orange, Acridine yellow, Ethydium bromide
(3). Base analogues: Example: 5-bromo uracil, 5-chloro uracil
(4). Others: example: Nitrous acid, hydroxy amine, sodium azides
They are different types of radiations which are categorized into two categories.
(1). Ionizing radiations: They include particulate radiations (α-rays, β-rays and fast neutrons) and non-particulate radiations (X-rays andγ-rays).
(2). Non-ionizing radiations: Ultraviolet (UV) radiation.
The utilization of induced mutation in crop improvement is called mutation breeding. In mutation breeding, desirable mutations are induced in crop plants with the use of physical or chemical mutagens. The variability generated through induced mutations are either released as new variety or used as the parent for subsequent hybridization programmes. Treating of biological materials with mutagens to induce mutation is called mutagenesis. If any class of radiations are used as a mutagen to induce mutation in crop plants, the exposure of biological organism to the radiation is called irradiation. Mutation breeding programme should be clearly planned and should be large enough with sufficient facilities to screen large population.
Steps in mutation breeding:
(1). Objectives of the programme:
Ø Mutation breeding should have well defined and clear cut objectives.
(2). Selection of the varieties for mutagen treatment:
Ø The variety selected should be the best variety available
(3). Part of the plant to be treated:
Ø Seeds, pollen, vegetative propagules, sometimes complete plant as treated with mutagen
Ø The selection of plant part varies with crop plant.
Ø Seeds are best part in sexually reproducing plants.
Ø Seed treatment is actually the treatment of embryo.
(4). Dose of mutagen:
Ø The mutagen treatment reduces germination, growth rate, vigour and fertility of organism.
Ø The mutation also increases frequency of chromosomal changes, mitotic and meiotic irregularities in the organism
Ø All these damages increase with increase in the dose mutagen and duration of exposure.
Ø Thus, the dose should be optimized for a maximum success rate
Ø The dose and treatment duration of mutagens varies with crop and plant parts and also with the type of mutagen used.
Ø The optimum dose is the dose at which maximum frequency of mutation will occurs with minimum killing of the organism.
Ø The optimum dose of mutagen is expressed as LD50.
Ø LD50: Dose of mutagen which will kill 50% of treated individuals.
Ø LD50 varies with crop plants and type of mutagen used.
(5). Giving mutagen treatment:
Ø M1: generation produced directly from mutagen treated plant parts.
Ø M2, M3 & M4 are subsequent generation derived from M1, M2 and M3.
Ø M2, M3 & M4 are produced by selfing or clonal propagation.
(6). Handling mutagen treated population:
Ø Mutation treatment in seeds and vegetative propagules produce chimeras.
Ø Mutation usually occurs in small section of plant parts such as seeds or meristem.
Ø One or more clonal or sexual generations with selection are necessary for stable mutant phenotype.
Ø Mutant alleles are generally recessive. Dominant mutation do occurs, however, the chance of dominant mutation is very less.
Ø In sexually reproducing plants dominant and recessive mutations are useful.
Ø However in clonal propagated plants, the dominant mutations are beneficial.
Steps in Mutation Breeding for Oligogenic Traits in crop plants
Ø Mutation breeding is most commonly used to improve the qualities of a crop plant which are controlled by oligogenic traits.
Mutation breeding for Polygenic Traits:
Ø Mutagenesis also produces genetic variations in polygenic traits.
Ø This variation is however 50% less than that generated in F2 generation
Application / Advantages of induced mutations in crop improvements:
Ø Mutation breeding can be used for both oligogenic and polygenic traits in plants.
Ø It improves morphological and physiological characters of cultivated crops.
Ø Mutation breeding can improve the disease resistance of crop plants.
Ø Induced mutations can induce desirable mutant alleles in crop plants.
Ø Mutation breeding can be used to improve the specific characters of a well-adapted high yielding variety.
Ø Quantitative characters characteristics of crop plants including yield can be improved by induced mutations.
Ø The F1 hybrids obtained from inter varietal cross are treated with mutagen to increase variability.
Ø Mutation breeding can effective to disseminate an undesirable character from a crop variety.
Limitations / Disadvantages of Mutation Breeding
Ø The frequency of desirable mutation will be very low (0.1 % of total mutations)
Ø The breeder has to screen a large population to select a desirable mutation.
Ø Desirable mutations are commonly associated with undesirable side effects.
Ø Mutations often produce pleiotropic effects.
Ø Mutation in quantitative traits is usually in a direction away from the selection history of the parent variety.
Ø There may be problems in registration of mutant variety in many parts.
Ø Most of the mutations are recessive and their effects are not expressed due to the dominance of its allelic counterpart.
Achievements of Mutation Breeding
Ø A large number of crop varieties have been produced by mutation breeding all over the world.
Ø A brief list is given below
Ø Cereals : 350 varieties
Ø Legumes : 62 varieties
Ø Fruits : 40 varieties
Ø Ornaments : 462 varieties
Among seed plants:
Rice : 278 varieties
Barley : 229 varieties
Wheat : 113 varieties
Ø China has produced 281 varieties (Top position)
Ø India has produced 116 varieties (Second position)
Ø USSR has produce: 82 varieties (Third position)
Ø Japan has produced: 65 varieties (Fourth position)
Mutation breeding in India:
Ø Till 1990, 219 mutant varieties of crop plants have been produced in India.
Ø Among which 116 are seed propagated and 103 vegetative propagated plants.
Ø The number of varieties of crop plants produced by mutation breeding in India are given below:
Rice : 24 varieties
Barley : 12 varieties
Cotton : 8 varieties
Ground nut : 8 varieties
Crop varieties produced in India by Mutation Breeding:
Rice : Jagannath
Wheat : NP836
Sugar cane : Co 8152, Co 8153
Cotton : Indore 2
Jute : JRO 514, JRO 412
Ø In rice, Jagannath is a gamma semi dwarf mutant from tall cultivar T141.
Ø Jagannath has improved resistance to lodging, high yield, more responsive to fertilizers than its parent.
Ø In wheat, NP836 is an awned mutant from the awneless seed variety NP799.
Ø Sugarcane Co8152 is a gamma induced mutant from Co527.
Ø Co8152 has 40% more yield than the parent.
Learning objectives: Mutagens and crop improvement, Spontaneous and induced mutations, Effects of mutation, Physical and chemical mutagens, Methods of mutation breeding, Mutations in oligogenic traits, Mutations in polygenic traits, limitations of mutation breeding, Achievements of mutation breeding, Role of mutations in Plant Breeding