Authors: Pallavi Neha*1 and Vinod Jatav 2
1Division of Postharvest Technology,
2Division of Vegetable Sciences
Indian Institute of Horticultural Research, Bengaluru, Karnataka-560089
*Email of correspondence: Pallavinehasingh@gmail.com
Introduction
Mutation is a sudden heritable change in DNA sequence which leads to change in characteristics of an organism as classically defined by Hugo de Vries in 1900. It is the phenomenon of a sudden intermittent change in the hereditary character. In vegetable, breeding Mutagenesis has a great importance. Mutagenesis is all about treating a biological material with a mutagen in order to induce mutations. The entire operation of the mutation induction & isolation of mutants for crop improvement is called as ‘Mutation breeding’.
Classification of mutation –
On the basis of occurrence, mutation can be classified further as
Spontaneous mutation-
- Mutation occurs in natural populations (without any treatment by man) at a low rate; known as Spontaneous mutation.
- The frequency of Spontaneous mutation is generally one in 10 lacs, i.e. 10-6.
- Spontaneous mutation rates of genes may be considerably affected by the genetic background; some mutator gene may promote mutation of other genes.
- The mutation may be artificially induced by a treatment with certain physical or chemical agents known as Induced Mutation.
- Agents used for producing them termed as Mutagens.
- The utilisation of induced mutation for crop improvement is known as Mutation Breeding.
- Induced mutation have a great advantage over the spontaneous ones; they occur at a relatively higher frequency so that it is practical to work with them.
Based on the structural change occur due to mutation it can be classified under-
Genomic mutation
- Change in chromosome number (gain or loss in complete sets of chromosomes or parts of a set)
- Change in chromosome structure (duplications of segments, translocation of segments)
- Changes in the nucleotide constitution of DNA (by deletion or substitution)
Dominant mutation
- Change of a recessive allele to a dominant allele (aA).
- Change of a dominant allele to a recessive allele (Aa).
- Mutations are generally recessive.
- Mutations are generally harmful to the organism. But a small proportion (0.1 percent) of them are beneficial.
- Mutations are random.
- Mutations are recurrent.
- Induced mutation commonly show pleiotropy.
Variations in chromosome structure due to mutation
Mutations involving changes in chromosome structure occur in four common types:
- Deletions.
- Duplications.
- Inversions (changing the orientation of a DNA segment).
- Translocations (moving a DNA segment).
Mutagents Agents which induces mutations are known as mutagens
Types of mutagens
Physical mutagen
- Ionising radiation: Non-particulate: Electromagnetic, X-ray, Gama ray Particulate and radiation: Alpha ray, Beta ray, Fast & Thermal neutrons
- Non- ionising radiation: U.V rays
- Alkylating agents: Sulphur mustard, nitrogen mustard, EMS, MMS
- Acridinedyes : acriflavine, proflavine, acridine orange etc.
- Base analogues: 5-bromouracil, 5-chlorouracil
- Others: Nitrous acid, hydroxyl amine, sodium azide
- The dose of radiation is measured as the amount of energy absorbed per unit mass of the irradiated object and is commonly expressed as rad.
- Exposure to radiation is measured in Roentgen units.
- The unit of radiation dose is called Gray (Gy), 1 Gy equals 100 rad or 1 J per kg of the irradiated object.
- Many workers feels that a dose close to LD50 should be optimum. LD50 is that dose of a mutagen which would kill 50% of the treated individuals.
- Lethal: Kill each & every individual carrying them
- Sublethal&Subvital : Sublethal kill more than 50% of the individuals, while sub vital kill much less than 50%.
- Vital : Do not reduce viability of the individual carrying them
- Induction of desirable mutant alleles, which may not be present in the germplasm or which may be present but may not be avoidable to the breeder due to political or geographical reasons.
- It is useful in improving specific characteristics of a well adapted high yielding variety.
- Mutagenesis has been successfully used to improve various quantitative characters, including yield.
- F1 hybrids from intervarietal crosses may be treated with mutagens in order to increase genetic variability by inducing mutations and by facilitating recombination among linked genes.
- Irradiation of distant hybrids has been done to produce translocations. This is done to transfer a chromosome segment carrying a desirable gene from the alien chromosome to the chromosome of a cultivated species
- Objectives of the programme
- Selection of the variety for mutagen treatment
- Part of the plant to be treated
- Dose of the mutagen
- Giving mutagen treatment
- Handling of the mutagen-treated population
- Dwarf mutants
- Leaf mutants
- Flower mutants
- Mutants for earliness
- High yielding mutants
- Male sterile mutants
- Mutants with changed quality traits
- Mutant with changed post-harvest life
- Mutants released as new varieties.
Mutant varieties in Vegetables
| Crop | Variety | Mutant Type |
| Okra | EMS-8 | EMS-treated mutant of PusaSawani |
| French bean | PusaParvati | X-ray mutant of Wax pod |
| Tomato | S-12 Maruthan (Co-3) PKM-1 PusaLalMeeruti | X-ray mutant of Sioux Mutant of Co-1 Mutant of Annagi Gamma ray mutant of Meeruti |
| Chilli | MDU-1 | Gamma ray mutant of K-1 |
| Hyacinth bean | Co-10 | Gamma ray mutant of Co-6 |
| Bitter gourd | MDU-1 | Gamma ray mutant of MC-103 |
| Palak | Jobner Green | A spontaneous mutant from local cv. |
Limitation of Mutation Breeding
- The frequency of desirable mutation is very low, about 0.1% of the total mutations.
- The breeder has to screen large populations to select desirable mutations.
- Desirable mutations are commonly associated with undesirable side effects due to other mutations, chromosomal aberrations etc.
- Often mutation produces pleiotropic effects.
- There may be problems in the registration of a mutant variety.
- Most of the mutations are recessive.
About Author / Additional Info:
Ph.D. scholar at IARI-IIHR