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. The genomes of organisms are all composed of DNA, whereas viral genomes can be of DNA or RNA. Mutation in the DNA of a body cell of a multicellular organism (somatic mutation) may be transmitted to descendant cells by DNA replication and hence result in a sector or patch of cells having abnormal function, an example being cancer.
Mutations in egg or sperm cells (germinal mutations) may result in an individual offspring all of whose cells carry the mutation, which often confers some serious malfunction, as in the case of a human genetic disease such as cystic fibrosis. Mutations result either from accidents during the normal chemical transactions of DNA, often during replication, or from exposure to high-energy electromagnetic radiation (e.g., ultraviolet light or X-rays) or particle radiation or to highly reactive chemicals in the environment. Because mutations are random changes, they are expected to be mostly deleterious, but some may be beneficial in certain environments. In general, mutation is the main source of genetic variation, which is the raw material for evolution by natural selection.
Characteristics of Mutations:
Mutations have several characteristic features.
Some of the important characteristics of mutations are briefly presented below:
i. Nature of Change:
Mutations are more or less permanent and heritable changes in the phenotype of an individual. Such changes occur due to alteration in number, kind or sequence of nucleotides of genetic material, i.e., DNA in most of the cases.
ii. Frequency:
Spontaneous mutations occur at a very low frequency. However, the mutation rate can be enhanced many fold by the use of physical and chemical mutagens.
The frequency of mutation for a gene is calculated as follows:
Frequency of gene mutation = M / M + N
where, M = number of individuals expressing mutation for a gene, and
N = number of normal individuals in a population.
iii. Mutation Rate:
Mutation rate varies from gene to gene. Some genes exhibit high mutation rate than others. Such genes are known as mutable genes, e.g., white eye in Drosophila. In some genomes, some genes enhance the natural mutation rate of other genes. Such genes are termed as mutator genes.
The example of mutator gene is dotted gene in maize. In some cases, some genes decrease the frequency of spontaneous mutations of other genes in the same genome, which are referred to as anti-mutator genes. Such gene has been reported in bacteria and bacteriophages.
iv. Direction of Change:
Mutations usually occur from dominant to recessive allele or wild type to mutant allele. However, reverse mutations are also known, e.g., notch wing and bar eye in Drosophila.
v. Effects:
Mutations are generally harmful to the organism. In other words, most of the mutations have deleterious effects. Only about 0.1% of the induced mutations are useful in crop improvement. In majority of cases, mutant alleles have pleiotropic effects. Mutations give rise to multiple alleles of a gene.
vi. Site of Mutation:
Muton which is a sub-division of gene is the site of mutation. An average gene contains 500 to 1000 mutational sites. Within a gene some sites are highly mutable than others. These are generally referred to as hot spots. Mutations may occur in any tissue of an organism, i.e., somatic or gametic.
vii. Type of Event:
Mutations are random events. They may occur in any gene (nuclear or cytoplasmic), in any cell (somatic or reproductive) and at any stage of development of an individual.
viii. Recurrence:
The same type of mutation may occur repeatedly or again and again in different individuals of the same population. Thus, mutations are of recurrent nature.
Classification of Mutations:
Mutations can be classified in various ways. A brief classification of mutations on the basis of:
(1) Source,
(2) Direction,
(3) Tissue,
(4) Effects,
(5) Site,
(6) Character, and
(7) Visibility is presented in Table 14.1.
Types of Mutants:
The product of a mutation is known as mutant. It may be a genotype or an individual or a cell or a polypeptide.
There are four main classes of identifiable mutants, viz:
(i) Morphological,
(ii) Lethal,
(iii) Conditional and
(iv) Biochemical.
These are briefly described below:
i. Morphological:
Morphological mutants refer to change in form, i.e., shape, size and colour. Albino spores in Neurospora, curly wings in Drosophila, dwarf peas, short legged sheep are some examples of morphological mutants.
ii. Lethal:
In this class, the new allele is recognized by its mortal or lethal effect on the organism. When the mutant allele is lethal all individuals carrying such allele will die; but when it is semi-lethal or sub-vital some of the individuals will survive.
iii. Conditional Lethal:
Some alleles produce a mutant phenotype under specific environmental conditions. Such mutants are called restrictive mutants. Under other conditions they produce normal phenotype and are called permissive. Such mutants can be grown under permissive conditions and then be shifted to restrictive conditions for evaluation.
iv. Biochemical Mutant:
Some mutants are identified by the loss of a biochemical function of the cell. The cell can assume normal function, if the medium is supplemented with appropriate nutrients. For example, adenine auxotroph’s can be grown only if adenine is supplied, whereas wild type does not require adenine supplement.
Agents of Mutations:
Mutagens:
Mutagens refer to physical or chemical agents which greatly enhance the frequency of mutations. Various radiations and chemicals are used as mutagens. Radiations come under physical mutagens. A brief description of various physical and chemical mutagens is presented below:
Physical Mutagens:
Physical mutagens include various types of radiations, viz. X-rays, gamma rays, alpha particles, beta particles, fast and thermal (slow) neutrons and ultra violet rays (Table 14.2).
A brief description of these mutagens is presented below:
Detection of Mutation:
Detection of mutations depends on their types. Morphological mutations are detected either by change in the phenotype of an individual or by change in the segregation ratio in a cross between normal (with marker) and irradiated individuals. The molecular mutations are detected by a change in the nucleotide, and a biochemical mutation can be detected by alteration in a biochemical reaction.
The methods of detection of morphological mutants have been developed mainly with Drosophila. Four methods, viz., (1) CIB method, (2) Muller’s 5 method, (3) attached X-chromosome method, and (4) curly lobe plum method are in common use for detection of mutations in Drosophila.
A brief description of each method is presented below:
i. CIB Method:
This method was developed by Muller for detection of induced sex linked recessive lethal mutations in Drosophila male. In this technique, C represents a paracentric inversion in large part of X-chromosome which suppresses crossing over in the inverted portion. The I is a recessive lethal. Females with lethal gene can survive only in heterozygous condition.
The B stands for bar eye which acts as a marker and helps in identification of flies. The I and B are inherited together because C does not allow crossing over to occur between them. The males with CIB chromosome do not survive because of lethal effect.
The important steps of this method are as follows:
(a) A cross is made between CIB female and mutagen treated male. In F1 half of the males having normal X-chromosome will survive and those carrying CIB chromosome will die. Among the females, half have CIB chromosome and half normal chromosome (Fig. 14.2). From F1, females with CIB chromosome and male with normal chromosome are selected for further crossing.
(b) Now a cross is made between CIB female and normal male. This time the CIB female has one CIB chromosome and one mutagen treated chromosome received from the male in earlier cross.
This will produce two types of females, viz., half with CIB chromosome and half with mutagen treated chromosome (with normal phenotype). Both the progeny will survive. In case of males, half with CIB will die and other half have mutagen treated chromosome.
If a lethal mutation was induced in mutagen treated X-chromosome, the remaining half males will also die, resulting in absence of male progeny in the above cross. Absence of male progeny in F2 confirms the induction of sex linked recessive lethal mutation in the mutagen treated Drosophila male.
ii. Muller 5 Method:
This method was also developed by Muller to detect sex linked mutation in Drosophila. This method is an improved version of CIB method. This method differs from CIB method in two important aspects. First, this method utilizes apricot recessive gene in place of recessive lethal in CIB method. Second, the female is homozygous for bar apricot genes, whereas it is heterozygous for IB genes in CIB method.
In this method, the mutation is detected by the absence of wild males in F2 progeny.
a. A homozygous bar apricot female is crossed with mutagen treated male. In F1 we get two types of progeny, viz., heterozygous bar females and bar apricot (Muller) males.
b. These F1 are inter-mated. This produces four types of individuals. Half of the females are homozygous bar apricot, and half are bar heterozygous. Among the males, half are bar apricot (Muller 5) and half should be normal. If a lethal mutation is induced, the normal male will be absent in the progeny.
iii. Attached X-Method:
This method is used to detect sex linked visible mutations in Drosophila. In this method a female in which two X-chromosomes are united or attached together is used to study the mutation (Fig. 14.4). Therefore, this method is known as attached X-method. The attached X females (XXY) are crossed to mutagen treated male. This cross gives rise to super females (XX-X), attached female (XXY), mutant male (XY) and YY.
The YY individuals die and super female also usually dies. The surviving male has received X-chromosome from mutagen treated male and Y chromosome from attached X-female. Since Y chromosome does not have corresponding allele of X-chromosome, even recessive mutation will express in such male which can be easily detected.
iv. Curly Lobe-Plum Method:
This method is used for detection of mutation in autosomes. In this method curly refers to curly wings, lobe to lobed eye and plum to plum or brownish eye. All these three genes are recessive lethal. Curly (CY) and lobed (L) genes are located in one chromosome and plum (Pm) in another but homologous chromosome.
Crossing over between these chromosomes cannot occur due to presence of inversion. Moreover, homozygous individuals for CYL or Pm cannot survive because of lethal effect. Only heterozygotes survive. Thus, this system is also known as balanced lethal system.
a. A cross is made between curly lobe plum (CYL/Pm) female and mutagen treated male. This produces 50% progeny as curly lobe and 50% as plum.
b. In the second generation cross is made between curly lobe female and curly lobe plum male. This will give rise to curly lobe plum, curly lobe and plum individuals in 1 : 1 : 1 ratio and homozygous curly will die due to lethal effect. From this progeny, curly lobe females and males are selected for further mating.
c. In third generation, a cross is made between curly lobe female carrying one mutagen treated autosome and curly lobe male also carrying treated autosome. This results in production of 50% progeny as curly lobe, 25% homozygous curly lobe which die and 25% progeny homozygous for treated autosomes.
This will express as autosomal recessive mutation and constitute one third of the surviving progeny. A comparison of different methods of detection of mutation in Drosophila is given in Table 14.4.
Applications of Mutations in Crop Improvement:
Induced mutations are useful in crop improvement in five principal ways, viz:
(1) Development of improved varieties,
(2) Induction of male sterility,
(3) Production of haploids,
(4) Creation of genetic variability, and
(5) Overcoming self-incompatibility.
These are briefly discussed below:
i. Development of Improved Varieties:
More than 2000 improved varieties (some directly and some by use of mutants in hybridization) have been developed through induced mutations in various field crops all over the world.
In India, induced mutations have been instrumental in developing improved varieties in wheat (NP 836, Sarbati Sonor’a, Pusa Lerma), barley (RDB 1), rice (Jagannath, IIT 48, NT 60), tomato, castor bean (Aruna, Sobhagya), cotton (MCU 7, MCU 10, Indore 2), groundnut (TGI), sugarcane (Co 8152, 8153) and several other crops.
Besides high yield, varieties have been developed with better quality, earliness, dwarfness, disease resistance and low toxin contents in various crops.
Improvement in quality has been achieved for protein content in wheat and rice, oil content in mustard and sugar content in sugarcane. Earliness has been achieved in castor (from 270 days to 140 days), rice and soybean. Dwarf varieties have been developed through the use of mutant parents in wheat, rice, Sorghum and pearl millet.
Disease resistance has been induced in oats to Victoria blight and crown rust; in wheat for strip rust; in barley for mildew; in groundnut for leaf spot and stem rust; in sugarcane for red rot; in apple for mildew, etc. Low toxin content varieties have been developed in rapeseed and mustard for erusic acid and in Lathyrus sativa for neurotoxin content.
ii. Induction of Male Sterility:
Induced mutations have been useful in induction of male sterility in some crop plants. Genetic male sterility has been induced in durum wheat using radiations. CMS mutants have been induced in barley, sugarbeet, pearl millet and cotton. Use of GMS and CMS lines helps in reducing the cost of hybrid seed production.
iii. Production of Haploids:
Use of X-ray irradiated pollens has helped in production of haploids in many crops. Chromosome doubling of these haploids results in the development of inbred lines which can be utilized in the development of commercial hybrids.
iv. Creation of Genetic Variability:
Induced mutations are very effective in creating genetic variability for various economic characters in crop plants. Induced mutations have been used for increasing the range of genetic variability in barley, oats, wheat and many other crops. In asexually propagated crops like sugarcane and potato, somatic mutations may be useful, because the mutant plant can be multiplied as a clone.
v. Overcoming Self-Incompatibility:
Mutation of S gene by irradiation offers a solution to the production of self-fertile plants in self-incompatible species. This has been successful in case of Prunusovium. Besides this practical application in crop improvement, induced mutations are of fundamental interest in genetical studies.
Induced mutations have some limitations also. 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 mutations are produced at a very low frequency, a very large plant population has to be screened to identify and isolate desirable mutants.
