Mutation: Meaning, Kinds and Frequency

In this article we will discuss about:- 1. Meaning of Mutation 2. Kinds of Mutation 3. Frequency 4. Evolution 5. Role in Plant Breeding 6. Limitations.

Meaning of Mutation:

The credit of discovery of mutation goes to Wright (1791) when he observed it in male sheep. Any permanent or stable hereditary change other than one due to Mendelian segregation and recombination may be termed in its broader sense as mutation. Thus, the term ‘mutation’ in this sense ‘a sudden heritable change in a characteristic of an organism’ was primarily introduced by Hugo de Vries in 1900.

Mutation may be defined as an abrupt or sudden or discontinuous chromosomal change with genetic effect. —Mayer

“A mutation is a change in gene potentially capable of being transmitted”. —Synder

OR

“A mutation is a sudden and discontinuous change in a gene occurring rarely for any particular gene and capable of producing a change great or small in some part of body.” —Collins

OR

Mutation in the broad sense include all the changes in the hereditary material which can alter the character of any individual.

The word mutation is derived from Latin word mutate, meaning to change. Thus, mutations are the permanent changes in the genes. Mutant genes do not become expressed immediately because most of them are recessive.

Its phenotypic effects are seen only after one or more generations when the mutant gene is able to recombine with another similar recessive gene. Thus, mutation may be defined as an event that gives rise to a heritable alternation in the genotype. A sudden appearance of new hereditary character in a population is said to be due to mutation.

The mutation can be classified as follows:

1. Changes in genes

2. Changes in chromosomal number (polyploidy, haploidy, heteroploidy)

3. Changes in the arrangement of the chromosomal segments due to

(а) Intra-chromosomal segmental rearrangements (Inversions)

(b) Inter-chromosomal segmental rearrangements (Translocations)

(c) Losses and duplication of chromosomal segments (Deletions and Deficiency)

Kinds of Mutations:

According to the phenotypic expression the mutation may be classified in the following types:

(i) Somatic Mutations:

When changes in genes occur in the somatic or vegetative cells of the individuals, these are referred to as somatic mutation. Hugo de-Vries termed it as sports are saltation’s. It has been found by Emerson in endosperm of maize and in many tissues of plants. Chimeras have been developed of such nature.

(ii) Spontaneous Mutations:

These spontaneous or gene mutation generally are developed by natural agencies like light, temperature etc. There are various characters which are gene mutation. In mice spontaneous gene mutations determine coat colour which may be variously coloured like black, brown, spotted etc. In Drosophila there are many wild or normal type genes and their mutants like white eyes, pink eyes, yellow or black body colour and vestigial wing etc. Likewise, there are other gene affecting characters.

(iii) Germinal Mutations:

If the mutation occurs in the reproductive cells of gonads, then these are said as germinal mutation. Such type of mutation may be genetic occurring in the gametes of individuals or zygotic originating in the fused diploid gamete. Different sex linked mutations are of these types and pass from generation to generation.

(iv) Biochemical Mutation:

Such mutants influence the production of chemicals within an organism or causes the prevention of some enzymatic formation thus constituting biochemical mutation. Beadle and Tatum have studied in detail in Neurospora. Alcaptonuria and phenyl ketonuria described under gene nature are also biochemical mutations.

(v) Spurious Mutations:

These are hidden mutations appearing in the generation as a result of crossing over or other means. For example, in Drosophila, the gene for pink eyes remain usually hidden but it comes to light after crossing over. The appearance of recessive genes produced by crossing-over constitute spurious mutation.

(vi) Anomozygous Mutations:

These changes have developed due to structural (chromosomal aberrations) or numerical (polyploids) variations in the chromosomes.

(vii) Reverse Mutations:

It has been found in certain bacteria that are not capable to produce vitamins and other nutrients for their growth which normal type of bacteria can do efficiently. Sometimes such deficient mutants revert or change to normal condition is termed as reverse mutation. The chief cause of reverse mutation is radiation. Even then these reverse mutations as a rule occur rare.

(viii) Induced Mutations:

When gene changes are artificially produced or induced by means of experiments, such change constitutes induced mutation. The agent which cause these induced mutations are called mutagenic agents which may be x- ray’s radiation, various chemicals etc. Various chromosomal breaks make these changes.

Mutations have certain general characteristics which are summarised as below:

(i) Mutations are random i.e., they may come in a gene. However, some gene show higher mutation rates than others.

(ii) Mutations are generally lethal or harmful to the organism, a small proportion (0.1%) of all the induced mutations are useful.

(iii) Mutations are recurrent, i.e., the same mutation may occur repeatedly or again and again.

(iv) Induced mutations generally show pleiotropy (single gene affecting two or more different characters) often due to mutations in closely linked genes.

(v) Mutations provide the raw material for evolution.

(vi) Origin of mutation is unpredictable and haphazard.

(vii) Mutations are reversible i.e., an allele that arose through mutations of a gene can in turn mutate back to the original form of the gene. This is known as back mutation.

This can be represented as follows:

(viii) A number of different mutational possibilities exists for any particular gene. Different mutations at the same locus give rise to multiple allelic series. For example, in Drosophila, the sex-linked white-eye locus (w) is represented by a large number of different alleles. These include eosin and apricot as well as white and the wild type allele.

(ix) Some genes increase the spontaneous mutations rates of some other genes of the genome, such genes are called mutator genes. Some genes are termed as anti-mutator genes which suppress or prevent the mutation of other genes.

(x) Many agents, both physical and chemical increase the frequency of mutations, they are said as mutagenic agents.

(xi) Some mutant alleles do not mutate back. They do not exhibit reverse mutation. Such mutant alleles are believed to be formed by deletions.

Generally mutation have harmful effect on organisms. The individuals which carry them, reduce their viability.

Depending upon their effect on their viability of the individuals, it may be classified in to four groups:

(i) Lethal

(ii) Sub-lethal

(iii) Sub-vital

(iv) Vital

Muller (1927) firstly produced mutation successfully in Drosophila by x-ray treatment.

(i) Lethal:

Lethal mutations kill each and every organisms which carry them. Dominant lethals, therefore, cannot be studied because they can not survive even in the heterozygous state. Thus, we have to consider only recessive lethals. Recessive lethals would kill the individuals that carry them in homozygous state, e.g., albina chlorophyll mutation.

(ii) and (iii) Sub-lethal and Sub-vital.

It reduces the viability but do not kill the individuals carrying them. Sub-lethals will kill more than 50% individuals, where as sub-vitals less than 50%. A large majority of mutations are sub-lethals and sub-vitals, thus are of no value in crop improvement.

(iv) Vital:

Vital mutations do not reduce the viability of the individuals carrying them. Practically, crop improvement needs only such mutations. It occurs in very low frequency as compared to the other types.

Stages at which Mutations Occur:

Mutations may occur at any stage in the development of the organism. If mutation comes in the primordial germ cells, all the gametes derived from these primordial germ cells will be carrying the mutant character.

If it happens in one of the gamete, this leads a mutant individual in the progeny. If mutation takes place in one of the daughter chromosomes of the dividing zygote, one part of the body of the individual will be carrying the mutation.

The later it appears, smaller will be the part of the body carrying the mutation. This type of individual is called Mosaic, for example, in Drosophila normal red eye with a speck of white or with one white and one red eye. Mutations also take place in the somatic tissue of any part of the body.

Mutation appears suddenly and never occurs gradually in a single individual and transmits to its progeny. Mutations have been observed in Oenothera, maize, man and other plant and animal species like Drosophila. In recent years, micro-organisms have been found to be the most favourable material to study this phenomenon.

Frequency of Mutations:

Different genes have different rates of mutability. Mutation rate may be defined as the number of changes at one special locus from one allele to another, measured in a biological unit of time, i.e., generation in a given population.

The rate of mutation varies from one organism to another and even from one variety to another in the same organism. The mutation rate varies considerably from one locus to another in the same variety.

(a) Mutations in Drosophila:

In 1909, T.H. Morgan found in his normal red eyed strain of Drosophila culture, an exceptional male with white-eyes. He observed that the gene for eye colour is located in the sex chromosome and the character is transmitted as a sex-linked one. In the white-eyed flies, mutation occurred in the gene located in the sex chromosome and as a result the eye colour becomes white.

Thereafter, the eye colour remained as white generation after generation. So change in eye colour of Drosophila is due to sudden change in the gene which is located in the sex-chromosome. This sudden change is known as gene or point mutation.

Mutation may occur during at any developmental stage of organisms, but certain stages of cell division e.g., S phase may yield mutations at a higher rate with mutagens than those during other phases of cell-cycle.

(b) Mutation in Oenothera:

The Oenothera Lamarckian (evening primrose) is a native of America, de-Vries found it growing as a weed in Holland. In de Vries garden it produced a number of striking mutants. Of these a mutant called gigas differed from the parental form in its large size; another mutant called nanella was a small one; and still other different in colour, size and shape of various parts.

Now it has been traced out that what de-Vries had described as mutations are really changes in the chromosomal number. It has been proved that the parental forms have 14 chromosomes, and the mutant gigas have 28 chromosomes. So it is clear that the appearance of gigas is due to the change in the number of chromosomes only and not in the number of genes.

(c) Mutations in Bacteria:

Due to mutations bacteria may undergo the following changes:

(i) Bacteria may become resistant to the antibiotics such as Penicillin or they may become susceptible to it.

(ii) When Bacteriophages are introduced in a Bacteria culture, most of them die. But some of them become resistant to Bacteriophages. This resistance might have occurred in two ways (i) before the introduction of bacteriophages, mutations might have occurred and they would have become resistant or (ii) after the introduction of Bacteriophages mutation might have occurred in genes and they would have become resistant.

(d) Mutation in Man:

Biochemical Mutations:

Enzymes have a significant role in metabolism of every organism. But many of the enzymes are not present in the fertilized egg but gradually develop during the course of development under the influence of genes.

Chemical substances in the body are often of a complicated nature and are built up or broken down in a series of steps. Each enzyme is responsible just for one step and the following step would be taken in care of a totally different enzyme.

In the absence of enzyme ‘1’ A will not be converted in to B and in the absence of B, there will be no C and D. Each gene influences or is responsible for one particular enzyme and in the event of mutation in that gene the specific enzyme that influences is absent, and there would be a chemical block.

For example, if ‘1’ enzyme is in the chemicals produced by an organism or which prevent their production are called Biochemical mutations. These are known to occur both in man and other organisms.

Biochemical Mutations in Man:

1. Alcaptonuria:

The first recorded biochemical mutation in man is Alcaptonuria, described by Garrod in 1909. This is characterised by the fact that the urine of the patient turns black on exposure to air. Alcaptonuria is inherited as an autosomal recessive character. Garrod found that people who suffer from this disorder can not break down a certain chemical substance known as homogenetisic acid.

In normal people, homogenetisic acid is converted in to an intermediate substance known as acetoacetic acid which then is broken down in to CO₂ and H₂O. But in alcaptonurics, the step from homogenetisic acid to acetoacetic acid does not occur, with the result that the homogenetisic acid accumulates in the urine and gives black colour when exposed to air.

The normal reaction takes place under the influence of an enzyme present in the blood serum of normal persons, but seems to be absent in the sera of alcaptonurics.

Garrod was able to show that the chemical block, here was due to a recessive mutation.

2. Phenylketonuria:

It is a metabolic disorder more serious than Alcaptonuria, but chemically related to it. Children suffering from this disorder are mentally defective and are called phenyl pyruvic idiots. They have also a variety of other defects of a serious nature and die at an early age.

On the chemical side, they are unable to break down a certain chemical substance, known as phenyl pyruvic acid, in to another substance called hydroxy-phenyl-pyruvic acid which in turn would usually be broken down in to simple waste products, CO₂ and H₂O. Here again, the chemical block is due to a mutation.

3. Albinism:

Albinism in man is due to a mutation which interferes with a step in the building up of melanin pigment. All the three defects mentioned above are chemically related, in that they involve breaks in a metabolic chain which can be traced back to a substance known as phenylalanine.

4. Gout:

Gout is another disease in man. Biochemically it results from a defect of nucleic acid metabolism. This results in accumulation of uric acid in the blood. If the amount of uric acid exceeds too much, gouty arthritis may occur because of uric acid crystals forming in the joints.

5. Diabetes mellitus:

This is a well-known and important disease characterised by excretion of sugar in the urine. It is caused by failure of the pancreatic inlets to secrete insulin, a hormone which is essential to carbohydrate metabolism. The symptom of the disease is sugar in the urine, which in turn results from elevation of the blood sugar level above 180 milligrams percent.

Induced Mutations:

Mutations produced artificially due to the chemical or a physical agents (mainly radiations) are said as induced mutation. The agents capable of inducing mutations are called as mutagens and their capacity for inducing mutation is termed as mutagenic property. The process of inducing mutations through treatment of mutagen is called as mutagenesis, while use of induced mutations for improvement of crops is referred to as mutation breeding’s.

Detection of Mutation, CIB Method:

H.J.Muller (1927) showed that mutations could be artificially produced in Drosophila by X-rays. These mutations caused visible changes as from red-eye colour to white or lethal effects causing death. X-rays produce lethals in all the chromosomes of Drosophila, but here the discussion will be confined to X- chromosome.

There is a standard technique for the detection of new lethals in the X-chromosomes in Drosophila devised by Muller and known as CIB method. In the females (XX) one X-chromosome contains three genes: a dominant C is the cross-over suppressor; a recessive lethal gene; and the dominant gene B for bar eyes.

All the three genes CIB are in one X-chromosome and the other X- chromosome is normal. Such flies are known as CIB females. These bar eyed female (CIB) are mated with normal males (XY) that have been X-rayed.

As a result of mating, barred females were produced containing CIB (derived from their mother) and X-chromosome from father being treated with X-rays. This female (CIB X’) when mated with any normal male (XY), then in F₂ half of their sons receive the CIB chromosome and die.

The other half receive the treated X-chromosome, and these also die if the treated X- chromosome contains a lethal. But all the F₂ daughters will survive since they receive a normal X-chromosome from their father. Thus it is concluded that if F₁ CIB female produces only daughters and not sons, it means that she has received a lethal from treated father.

Some times a special technique ‘attached X method’ is used to detect visible mutation. In this female carrying attached X and Y chromosomes are mated with the males radiated by X-rays. Sons which will have two different X-chromosomes may be identified and mutation can be detected.

Evolution of Mutation:

It has been observed that Mendelian variation may arise by the recombination of the existing genes through hybridization. Therefore, fundamental change is provided for by the mutation. Mutations are not losses of genes but are only changes in genie structure. Mutations are almost recessive. Baur in Antirrhinum majus found only 9 to 10 dominants out of 300 mutations and the others proved to be recessive to the wild types.

The rate and direction of mutation vary in different genes. Mutations cause a large disturbance in the normal development, functioning and living of any body. Therefore, mutations are of great importance in evolution with small effects. These are some times said as micro mutations and their effects are often not observable. If these small variations prove to be of advantage to the organisms, the later have greater value in evolution.

The hereditary variations by mutations are directive and non-purposive. The mutation are not directed by the environment and these do not develop to fulfill any specified purpose for the organism.

As examples for mutation providing chances for evolution of new forms, varieties or species may be mentioned, Cicer gigas and Arachis hypogea var. gigantea. The very popular improved variety in paddy G.E.B. 24 is believed to be arisen by mutation in Kona Mani. Similarly, in ragi (Eleusine coracana) E.C. 3735 arose by mutation in E.C. 593. The former is earlier in duration than the latter.

Mutation in both quantitative and qualitative characters has been exploited for development of over 300 varieties of various crops. Some mutations are useful in crop improvement and have beneficial effects. Mutations have allowed the analysis of genes as well as the determination of relationship between genes and proteins as well as some features of the genetic code.

Induced mutations are being applied for analysis of the effects of the various types of known alternations in DNA or genes on the expression of the concerned characters. Such an analysis is often called reverse genetics as opposed to the classical genetics.

Fundamentally mutation in a gene allows that gene to be identified and studied. For example, white eye locus was observed only after the white eyed mutant of the Drosophila was discovered, the same holds true for other eye colour genes.

Role of Mutation in Plant Breeding:

It has been used for improving qualitative and quantitative characters including disease resistance and yielding ability of various crops.

The various applications of mutation breeding may be given as below:

(i) F₁ hybrids produced from hybridization may be treated with various mutagens to increase genetic variability and to facilitate recombination among linked genes. This method has not been extensively used.

(ii) Various mutagens have been used to improve different quantitative characters specially yield. By this technique various varieties have been developed so far which have shown high yielding performance.

(iii) In the case of clonal crops which are highly heterozygous in nature, mutagenesis is only the best method to bring about improved specific characteristic of clones without modifying their genetic make up. e.g., ‘red sports’ in apple etc. In other words, it is useful to improve specific characteristics of a well adopted high yielding variety.

(iv) Mutation breeding serves as a useful supplement to the available germplasm. It should be well understood that mutation breeding cannot minimise the necessity of collection of germplasm.

(v) Mutation have been found useful in certain specific characters like seed setting. Much more work has been done in Sweden by Gustafson 1954, 1960, Nybora 1954, Mackey, 1956, Smith 1951, where mutagenic agents are applied over many cultivated crop plants including garden trees.

Likewise, a huge amount of work has been done in east Germany on soya bean and barley by Scholz 1960, Zacharias 1956, Stubbe 1959 and in U.S.A. on Arachis hypogea. (pea-unit) by Gregory (1956). A variety of barely named Pallas and of pea named Stral has been developed by X-ray irradiation The mutant variety of barley is different in quality than its parent in following ways- (a) Early maturity (b) hard stem (c) more diastase activity (d) bold seeds and (e) disease resistance etc.

(vi) Irradiation of distant hybrids has been done to produce translocations. This is done to transfer a segment of chromosome having a desirable gene from the alien chromosome to the chromosome of a cultivated species of crop.

(vii) As a result of mutagenesis more than 335 varieties have been produced in different countries of the world. Such mutant varieties may be exemplified as in cereals, vegetables, millets, oil seeds, pulses, fruit trees etc. but paddy, barley, wheat account for 50% of the mutant varieties in all the crops. These crop varieties belong to diploid and polyploid, sexually and asexually reproducing species.

In India the work on inducing mutation has been started from ICAR, New Delhi. ‘Gamma garden’ has been developed for the same. Since 1930, a number of crop varieties have been developed through mutagenesis e.g., G.E.B. 24, Jagannath paddy, CO8152, CO8153 (sugarcane), wheat NP836, cotton Indore 2, Jute JRO 412 and 514, Gram T87 etc. Jagannath paddy is a gamma-ray induced semi-dwarf mutant from the tall variety T141. Jagannath has various qualities like resistance to lodging, higher yielding ability and more responsible to fertilizer application in comparison of parent variety.

Sugarcane variety CO8152 is a gamma-ray induced mutant from CO527. It was observed that CO8152 gave 40 percent higher yield than the parent variety. The mutant variety CO8152 has two chromosomes less than the parent variety CO527. This is an example of a change in chromosome number which has already produced a desirable mutant phenotype.

The mutant varieties (about 94%) developed till 1982 were due to following treatments like physical mutagens; through chemical mutagenesis (about 5%) and 1% through combined treatment (physical & chemical mutagens both). It was observed that physical mutagens were more effective in asexually propagated crops than the sexually reproducing ones as shown below-

Apsara in Bhabha atomic research centre, Trombe is now underutilization for inducing mutation since 1956. By application of neutron in Apsara various crop varieties have been produced for example, Pt B 10 and G.E.B. 24, Inbred lines like D4, N6, D65 & ITe 701 in maize etc.

Limitations of Mutation:

(1) Induction of mutation artificially is very costly. Therefore, this technique is not so useful.

(2) Mutations are not stable.

(3) Most of the mutations are lethal.

(4) The main target of the mutation is to develop variation but India is lucky where natural variation is in plenty.