What are the different types of mutations in plants ?

Mutation is defined as any sudden and drastic heritable change in gene which is not traceable or ascribable to segregation or recombination. According to Darwin sudden appearance of new hereditary character in the offspring of plants. Bateson said that mutation is a discontinuous variation. How even mutations play important role in evolution of a new species when a major set of characters is subjected to be mutated.

Mutations may be natural or induced and may be occur at chromosome level or at gene or molecular level or may takes place involving the cytoplasm or cytoplasmic organelles like plastids.

Classification of mutation:

Plant geneticists have classified mutations in different ways for shake of their own convenience

Ammato (1950) classified mutation as:

i) Gene mutation (mutation at gene level)

ii) Chromosomal mutation(structural level of chromosome)

iii) Genomatic mutation (Mutation in chromosome number)

According to Darlington and Mather mutations are following five types:

i) Gene mutation

ii) Structural mutation

iii) Plastid mutation

iv) Numerical mutation

v) Cytoplasmic mutation

Recent studies on mutation in plants have been classified under following types

Chromosomal mutation:

i) Chromosomal aberrations (structural mutation)

ii) Geomantic or numerical mutations (Poly ploides)

iii) Germinal mutation

iv) Somatic mutation

v) Gene mutation or point mutation

vi) Cytoplasmic mutation

Chromosomal mutation:

Chromosomal, utation refers to any change in the structure or gross morphology of chromosome and change in chromosome number. The former case involve loss or gain or any alteration in chromosome which called chromosomal aberrations and the later is the change in basic chromosomal aberrations and the later is the change is basic chromosome number of a species and called polyploidy or numerical mutation.

i) Chromosomal aberrations: These include structural changes which takes place during meiosis. These are following types:

a. Deletion or deficiency: In this case if a chromosome broken in to pieces and reunion takes place without taking one or more pieces with loss of a segment. It has great cytological and genetic effect on organism and may be lethal.

b. Duplication: If an extra chromosomal part is added to normal character, it is called duplication. Duplication takes place when delated part of a chromosome added to another chromosome.

c. Translocation: When a part of chromosome is broken into two or more pieces and some fragment is transferred from the broken chromatid and rejoin with the other chromatid then it is called translocation. It may be simple or reciprocal causing disturbance in position of genes. It may cause sterility in plants.

d. Inversion: In this process a piece of chromosome is removed and reinserted in reverse order changing the gene sequences. It may be paracentric or pericentric type. It causes position effect resulting alteration in geneaction.

ii) Poly ploidy : (Numerical mutation)

In this type of mutation basic genomic set changed due to disjunction or defective meiosis or nuclear division. Thus changed into 3n or 4n or 6n etc. these polyploidy can be classified into Autopolyploidy and allopolyploidy.

a) Autopolyploidy:

When the basic chromosome set of an individual is multiplied due to union of two diploid gametes or somatic doubling of chromosomes or union of one haploid and one diploid gamets. It may be autotriploid, autotetraploid etc. these are found in avaena, coffee, apple, banana, sugarcane, chili etc.

(b) Allopolyloids:

Allopolyploids formed by multiplication of chromosome sets of a hybrid of two diploid species. Suppose in a cross between a species – X (AA) and species Y(BB) is made it will give rise to F1 hybrid with chromosome doubling and a alloteraploid having (A,A,B,B)genomic set will be obtained.

Allopolyploidy can be induced and have played important role in evolution of new species.

Example: Raphanobrassica, Gossipium hissutum etc.

ii) Germinal Mutation:

Those mutations occur in the germplasm of an organism and may occur at any time during life cycle are called germinal mutation. The effect of these mutations expressed in the progenies. This mutation is found in wheat as dwarfism. Expression of germinal Mutation depends upon whether it is of dominant or recessive form. The progeny produced will be mutant if the gamete posses mutant gene.

iii) Somatic mutation:

Somatic mutations are the mutation occurs in the somatic cells of an individual, which cannot be passed to the offspring through the gamets. This mutated genes lost with the death of the individual.

In somatic mutations, the extent of phenotypic effect depends on various factors, like the stage of life cycle. It can not be pass on to the germ cells and only inherited to the next generation of progeny which produced from the mutated somatic cell.

This mutation cause cancerous growth and defected metabolism of cells and tissues produced by the mutated cell. It is found in vegitatively and asexulally reproducing plants and animals.

Such mutations used in production of delicious apples, navel orange and many other fruits and flower production.

Now it is commercially used in horticultural practices.

iv) Gene mutation or point mutation.

These gene mutations are intragenic in which alteration in the structure of DNA molecule within a gene occurs. These occurs change in the normal base sequence of DNA molecule which leads to modification of structural characteristic or enzymatic activity of an individual. The unit of gene mutation, i.e. a specific nucleotide or nucleotides called muton.

In this type of mutation tautomerism occur in the structure of base molecule, which enable it for unusual pairing. “Adenine instead of bonding with “Thymine” binds with guanine or other bases or other tautomeric forms of bases.

Gene mutations may be spontaneous or may be induced by application of mutagenic agents.

Gene mutations may occur under following types on the basis of their mode of occurrence.

i) Frame shift mutations

ii) Deletion mutations

iii) Insertion mutations

iv) Substitution mutations

v) Transition mutation

vi) Transition mutation

vii) Transversion mutation

viii) Inversion mutation

ix) Missense mutation (Formation of codon carried other amino acid in sequence)

x) Nonsence mutation (Forming nonsence codons at the placed of normal codons)

xi) Silent mutations (Point mutation having no phenotypic expression)

v) Plastid mutation:

Mutations in the genetic materials in plastids are called plastid mutation and it is governed by the self duplicating non-med3elian genetic material called plastogenes. Plastogenic mutation cause defective plastid characters which traced in maize, barley and rice.

vi) Cytoplasmic mutation:

Mutation which takes place in the nucleus free or extra chromosomal genetic material is called cytoplasmic mutation. These mutations effect cytoplasmic inheritance chlorophyl deficiency in algae, like chlamydomonas, enzmatic alteration in yeast and antibiotic resistant in bacteria are the consequences of cytoplasmic mutation. These mutations occure at plasmogenes or cytogenes or plasmones or plasmid.