⚗️ Procedure for Mutation Breeding

This lesson covers core principles and exam-focused points from this topic in plant breeding.

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PROCEDURE FOR MUTATION BREEDING

Treating a biological material with a mutagen in order to induce mutations is known as

mutagenesis. Exposure of a biological material to a radiation like X rays,_gamma - rays, etc. is

known as irradiation. When mutations are induced for crop improvement, the entire operation of

the induction and isolation, etc. of mutants is termed as mutation breeding. A mutation breeding

programme should be clearly planned and should be large enough with sufficient facilities to

permit an effective screening of large populations. The various steps involved in mutation

breeding are briefly discussed below.

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Objectives of the Programme

A mutation breeding programme should have well defined and clear-cut objectives. If

the experimenter starts a mutagenesis programme just with the hope that he will discover

something useful, he is most likely wasting his time and resources. This is because the ratio of

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beneficial to useless mutations is very small (1 in 800 mutations, that is, about 0.1 % of the

mutations), and identifying desirable mutations from among the undesirable ones is a very

difficult task indeed. Further, if a character governed by oligogenes is to be improved, the

procedure for the handling of treated populations would be different from that when a polygenic

trait is the target for improvement.

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Selection of the Variety for Mutagen Treatment

Generally, the variety selected for mutagenesis should be the best variety available in the

crop. This is particularly so when polygenic traits are to be improved. It serves no purpose to

isolate desirable mutants in a less adapted inferior variety only to discover that the mutant lines

have no agricultural worth, or that the mutants have to be used in a hybridization programme for

transferring the mutant characteristics to a superior variety. In certain situations; however, it may

be desirable to isolate mutants in varieties other than the best one. For example, an extensive

search is being made for alternative dwarfing genes in cereals, particularly in wheat and rice ( O.

sativa ). In this situation, dwarf and semi dwarf mutants would have to be isolated from tall

varieties, which obviously would not be the best varieties of these crops.

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Part of the Plant to be Treated

Seeds, pollen grains or vegetative propagules (buds and cuttings) or even complete plants

may be used for mutagenesis. Which plant part should be used for mutagen treatment depends

primarily on whether the crop is sexually or asexually propagated and on the mutagen to be used.

In sexually propagated crops, seeds are the most commonly used plant part. Dry dormant seeds

are biologically almost inert and they can stand a range of extreme environmental conditions,

such as, soaking, desiccation, heating, freezing, oxic or anoxic regimes, etc. Mutagenic treatment

of seeds is essentially a treatment of embryo meristems .

Since mutation is a single cell event, the M1 plants will carry an induced mutation only

in parts of the shoot, i.e., they will be chimaeras. Pollen grains may be used, but they are

infrequently used because (1) it is difficult to collect large quantities of pollen grains in most

crop species, (2) hand pollination (with treated pollen) is difficult, and (3) pollen al is relatively

short. Pollen grains are the only plant part, which can be successfully treated with UV radiation.

A pollen monolayer is exposed to UV rays of 250 to 290 nm; of the biological effects induced by

UV rays are almost comparable to those produced sparsely ionizing radiation. In case of clonal

crops, buds or cuttings are used for mutagenesis.

Radiation (except UV) is suitable for use with all the three plant parts and even with the

plants. Whole plants are generally irradiated during the flowering stage so that it is equivalent to

the irradiation of pollen grains and egg cells. However the treatment of whole requires special

facilities (a gamma garden) and is possible in a few places only. Chemical mutagens are best

used with seeds, but some workers have used them with vegetative propagules as well.

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Dose of the Mutagen

The usefulness of a mutagen and the type of treatment required to obtain a high

efficiency pendent upon specific properties of the mutagenic agent employed (its effectiveness,

effect relationship and mode of application) as well as on specific characteristics of the

biological system to be treated (the sensitivity of the treated tissues depending upon anatomical,

physiological, biochemical and genetic peculiarities). The most appropriate plant or stage to be

treated requires a thorough knowledge of the organisms and a clear definition of experimental

objectives.

Mutagen treatments reduce germination, growth rate, vigour and fertility (pollen as well

lie). There is considerable killing of plants during the various stages of development after

mutagen treatment; thus survival is reduced considerably. Mutagens generally induce a high

frequency of chromosomal changes and mitotic and meiotic irregularities. Usually, the damage

increases with the mutagen dose, but it may not necessarily be proportional. An optimum dose is

the one, which produces the maximum frequency of mutations and causes the minimum killing.

The dose required for high mutagenic efficiency depends on the properties of the

mutagenic agent, of the solvent medium and of the biological system. Many workers that a dose

close to LDso should be the optimum. LDso is that dose of a mutagen, which would kill 50 per cent

of the treated individuals. LDso varies with the crop species and with mutagen used. A

preliminary experiment is generally conducted to determine the suitable mutagen dose. In

general, an overdose is likely to kill too many treated individuals, while an underdose would

produce too few mutations.

Dose of the mutagen may be varied by varying the intensity or the treatment duration. In

case of radiation, intensity may be varied by changing the radiation source or by changing the

nee from the radiation source of the material being irradiated. Intensity in the case of chemical

mutagens may be varied by changing the concentration of mutagens.

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Giving Mutagen Treatment

The selected plant part is exposed to the desired mutagen dose. In case of irradiation, the

plant parts are immediately planted to raise M1 plants from them (pollen grains are used for

pollination). In case of chemical mutagens, seeds are usually presoaked for a few hours to initiate

metabolic activities, exposed to the desired mutagen and then washed in running tap water to

remove the mutagen present in them. The treated seeds are, usually, immediately planted in the

field to raise the M1 generation. M1 is the generation produced directly from the mutagen-treated

plant parts without a recourse to sexual or asexual reproduction. But when pollen grains are

treated, the generation resulting from the seeds that were produced by pollination with the treated

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pollen grains would be the M1 generation. M2, M3,M4, etc. are the subsequent generations

derived from M1, M2, M3, etc. plants through selfing or clonal propagation.

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Handling of the Mutagen-Treated Population

Treatment of seeds and vegetative propagules commonly produces chimaeras. A chimera

is an individual with one genotype in some of its parts and another genotype in the others. Shoot

tip meristem usually has three functional layers as follows: (1) L1 gives rise to epidermis, (2) L2

produces a part of leaf mesophyll and gametes, and (3) L3 yields the rest of plant body. When

the whole of Ll, L2 or L3 layer is affected, the chimaera is known as periclimal chimaera,

while in a sectorial chimaera only a part of Ll, L2 or L3 layer is affected.

In sexually reproducing species, only the L2 chimaera (periclinal or sectorial) will be

transmitted to the next generation; other chimaeras will not be recovered since these layers do

not contribute to the production of gametes. In clonal crops, however, all chimaeras can be

utilized either as periclinal chimaeras or by producing homogeneous individuals through sexual

reproduction (only if the L2 layer is affected), tissue culture or certain other horticultural

manipulations, e.g., wounding, etc., which induce production of adventitious shoot buds (all

chimaeras are utilized). Sectorial chimaeras are unstable in clonal crops and have to be made

periclinal through successive clonal propagation and selection for stability.

Mutations usually occur in small sectors of the meristem and, as a result, only a part of

the plant is affected. One or more sexual or clonal generations coupled with selection are

necessary to obtain a stable mutant phenotype. Mutant alleles are generally recessive, but some

dominant mutations may also occur. In case of sexually reproducing crops, mutation breeding

utilizes both recessive and dominant mutations and, in addition, excellent opportunities exist for

mutation breeding for polygenic traits. Mutation breeding in clonal crops, however, primarily

depends on dominant mutations; recessive mutations may also he utilized provided the clone

used for mutagen treatment was heterozygous for the gene in question. For example, if recessive

mutant allele a is to be useful in a clonal crop, the clone used for mutagenesis has to have the

genotype Aa. Such situations are, however, rare; more frequently, the mutants useful in the

improvement of clonal crops contain dominant mutations, and they may even include changes in

chromosome structure or even number.

Mutations are called macro or micro-mutations depending on the magnitude of phenotypic

effect produced by them. A macromutation produces a large phenotypic effect recognizable on

individual plant basis; obviously, such mutations are oligogenic in nature and can be easily

selected in the M2 generation. In contrast, a micromutation has a small phenotypic effect that

cannot be recognised on individual plant basis; it can be detected only in a group of plants and

often statistical treatment of data may be necessary. Obviously, macromutations are polygenic in

nature and selection for them is delayed till M3 or a later generation.

The detailed procedure for handling of M2, M3, etc. generations will differ depending

mainly on the oligogenic or polygenic trait to be improved and on the mode of reproduction crop

species. The following discussion is based on sexually reproducing species, more particularly,

self-pollinated species. Since dominant mutations are able to express themselves in heterozygous

state, mutant plants are selected in M1 and often in M 2 and M3, individual plant progenies are

raised and homozygous mutants are selected. Selection for recessive mutations, however, can be

taken up in M2 only, but the mutant allele will be homozygous in the M2 itself. Selection for

polygenic traits is delayed till M3 generation, and is based on individual plant progenies rather

than on individual plants. Generalized schemes handling the mutagen treated populations for

oligogenic and polygenic traits are outlined in the next section.

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Summary Cheat Sheet

Quick Recall Points

• This lesson focuses on key plant breeding concepts, terminology, and exam-relevant applications.
• Review major definitions, classifications, and method-wise distinctions from the sections above.
• Revise tables and examples from this lesson for fast pre-exam recall.

Exam Traps

• Do not confuse similarly named breeding methods without checking their core selection logic.
• Pay attention to crop-specific examples because the same principle can behave differently by species.

References

1 source • [1]

[1]

Standard Plant Breeding Class Notes (GPBR211)

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