Top 2 Methods of Plant Breeding | Botany

The following points highlight the top two methods of plant breeding. The methods are: 1. Mutation Breeding 2. Haploid/Monoploid Breeding.

Method # 1. Mutation Breeding:

Mutation is a sudden heritable change in the genotype of an organism. The process by which the change does occur is called mutation and the individual in which it is observed is called mutant.

Mutation occurs in natural population but at a very low rate, which is called spontaneous mutation, but the mutation can be induced by chemical or physical mutagen is called induced mutation. Utilisation of characters which are the result of induced mutation in crop improvement programme is known as mutation breed­ing.

Objectives:

As in mutation process the ratio of beneficial and harmful character is very small, so it is very difficult to get the desirable trait within a small population. Any mutation breeding programme should have well defined and clear-cut objectives-on which the ma­terial type and also the type of mutagen to be used will depend. Furthermore it should be kept in mind that whether the character is governed by oligogene or polygene – the han­dling of treated material will be different.

Selection of Material:

The mutation should be attempted in the variety which is the best one in that species, as getting a desirable mutant in a poor variety is not useful for breeding programme. Though sometimes it may happen that for selection of particular character poor variety is selected, such as, for searching a ‘dwarf’ mutant some tall varieties should be considered which may not be best one.

Many plant parts, such as, seeds, pollen grains; vegetative propagules (buds or cut­tings) may be used for mutagenesis. Plant parts to be used for mutagen treatment depend on the mode of reproduction of plant and the type of mutagen to be used for mutagenesis.

Selection of Mutagen:

The choice of mutagen depends on the material type used; its effectiveness and the proper dose should be considered during treatment. The mutagen may be chemical, such as, base analogues (5-Bromo-uracil), antibiotics (azasorine), alkylating agents (EMS, MMS, DES) and also miscellaneous compounds (dyes, hydroxylamine), etc. or may be physical, such as, ionising (X-ray, P-ray, y-ray) or non-ionising (UV) radiations.

An optimum dose is the one which produces the maximum frequency of mutation and causes the minimum killing. Overdose will cause death or detrimental to most whereas the under-dose will produce few mutations.

Procedure of Mutation Breeding:

A. In Self Pollinated Crops:

First Treated Generation (M₁):

The common effects of mutations are death of seed­ling, growth inhibition, morphological or developmental abnormalities, and the heritable changes in qualitative or quantitative characters. Among these only the heritable changes either in cytoplasm or chromosomes or genes which are observed in M₁ generation are selected as mutants and these are promoted to M₂ generation.

M₂ Generation:

In this generation a large population is essential and maximum number of mutants is recovered due to recombination and segregation. The process of recovery would depend on the efficiency of screening procedure in M₂ and also in the later generations.

Mutagen treatment of seeds and vegetative propagules produce chimeras, where some parts of plant get changed or mutated but other parts remain unchanged. Chimera may be periclinal or sectorial, the inner chimera only will be transmitted to next generation but not the outer one.

When seeds are produced in mutated M₁ plants, the mutated tissue has to pass through two sieves of selection pressure. Mutated tissue should compete with normal tissue during formation of vegetative stage and reproductive organs.

This selection is called diplontic selection. If the M₁ flower is chimeric, mutant or normal pollen will be formed and the stigma will receive two types of pollen and both will compete for fertilizing the ovule — this is called as haplontic selection.

After competing in both the selection pressure, i.e., diplontic and haplontic selec­tion, the mutant type would appear in M₂ generation.

M₃ and later Generation:

Selected plant progenies of M₂ generation are grown and evaluated critically in M₃ generation.

Progenies of selected M₃ plants are grown as M₄ generation for further purification, evaluation and multiplication.

Most mutants get purified by M₅ generation and those are then evaluated in field trials along with parental variety as standard check variety.

B. In Cross-Pollinated Crops:

In this type of crops genetic variability occurs more naturally, so there is minimum need for mutation breeding. Also as these are opening pollinated, the detection of mutants is more difficult.

C. Asexually Propagated Crops:

Mutation breeding is more useful in this type of crops as other conventional breeding methods cannot be applied due to lack of their sexual cycle. Somatic mutant can easily be created in this type of plants, propagated and directly used.

Since such plants are more heterozygous, any mutation from dominant to recessive may be detected and used. Any phenotypic effect of chromosomal arrangements may also be utilised due to vegetative mode of reproduction.

After mutagen treatment, formation of chimera is regular occurrence. This can be avoided by irradiating the youngest possible stage of bud, i.e., less differentiated primordia. Various propagation methods like budding, grafting and prunning are used for selec­tion of mutated parts.

Merits:

1. De Novo (New) Creation of Genetic Variability:

When the desired character does not exist, mutations are used to get the new character.

2. Breaking Undesirable Linkage:

When two undesirable characters are linked together, mutagen treatment may be used to break the linkage.

3. Production of Haploids:

Haploids may be produced using X-rayed pollen for pollination; un-fertilised egg develops into haploid which are variously used.

4. Increase or Decrease of Chiasma Frequency:

Mutation may change the chiasma frequency which is directly related with recombination and segregation of characters.

5. Production of Transitory Sexuality in Apomicts:

Apomictic plants breed like asexually propagated plants and breeding is difficult. Transitory sexuality can be induced so the better apomictic type can be selected after crossing with various sexual types.

6. Reduction of Incompatibility in Wide Crosses:

Though actual mechanism is not known but irradiation induces better pollen tube growth in few genera which enables inter-specific crosses with the irradiated pollen.

7. Variation in F₁ Hybrids:

F₁ hybrids from inter-varietal crosses may be treated with mutagens in order to increase genetic variability by inducing mutation and to facilitate recombination of linked genes.

8. Production of Distant Hybrid with Translocation:

Irradiation of inter-spe­cific (distant) hybrid causes chromosomal segment translocation carrying some desirable genes which may help in transfer of character from one species to another.

Demerits:

1. Unpredictability:

The frequency of desirable mutants is very low, 0.1% of total mutations. So it is required to grow a large population of M₂ generation and it is laborious to screen out a few economically wanted mutated prog­enies.

2. Difficulty in Detection:

Mutation breeding is difficult if the detection proce­dure is elaborate. The disease resistance or quality characters cannot be de­tected easily which needs elaborate tests. Thus mutation breeding is not easily applicable to improve this type of character in crop. Moreover most of the mutations are recessive in nature; it is very difficult to detect it in polyploid species.

3. Undesirable Expression:

Desirable mutations are sometimes associated with undesirable side effects or chromosomal aberrations. To remove these kinds of deleterious effects, back-crossing is done which requires more time and expenses.

4. Unrepeatability:

Since the process of mutation is not well understood, so there is no control on the outcome or result of mutagen treatment. Unless directed mutagenesis is applied, it is difficult to obtain the same or desired mutants every time.

Achievements:

Point mutation as well as chromosomal mutation both has been used to produce many improved varieties which represent the improvement in oligogenic as well as poly­genic characteristics.

‘Sonora 64’, a good variety of wheat had red colour grain, ϒ-radiation produced ‘Sharbati Sonora’ with white colour grain. Chromosome segments of Aegilops, Secale, Agropyron carrying rust resistance genes have been trans-located to wheat by using X-rays.

High yielding groundnut variety NC₄ X modified into NC₄ by micro mutation which looks like the parent.

Point mutations have produced superior sugarcane varieties TS-1 and TS-2 which are the mutants of CO 419. In sugarcane, CO 8152 is a ϒ-ray induced mutant from CO- 527 which gives 40% higher yield, but CO 8152 has two chromosomes less than CO-527.

Duplication in a segment of chromosome 6 of barley carrying gene ‘orange lemma’ associated with a-amylase activity improves the matting quality of barley.

‘Jagannath’ rice variety is a ϒ-ray induced semi-dwarf mutant from tall cultivar T- 141, which has the improved resistance to lodging, high yield, responsive to fertiliser.

JRO-3690, a high yielding jute variety has been developed by hybridisation between two low yield mutants of jute.

Polyploidy Breeding:

Irregularities occurring during mitotic or meiotic division may lead to changes in chromosome number which might have some role in improvement of crop plants.

The type of changes in chromosome number can be broadly subdivided into two categories:

1. Aneuploidy:

Addition or loss of one or few chromosomes.

2. Euploidy:

Addition or loss of total genome set.

Aneuploids are either hypoploid (Monosomy or Nullisomy) or hyperploid (Tri­somy or Tetrasomy). Euploids may be haploid, diploid, triploid, tetraploid, etc. Polyp­loids are euploids above the level of diploid which are either auto-polyploids or allopoly­ploids.

Effects of Polyploidy:

Primary Effect:

The increase in cell size in growing tissue is the primary effect of polyploidy, though the magnitude of cell elongation and increase in cell number depend on the type of plant parts. It is reflected in the plant parts where there is determinate growth habit (sepals, petals, anthers, fruits, etc.).

Secondary Effects:

Changes in Growth Rate:

Polyploidy results in slower growth rate associated with reduction in branching, etc.

Effects on Fertility and Genetic Behaviour:

Reduction in pollen viability and seed fertility occurs due to irregular chro­mosome distribution during meiosis.

Changes in Cell Composition:

As polyploidy is associated with cell volume expansion, so it is associated with high water con­tent and lower somatic tension.

Effects on Segregation:

Multivalent formation causes differen­tial behaviour in meiosis which effects the segregation pattern of genes.

Mating Barrier:

Polyploidy creates a barrier on crossing be­tween a polyploid and the diploid progenitor.

Induction of Polyploidy:

Polyploidy is induced artificially with the help of agents like:

1. Colchicine Drug:

It is a non-toxic water soluble alkaloid obtained from Colchicum autumnale which is applied by different methods:

(a) Tube method — Treatment of shoot apex and cotyledons of rooted seed­lings through a tube.

(b) Dropper method — A small drop of colchicines solution is applied on the shoot apex situated between cotyledons.

(c) Immersion method— The seeds are allowed to germinate in immersed condition in colchicine solution.

(d) Wilk method — The young seedlings are allowed to soak colchicine so­lution through roots.

2. Physical Agents:

Heat or cold treatment, X-ray or ϒ-ray irradiation may pro­duce polyploids in low frequencies.

3. Decapitation:

In some plants decapitation induces polyploid cell develop­ment at cut ends and the adventitious bud formation.

4. In Vitro Culture:

Ploidy level may change during in vitro cultural condition. During protoplast culture it is a normal feature to get the auto-fused product. In another culture also, the polyploidy can be induced.

Breeding Procedure:

The procedure for polyploid breeding is mainly based on following aspects:

1. Artificial induction and collection of naturally occurring polyploids.

2. Detection of different kinds of polyploids.

3. Hybridization and selection of polyploids.

Method # 2. Haploid/Monoploid Breeding:

Haploidy is very much important in plant breeding as it is the quickest procedure of attaining homozygosity, so it helps to get the expression of recessive genes in double haploids. Haploids are helpful to get the expression of induced mutations and can be used to determine the homology between genomes.

Haploidy can be induced by parthenogenesis and through anther or pollen culture technique. Through wide hybridization between species and through chromosome elimi­nation technique haploidy can also be attained.

The haploidy breeding is concentrated on three items:

(a) High frequency of haploids produced.

(b) Easy detection of haploids.

(c) Diploidisation of desirable haploids.

Autopolyploid Breeding:

The production of triploids (3n) and tetraploids (4n) are most important among auto-polyploids. Triploids are obtained through crossing between tetraploid and diploid (4n x 2n → 3n).

Owing to their self-sterility seed yield is low, but in case of seedless fruits (watermelon) or where the vegetative parts are commercially used, the triploids can be exploited successfully. The triploids or tetraploids can easily be detected by their greater vigour for which they are economically viable.

Amphidiploid or Allopolyploid Breeding:

When the hybrids are developed by inter-crossing between two distantly related taxa then to restore the normal segregation and chromosome balance doubling is needed. Amphidiploid breeding offers a short-cut method of transfer of alien gene/chromosome into cultivars by overcoming the natural crossing barrier. One of the most important example is Triticale (Triticum aestivum x Secale cereal).

Aneuploid Breeding:

Though nullisomics or trisomics have no direct value in plant breeding but they are extensively used to bring the alien variation into the cultivated varieties. Tertiary trisomics have been used successfully in production of hybrid barley seeds.

Applications of Allopolyploidy in Crop Improvement:

Monoploids:

These are weaker than diploids and have no direct use in agriculture. But they are of great interest for crop improvement to develop diploid homozygous line, for isolation and detection of mutant line.

Triploids:

Triploids are obtained after crossing between 4n and 2n plants, as they are highly sterile to produce seedless fruit like watermelon, thus have been used successfully. Another example is sugarbeet where triploidy has contributed more sugar content than diploids.

Tetraploids:

Autotetraploids have been produced in large number of crop species, these are very much useful in breeding for improved quality, overcoming self-incom­patibility and helpful in making distant crosses.

Autotetraploid maize has 43% more carotenoid pigments and vitamin A than dip­loid, but the performance of tetraploids may not always be superior than diploids.

Sometimes the distant hybridization programme fails due to self-incompatibility in case of diploids. But autotetraploids can overcome this problem as experimented in Brassica oleracea, or in Solanum tuberosum.

The most successful autotetraploids used in agriculture are tetraploid red clover, ryegrass, berseem, etc. There are also attempts for production of larger grains with high protein content and higher yield in case of Jowar and Barley.

Application of Allopolyploidy in Crop Improvement:

In nature many crop species have been evolved due to successful production of al­lopolyploids.

So the allopolyploids have immense importance in plant breeding as this can be used for three broad objectives:

(a) Utilisation as Bridging Species:

Amphidiploids can serve as a bridge in transfer of characters from one species to a related species, may be from wild species to cultivated species. In most of the cases, the F₁ hybrid between two widely related species may become sterile which on diploidization produces the amphidiploid fertile plants.

For example, F₁ hybrid of Nicotiana tabacum and N. sylvestris is sterile, diploidization of which produces allohexaploid N. digluta which is fertile. This new bridging species is back-crossed with N. tabacum to produce ‘Tabacum’ like plants which is TMV resistant. The objective was to transfer TMV resistant character from N. sylvestris to N. tabacum.

(b) Creation of New Species:

Triticale is the most successful synthetic allopoly­ploid which is produced by crossing wheat and rye. This has the combination of characters like yielding ability and grain qualities of wheat and the hardi­ness of rye. The successful commercial variety of Triticale has the yield as much as the best varieties of common wheat.

Raphanobrassica is another promising allopolyploid obtained from the cross­ing between Brassica oleracea and Raphanus sativus. There is possibility of improvement of Raphanobrassica by further hybridization and selection at the polyploid level.

‘Varalakshmi’, a hybrid variety of cotton which is the interspecific hybrid between Gossypium hirsutum (American cotton) and G. barbadense (Egyp­tian cotton), made by diploidisation of the F₁ hybrid.

(c) Widening the Genetic Base of Existing Allopolyploids:

The genetic base of some natural allopolyploids may be narrow, and it may be useful to introduce variability by producing new allopolyploids. Brassica napus is a natural allopolyploid which has very narrow range of genetic variability. The synthetic allopolyploid B. napus can be obtained by crossing B. campestris (n = 10, AA) and B. oleracea (n = 9, CC) to produce the amphidiploid, B. napus (n = 19, AACC).

Application of Aneuploidy in Crop Improvement:

1. Aneuploids are useful in studying the effects of loss or gain of an entire chro­mosome on the phenotypic character expression of any individual.

2. By using secondary or tertiary trisomics, the location of a particular linkage group can be detected on a particular chromosome.

3. Homologous chromosomes can be identified among the different genomes of a particular crop, i.e., nullisomic condition of a particular chromosome can be compensated by tetrasomic condition of another chromosome of other ge­nome set.

4. Aneuploids are useful in production of substitution line which are helpful to study the effect of individual chromosome.

5. Aneuploids can also be helpful to identify the trans-located chromosomes in an individual.