Hybridization Method of Crop Improvement

In this article we will learn about the Meaning of Hybridization Method of Crop Improvement:- 1. Meaning of Hybridization 2. Objectives of Hybridization 3. Types 4. Procedure 4. Hybridization Methods of Plant Breeding in Self-Pollinated Groups 5. Hybridization Methods of Plant Breeding in Cross-Pollinated Crops.

Meaning of Hybridization:

Individual produced as a result of cross between two genetically different parents is known as hybrid. The natural or artificial process that results in the formation of hybrid is known as hybridization.

Or

The production of a hybrid by crossing two individuals of unlike genetical constitution is known as hybridization. Hybridization is an important method of combining characters of different plants. Hybridization does not change genetic contents of organisms but it produces new combination of genes.

The first natural hybridization was recorded by Cotton Mather (1716) in corn. The first artificial interspecific plant hybrid was produced by Thomas Fairchild in 1717. It is commonly known as ‘Fairchild Mule’.

Hybridization was first of all practically utilized in crop improvement by German botanist Joseph Koerauter in 1760. Mendel onward, the hybridization had become the key method of crop improvement. Today, it is the most common method of crop improvement, and the vast majority of crop varieties have resulted from hybridization.

Objectives of Hybridization:

I. To artificially create a variable population for the selection of types with desired combination of characters.

II. To combine the desired characters into a single individual, and

III. To exploit and utilize the hybrid varieties.

Types of Hybridization:

Hybridization may be of following types:

(i) Intra-varietal hybridization:

The crosses are made between the plants of the same variety.

(ii) Inter-varietal or Intraspecific hybridization:

The crosses are made between the plants belonging to two different varieties.

(iv) Interspecific hybridization or intragenric hybridization:

The crosses are made between two different species of the same genus.

(v) Introgressive hybridization:

Transfer of some genes from one species into the genome of the other species is known as introgressive hybridization. The crosses between different species of the same genus or different genera of the same family are also known as distant hybridization or wide crossing. Such crosses are called distant crosses.

Procedure of Hybridization:

It involves the following steps:

(i) Selection of parents.

(ii) Selfing of parents or artificial self-pollination.

(iii) Emasculation.

(iv) Bagging

(v) Tagging

(vi) Crossing

(vii) Harvesting and storing the F, seeds

(viii) Raising the F₁ generation.

(i) Selection of parents:

The selection of parents depends upon the aims and objectives of breeding. Parental plants must be selected from the local areas and are supposed to be the best suited to the existing conditions.

(ii) Selfing of parents or artificial self-pollination:

It is essential for inducing homozygosity for eliminating the undesirable characters and obtaining inbreeds.

(iii) Emasculation:

It is the third step in hybridization. Inbreeds are grown under normal conditions and are emasculated. Emasculation is the removal of stamens from female parent before they burst and shed their pollens.

It can be defined as the removal of stamens or anthers or the killing of the pollen grains of a flower without affecting in any way the female reproductive organs. Emasculation is not required in unisexual plants but it is essential in bisexual or self-pollinated plants.

Various methods used for emasculation are:

(a) Hand Emasculation or Forceps or Scissor Method:

This method is generally used in those plants which have large flowers. In this method the corolla of the selected flowers is opened and the anthers carefully removed with the help of fine-tip forceps.

Following are the important precautions while performing this method:

i. Flowers should be selected at proper stage.

ii. Stigma should be receptive and anthers should not have dehisced.

iii. All the anthers should be removed from the flowers without breaking (Fig. 6).

iv. Stigma and ovary of the flower should not be damaged.

(b) Hot Water Treatment:

Removal of stamens with the help of forceps is very difficult in minute flowers. In such small hermaphrodite flowers (e.g., Bajra, Jowar) emasculation is done by dipping the flowers in hot water for a certain duration (1-10 minutes) of time.

The time varies from species to species. This method Is based on the fact that gynoecia can withstand the hot temperature at which the anthers are killed. In this method an equipment is used which is placed on a simple heavy stand.

It consists of a cylindrical metallic container of 60 cm length, with one hole of 5 cm to 16 cm diameter on one end to pass over a bajra or jowar head. After inserting the panicle inside the container a cork is fitted in the hole to close it.

A 35 cm long rubber tube or belt is stretched over the side of the container, and when in use this tube is tied around the peduncle of the head. To measure the temperature, in the upper side of the container a thermometer is placed. In the field water is carried in a thermos jug (Fig. 7).

The panicle is inserted in the container prior to blooming for a particular duration of time. It has been observed that pollen grains of rice are killed by immersing the inflorescence for 5 to 10 minutes in the hot water maintained at 40-44°C in a thermos flask.

(c) Cold Water Treatment:

Like hot water cold water also kills pollen grains without damaging the gynoecium. In rice 0-6°C temperature is maintained to kill the pollen grains. This method is less effective than hot water treatment.

(d) Alcohol Treatment Method:

This method is not commonly used for emasculation because duration of treatment is an important factor since a very short duration is required failing which even the gynoecium may be damaged. Flowers or inflorescences are immersed in alcohol of a suitable concentration for a brief period. In alfa-alfa, a treatment of even 10 seconds with 57 % alcohol is sufficient to kill the pollen grains.

(e) Suction Method:

It is a mechanical method and is suitable for the crops having minute flowers. In this method the amount of pressure is applied in such a way that only anthers are sucked out and other parts of the flower like gynoecium remain intact. However, in this method 10-15% self pollination takes place. It is one of the major drawback of this method.

(f) Male Sterility or Self-incompatibility Method:

Emasculation option can be eliminated by the use of male-sterile plants, In some self-pollinated plants for example, Sorghum, Onion, Barley etc. anthers are sterile and do not produce any viable pollens! Similarly self-incompatibility may also be used to avoid emasculation.

(g) Chemical Gametocides:

Certain chemicals are capable of causing male sterility, when sprayed before flowering e.g., 2, 4-D, naphthalene acetic acid (NAA), maleic-hydrazide (MA), tribenzoic acid etc. FW450 in cotton may be used for bringing about emasculation.

(iv) Bagging:

It is the fourth step and completed with emasculation. The emasculated flower or inflorescence is immediately bagged to avoid pollination by any foreign pollen. The bags may be made of paper, butter paper, glassine or fine cloth. Butter paper or vegetable parchment bags are most commonly used.

The bags are tied to the base of the inflorescence or to the stalk of the flower with the help of thread, wire or pins. The bagging is done with the emasculation in bisexual plants and before the stigma receptivity and dehiscence of the anthers in unisexual plants. Both male and female flowers are bagged separately to prevent contamination in male flowers and cross-pollination in female flowers (Fig. 8).

(v) Tagging:

The emasculated flowers are tagged just after bagging. Generally circular tags of about 3 cm or rectangular tags of about 3 x 2 cm are used. The tags are attached to the base of flower or inflorescence with the help of thread.

The information on tag must be as brief as possible but complete bearing the following information:

(i) Number referring to the field record

(ii) Date of emasculation

(iii) Date of crossing

(iv) Name of the female parent is written first followed by a cross sign (x) and then the male parent, e.g., C x D denotes that C is the female parent and D is the male parent.

(vi) Crossing:

It is the sixth step. It can be defined as the artificial cross-pollination between the genetically unlike plants. In this method mature, fertile and viable pollens from the male parent are placed on the receptive stigma of emasculated flowers to bring about fertilization.

Pollen grains are collected in petridishs (e.g., Wheat, cotton etc.) or in paper bags {e.g., maize) and applied to the receptive stigmas with the help of a camel hair brush, piece of paper, tooth pick or forceps. In some crops (e.g., Jowar, Bajra) the inflorescences of both the parents are enclosed in the same bag.

(vii) Harvesting and Storing the F₁ Seeds:

Crossed heads or pods of desirable plants are harvested and after complete drying they are threshed. Seeds are stored properly with original tags.

(viii) Raising the F₁ generation:

In the coming season, the stored seeds are sown separately to raise the F₁ generation. The plants of F₁ generation are progenies of cross seeds and therefore are hybrids.

Hybridization Methods of Plant Breeding in Self-Pollinated Groups:

There are several methods of improvement of self-fertilized crops by hybridization. These are:

1. Pedigree method or breeding

2. Bulk method or breeding

3. Single seed descent method

4. Back cross method.

5. Multiple cross method

1. Pedigree Method:

Record of the ancestry of an individual selected plant for various generations is known as pedigree. A selection method, which is used in segregating population of self-pollinated species and keeps proper record of plants and progeny selected in each generation is known as pedigree breeding. This method is widely used for the development of varieties in self-pollinated crops.

In this method individual plants are selected till the progenies become homozygous. Selection for plants in the desired combination of characters is started in the F₂ generation and continued in succeeding generations until genetic purity is reached.

The method is as follows (Fig. 9):

I Year:

Plants are chosen for hybridization and F₁ seeds are produced.

II Year (F₁ generation):

F₁ plants are space planted to produce maximum number of F₂ seeds (see Fig. 9).

III Year (F₂ generation):

2000-10000 F₂ plants are space planted. About 200-500 desirable superior plants are selected.

IV Year (F₃ generation):

Selected superior plants in III year are space planted to study the individual plant. 3 to 5 best plants in these rows are selected and harvested (F₄)

V Year and VI Year (F₄, F₅ generation):

Process is continued as in F₃ generation. Normally 20-50 families may be retained at the end of F₅ generation.

VII Year (F₆ generation):

Due to successive self-pollination most of the lines become homozygous and uniform. The plants uniform in desired characters are harvested and the seed, bulked together to constitute the variety.

VIII Year (F₇ generation):

Preliminary yield trials are conducted.

IX to XI year (F₈ – F₁₀ generation):

Trials of superior lines are confirmed. During the testing period observations are made on height, tendency to lodge, maturity, disease resistance and quality.

XII to XIII Year (F₁₀, F₁₁ generation):

Seeds are multiplied and distributed to the farmers.

Merits:

(i) It is the quickest method.

(ii) Plant breeders can also obtain the genetic information.

(iii) There are chances of recovering transgenic segregation by this method.

Demerits:

Maintenance of accurate pedigree record is not easy. It takes much time.

Selected material becomes so large that handling of the same becomes very difficult.

Success of this method depends upon the skill of the breeder.

Mass pedigree method:

It is a modified form of pedigree method in which segregating material is handled by bulk (mass) method when conditions are unfavorable for selection and by pedigree method when conditions are favourable for selection.

2. Bulk Method or Breeding:

A selection procedure which is used in segregating population of self-.pollinated species in which material is grown in bulk plot from F₂ to F₅ with or without selection, next generation is grown from bulk seed and individual plant selection is practiced in F₆ or later generations is called bulk method or breeding.

This method is also known as the mass or population method. Nilsson-Eule of Sweden was first to use the bulk method and it is in use ever since. This method differs from the pedigree method in that no selection is practiced in F₂-F₅ generations (Fig. 10).

The method is as follows:

I Year:

Plants are chosen for hybridization and F₁ seeds are produced.

II Year (F₁ generation):

50-100 F₁ plants are grown and their F₂ seeds are harvested in bulk,

III Year (F₂ generation):

F₂ plants are grown and their F₂ seeds are harvested in bulk.

IV Year (F₃ generation):

F₃ plants are grown and their F₄ seeds are harvested in bulk.

V Year (F₄ generation):

F_A plants are grown and their F₅ seeds are harvested in bulk.

VI Year (F₅ generation):

F₅ plants are grown and their F₆ seeds are harvested in bulk. (The process may be repeated until the desired period of homozygosity is achieved. In general bulk period is allowed up to F₅ generation)

VII Year (F₆ generation):

Seeds are space planted and single plant selection is done (F₇ generation).

VIII Year (F₇ generation):

The progeny of each single plant is grown separately and superior progeny are selected and isolated (F₈).

IX Year (F₈ generation):

Preliminary yield test are conducted (F₉).

X-XII Year (F₉-F₁₂ generations):

Multi-locations field trials are carried out, best performing strain is multiplied for seed distribution.

Merits:

(i) The bulk method is simple, convenient, inexpensive and less labour consuming (no pedigree record is to be kept).

(ii) During early segregating generations, very little work and attention is needed, which gives the breeder more time to concentrate on other breeding projects.

(iii) Selection is done by nature only and it increases the frequency of superior types in the population.

(iv) This method is suitable for studies on the survival of genes and genotypes in populations.

Demerits:

(i) This method takes much longer time to develop a new variety.

(ii) The breeder is enable to exercise his skill and judgement in selection and therefore the method is less satisfying to him.

(iii) Information on the inheritance of characters cannot be obtained.

(iv) This method is totally dependent on natural selection to select the superior types. These types may not be necessarily the best yielding types.

3. Single Seed Descent Method:

This method was suggested by Coulden (1939) for advancing segregating generation of self-pollinated crops. A breeding procedure used with segregating populations of self-pollinated species in which plants are advanced by single seeds from one generation to the next is referred to as single seed descent method.

The procedure is as follows (Fig. 11):

4. Back Cross Method:

This method was first proposed by Harlan and Pope (1922). Now-a-days this method is employed in improvement of both self and cross-pollinated crops where varieties are deficient in one or two aspects. This method is used particularly for transferring a single simply inherited character like disease, frost or drought resistance and earliness from an undesirable variety to a good commercial variety.

The desirable variety is called as recurrent or recipient parent and it is crossed to an undesirable variety, called as donor or non-recurring parent (called donor because the desirable genes are transferred).

F₁ plants instead of permitting to self-pollination as in pedigree or bulk method care crossed with the recurring parent and therefore, it is called as back cross method (A back cross may be defined as a crossing of F₁ hybrid with any of its parents).

The procedure is as follows:

Suppose there are two varieties A and B. A is very good in all characters but disease susceptible and the variety B is disease resistant but very poor in all characters. It is needed to transfer the disease resistance from B to A without adversely affecting the good qualities of A. B is then donor or non-recurring parent and A is recipient or recurring parent.

Selected plants of A and B are crossed to raise the F₁ Here A is treated as the female and B as male. F₁ plant is backcrossed with the recurrent parent; variety A and BC_: plants (back cross first generation) plants are raised. Here B is treated as female and A as male. Select the BC₁ plants possessing desired characters of A with disease resistance of B.

The selected BC₁ plants again backcrossed to A till a desirable type having good qualities of A and disease resistance of B is obtained (Fig. 12). Disease resistant plants are selfed and individual plant progenies from the selfed seeds of selected plants are grown.

New variety is tested in replicated field trials along with the variety A as check. Seeds are multiplied and released for cultivation. The method is used for the transfer of a dominant gene. In case of recessive gene transfer, a slightly different procedure is followed. It is summarized in Figs. 12, 13.

Merits:

1. This is the most common method used for transfer for oligogenic character like disease resistance.

2. It is the only method used for the interspecific transfer of characters, transfer of qualitative characters, quantitative characters, transfer of cytoplasm, particularly for male sterility and for the production of isogenics lines.

3. Role of environment is almost negligible. So time required for developing new variety may be drastically reduced because off-season nurseries and greenhouses can be used for developing 2-3 generations each year.

4. Extensive yield tests are not required because the performance of the recurrent parent is already known.

5. Breeder does not require to handle large populations.

6. In this method all desirable characters of a popular variety (genotype known) are retained except the undesirable genes at a particular locus is replaced. So, the outcome of this method is known forehand, and plants can be reproduced any time in future.

Demerits:

(i) The new variety is superior over the already available variety only in the transferred character.

(ii) Hybridization has to be done for each backcross. It requires lot of time and money.

(iii) Sometimes undesirable genes are closely linked with desirable one and get transferred to the new variety.

5. Multiple Cross Method:

A cross-involving more than one inbred line is referred to as multiple cross. It is also known composite cross and is used to combine monogenetic characters from different sources into a single genotype. In this method, several pure lines are crossed together. The selected pure lines are first combined into crosses as A × B, C × D, E × F, G × H and so on.

The F₁ plants are mated together as (A × B) × (C × D) and (E × F) x (G × H). Finally, the F₁ plants of double crosses are crossed with each other to produce hybrids [(A × B) x (C x D)] x [(E x F) x (G x H)]. Further breeding in these hybrids is carried out according to either pedigree or bulk method.

Merits:

1. In self-pollinated crops this method is used when three or four monogenic characters scattered in three or four different varieties are to be combined into one.

2. These crosses generally have wider adaptation.

Demerits:

1. These crosses are generally less productive.

2. This method has limited utility except in high risk areas where severe disease damage occurs regularly from a highly specialized disease pathogen.

Hybridization Methods of Plant Breeding in Cross-Pollinated Crops:

Like self-pollinated crops, hybridization in cross-pollinated crops involves crossing of two or more inbreeds. The inbreeds are combined in any one of the following types of crosses and released as improved strains.

These crosses are:

(1) Single cross:

Crossing of two inbreeds or varieties are called single cross such as A x B or C x D. This cross was proposed by Shull (1909). The total number of possible different single crosses produced by a given number of inbreeds can be calculated by the formula n(n-1)/2 where n is the number of inbreeds. e.g., from four inbreeds six single crosses can be produced.

It can be calculated as:

Single crosses give the maximum degree of hybrid vigour but due to weak inbreeds, little amount of seed is produced.

2. Double Cross:

Commercial hybrids are produced by this method. These hybrids give very high yield in small land without any increase in the cost of production. All the hybrid seeds of maize to farmers for cultivation are nothing but double crosses.

Three-way cross:

It is a cross of a single cross (used as female) with another inbred (used as male), i.e., it involves three inbreeds [(A x B) x C], The single cross and the inbreeds are planted in the same way as the inbreeds in the single cross. The advantage of this cross is the use of vigorous hybrid of first generation as female in order to maximize the yield of hybrid seeds as well as to obtain seeds of normal grain size.

Top cross or inbred variety cross (A × variety):

It can be defined as a cross between an open pollinated variety and inbred line. Either the inbred or the variety may be used as female parent, but to use variety as female is preferable. It is used not only for the developing hybrid but to test the combining ability of the inbreeds.

Synthetic cross:

It can be defined as a cross among inbreeds, clones or sibbed lines without control of pollination. (Synonyms, poly-cross and strain building). This is the cross of a number (4-10) pretested hybrids and is done by open pollination in isolation.

Seeds of these inbreeds are mixed in equal proportion and sown in isolated plots. Natural cross pollination takes place and the harvested produce becomes synthetic cross. This cross is used in forage cross where floral structure causes difficulties in artificial pollination.

After any one of the above-mentioned crosses, seeds are tested, multiplied and distributed. The testing is done at various regional research stations by various research workers. The entire process takes about 5-7 years.

Hybridization Methods of Plant Breeding in Vegetatively Propagated Crops:

Improved clones of asexually propagating plants are selected and allowed to grow under conditions, which favour flowering and seed setting. Desirable clones are crossed together and the hybrids are multiplied by cloning. Each F₁ plant is a potential source for a new clone. This procedure has been used for developing many improved varieties of sugarcane, potato etc. (Fig. 14).