Hybridization in plant breeding

Hybridization

Hybrid is the individual or offspring produced through sexual reproduction between two genetically different parents. Hybridization is the natural or artificial process of production of hybrid. A hybrid may be fertile, partially fertile or sterile. This is the method of combining characters of two plants into one. However, a hybrid may be superior, in-between or inferior over both the parents. Joseph Kölrueter a German botanist in 1760 for the first time studied the hybridization in in tobacco.
Depending upon the genetic makeup of parent plants, hybridization may of following types.

• Intravarietal hybridization occurs between the two different individuals of same variety.
• Intervarietal or Intraspecific hybridization occurs between the individuals of two different varieties of species species.
• Interspecific or Intrageneric hybridization occurs between the individuals of two different species of a genus.

According to Kölrueter, interspecific hybridization is rare. This is because the species are reproductively isolated by means of various strong barriers.
Most of the present day crops are evolved through the process of hybridization occurred either in the nature or performed artificially.

Objectives

Main objectives of hybridization are:

To unite the different desired characteristics from two plants into one.

To produce the popullation with different traits.

To utilized the hybrid vigor.

Process of hybridization

Process of hybridization involves following steps:

1. Selection of parents
2. Selfing of parents
3. Emasculation
4. Bagging
5. Tagging
6. Crossing
7. Harvesting and storing F1 seeds
8. Raising the F1 generation

1. Selection of parents

This is the first step of hybridization, where a plant suited to the local prevailing environmental conditions. These plants are selected on the basis of desired characteristics.

2. Selfing of parents

Selfing among the parents incudes the homozygousity. Selfing produces inbreed and eliminates undesirable traits.

3. Emasculation

Emasculation is the removal of anthers from a bisexual folwer before the pollens get matured. Emasculation removes or kill the anther only without causing any damage to the other parts of the a flower.
Inbreeds obtained from the second steps are emasculated in order to make them unisexual. In unisexual flowers, emasculation is not needed.
Emasculation is done by one of the following methods:

a. Forceps method

This method is also known as "hand method" or "scissors method. In this method, anthers are carefully removed at budding stage of flower with the help of forceps and scissors. This is the most adapted method of emasculation in the big flowers.

b. Hot water treatment

In the crops with minute bisexual flowers such as, rice and bajra forceps method can not be applied. Hence, in such crops flowers are treated in hot water for a certain duration of time to kill the anther. This method is based on the fact that, gynoecium can withstand temperature at whichm anthers are killed.
Hot water treatment is given by an instrument, which contsists of metallic container attached to a stand. The dimension of container is according to the size of head of bajra. One end of this instrument is opened. The head/panicle of crop to be emasculated is inserted into the container and closed with the rubber lid. This end also contains a thermometer to monitor the temperature. The container is fixed with a stand without making any injury to the plant.
For emasculation of rice inflorescense, hot water treatment at 40-44°C is given for 5 to 10 minutes.

c. Cold water treatment

Cold water treatment also kills the pollen grains without without damaging the gynoecium. However, this method is less effective than hot water treatment.

d. Alcohol treatment

In this method, flowers are dipped in a suitable strenght of alcholo for a short period of time. High concentration or prolonged treatment duration can kill even gynoecium. Hence, this method is not of common used.

e. Chemical treatment

Certain gametocidal chemical are available, which can be used for emasculation. Common chemical gametocides are 2,4 D, Maleic Hydrazide (MA), Naphthalene Acetic Acid (NAA), FW450, Ethyl 4-fluorooxanilate and tribenzoic acid.

4. Bagging

Bagging is done in order to prevent the contamination of pollen grains. Both the male and female flowers are bagged separtely. In emasiculated bisexual flowers, bagging is done just after emasculation while in unisexual flowers, bagging is done stigma recpetivity and dehiscense of anther.
The bag may be made up of butter paper, fine cloth. A bag of butter is paper is more commonly used. Bag is tied to the base of flower/inflorescense with the help of a thread.

5. Tagging

In order to note the essential information, a tag is tied at the base of the bagg with the help of thread. Tag may be circular (3 inch diammeter) or rectengular (3 × 2 inch).
A tag contains atleast following essential informations:

1. Sr. No. of field records
2. Date of emasculation
3. Date of crossing
4. Name of the parents (name of the femal parent is written first, followed by × sign and then name of male parent)

6. Crossing

Crossing is the artificial cross-pollination between genetically unlike plants.
In this step, fertile and viable pollen grains are collected from male plant and placed on the receptive stigma of female plant with thee help of fine camel hair brush.

7. Harvesting and storing F1 seeds

The produce from the crossed plants are harvested and thrashed. Seeds are collected, dried and stored with proper tag.

7. Raising the F1 generation

The seeds harvested are sown in the comming seaso and raised as F1 generation. As these plants are raised from the crossed seeds, these are hybrids.

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First published on 13-04-2021
Last updated on 06-05-2021