Introduction

The mating or crossing of two plants or lines of dissimilar genotype is known as hybridization. In plants crossing is done by placing pollen grains from one genotype the male parent on to the stigma of flowers of the other genotype, the female parent. It is essential to prevent self-pollination as well as chance cross-pollination in the flowers of the female parent. The seeds as well as the progeny resulting from the hybridization are known as hybrid or F₁. The progeny of F₁, obtained by selfing or intermating of F₁ plants and the subsequent generations are termed as segregating generations. The term cross is often used to denote the products of hybridization i.e., the F₁ as well as the segregating generations.

Objectives of Hybridization

Combination Breeding
The main aim of combination breeding is the transfer of one or more characters into a single variety from other varieties. In this approach, increase in the yield of a variety is obtained by correcting the weakness in the yield contributing traits e.g., tiller number, grains per spike, test weight etc., of the concerned variety. One of the parents must have in a sufficient intensity the character(s) under transfer, while the other parent is generally a popular variety.

Transgressive Breeding
Transgressive breeding aims at improving yield or its contributing characters through transgressive segregation. Transgressive segregation is the production of plants in an F₂ generation that are superior to both the parents for one or more characters.

Hybrid Varieties
In most self-pollinated crops, F₁ is more vigorous and higher yielding than the parents. Wherever it is commercially feasible, F₁ may be used directly as a variety. In such cases, it is important that the two parents should produce an outstanding F₁.

Types

1. Intervarietal Hybridization

The parents involved in hybridization belong to the same species; they may be two strains, varieties or races of the same species. It is also known as intraspecific hybridization. In crop improvement programmes, Intervarietal hybridization is the most commonly used. The Intervarietal crosses may be simple or complex depending upon the number of parents involved.

• Simple cross

In a simple cross, two parents are crossed to produce the F₁. The F₁ is selfed toproduce F₂ or is used in a back cross programme, e.g. AxB----- F₁ (AxB)

• Complex cross

More than two parents or crossed to produce the hybrid, which is then used to produce F₂ or is used in a backcross. This crossed is also known as convergent cross because this cross bringing together, genes from several parents into a single hybrid. In breeding of highly improved self-pollinated crops like wheat and rice complex crosses are a common practice today.

2. Distant hybridization

Distant hybridization includes crosses between different species of the same genus or of different genera. When two species of the same genus are crossed, it is known as interspecific hybridization; but when they belong to two different genera, it is termed as intergeneric hybridization. Generally, the objective of such crosses is to transfer one or few simply inherited characters like disease resistance to a crop species.e.g Clinton oat variety was developed from a cross between Avena sativax A.byzatina (both hexaploid oat species), and CO 31 rice variety was developed from the cross Oryza sativa var. Indica x O. perennis. Almost all the present-day surgarcance varieties have been developed from complex crosses between Saccharum officinarum (noble canes), S.barberi (Indian cones) and other Saccharum species, e.g. S.spontaneum (Kans). The improvement in fiber length of Indian cotton (Gossypium arobreum) has been brought about by crossing it with American cultivated cotton (G. hirsutum). Many improved varieties have resulted from such crosses. Intergeneric hybridization may also be used to develop a new crop species, e.g. Triticale from a cross between Triticum sp. and Secale cereale (rye). Wild species often provide genes, which are not present in the cultivated species. For e.g. many of the genes for rust resistance in wheat are derived from related wild species.

Steps in hybridization

1. Choice of parents,


2. Evaluation of parents,


3. Emasculation,


4. Bagging,


5. Tagging,


6. Pollination, and


7. Harvesting and storage of F₁ seed.

1. Choice of parents

The choice of parents mainly depends upon the objective of breeding programme. Therefore, at least one of the parents involved in a cross should be a well-adapted and proven variety in the area for which the new variety is being developed. The other variety should be having the characters that are absent in this variety. In combination breeding, the genetic diversity of the parents is not important, but in transgressive breeding genetic diversity is of great importance. For transgressive breeding, it should be made sure that the parents differ for many genes affecting yield or some other character of importance. Some parents produce superior F₁s and F₂s, while others do not. This property of the parents is known as combining ability. Combining ability of the parents may serve as a useful guide in the selection of parents for a hybridization programme. The choice of parents is the basic step in a hybridization programme.

2. Evaluation of parents

Performance in the area is not known, it should be determined, particularly for the characters they are expected to contribute and for disease resistance. Disease reaction is important because an introduced parent may be susceptible to the new races of the pathogen occurring in the area, or even new diseases present in the area for which their reaction may not be known. New strains should also be checked for mechanical mixture and for heterozygosity if the crop species shows about 5% cross-pollination.

3. Emasculation

The removal of stamens or anthers or the killing of pollen grains of a flower without affecting in any way the female reproductive organs is known as emasculation. The purpose of emasculation is to prevent self-fertilization in the flowers of female parent. In dioecious plants, male plants are removed, while in monoecious species the male flowers, e.g., in castor, or the male inflorescence, e.g., in maize, are removed to prevent self-pollination. But emasculation is essential in bisexual flowers. In species with relatively large flowers, hand emasculation may be adequate in most hybridization programmes. The efficiency of an emasculation technique may be tested by bagging the emascualted flowers without pollination. The amount of seed thus set would indicate the frequency of chance self-fertilization during emasculation. If the seeds are to be used in genetic studies, there should be no self-pollination during emasculation.

Hand emasculation
Emasculation is done before the anthers are mature and the stigma has become receptive to minimise accidental self-pollination. Emasculation is generally done in the evening, between 4 and 6 p.m., one day before the anthers are expected to dehisce or mature and the stigma is likely to become fully receptive. Therefore, the flowers selected foremasculation are likely to open the next morning. Generally, it is desirable to remove the older and the younger flowers located close to the flower to be emasculated in order to avoid confusion in identification of crossed pods etc.

Suction method
This method is useful in species with small flowers. Emasculation is done in the morning just before or immediately after the flowers open. The petals are generally removed with forcepes exposing the anthers and the stigma. A thin rubber or glass tube attached to a suction hose is used to suck the anthers from the flowers. The tube is also passed over the stigmas to suck any pollen grains present on their surface. The suction may be produced by an aspirator attached to water tap or by a small suction pump. The suction should be enough to suck the stamens and pollen grains, but not the flowers or the gynoecium. Washing the stigma with a jet of water may help in reducing self-pollination.

Hot water emasculation
Pollen grains are more sensitive than the female reproductive organs to both genetic and environmental factors. This property is utilized to kill the pollen grains with hot water or other agents like alcohol treatment or cold water treatment without damaging the female reproductive organs. In the case of hot water emasculation, the temperature of water and the duration of treatment vary from crop to crop. For jowar-42-48⁰C for 10 minutes. Rice-40-44⁰C for 10 minutes. The hot water is generally carried in thermos flasks and the whole spike is immersed in the water.

Alcohol treatment
It is not a commonly used method of emasculation. The method consists of immersing the flower or the infloretion. The method consists of immersing the flower or the inflorescence in alcohol of a suitable concentration for a brief period, followed by rinsing with water. However, the duration of treatment is of utmost importance. Even a slightly, prolonged period of treatment, a few seconds more than the recommended, would greatly reduce seed set. This is because the female reproductive organs would also be killed by a longer treatment.

Cold treatment
Cold treatment, like hot water treatment, kills pollen grains without damaging gynoecium. In case of rice, treatment with cold water at 0-6⁰ C kills pollen grains without affecting gynoecium. Keeping wheat plants at 0-2⁰C for 15-24 hours kills the pollen grains. The amount of self-pollination is generally greater in cold treatment than in the case of hot water treatment.

Genetic emasculation
Genetic or cytoplasmic male sterility may be used to eliminate the necessity of emasculation. Many species are self-incompatible. In such cases, emasculation is not necessary because self-fertilization will not take place. For commercial hybrid seed production, male sterility is the most feasible method of emasculation. Protogyny facilitates crossing the anthers mature, hand pollination ensures seed set from cross pollination and prevent self-fertilization, i.e., in bajra (P.americanum).

4. Bagging

Immediately after emasculation, the flowers or the inflorescences are enclosed in suitable bags of appropriate size to prevent random cross-pollination. In cross-pollinated crops, like maize, the male flowers are also bagged to maintain the purity of pollen used for pollination. The bags may be made of paper, butter paper, glassine or fine cloth. Butter paper or vegetable parchment bags are the most commonly used. The bags are tied to the base of inflorescence or to the stalk of flower with the help of thread, wire or pins designed for the purpose. Fungus development on the fruit or the spike may be prevented by removing the bags after the danger of cross-pollination is over usually 2-3 days after pollination.

5. Tagging

The emasculated flowers are tagged just after bagging. The tags are attached to the flower or the inflorescence with the help of thread. The following information is recorded on the tags with a carbon pencil.

1. Date of emasculation
2. Date of pollination
3. Names of the female and the male parents. The name of the female parent is written first, and that of the male parent is written later.

6. Pollination

The two most important operations that determine the amount of seed set in hybridization are emasculation and pollination. In case of pollination, mature, fertile and viable pollen should be placed on a receptive stigma to bring about fertilization. The duration of pollen viability after anther dehiscence varies greatly from one species to another e.g., a few minutes in that fresh pollen from mature anthers should be used for pollination. The time of anther dehiscence falls within the duration of stigma receptivity and both generally coincide with the opening of flowers.

1. Pollen grains are collected in a bag, and are used for dusting stigmata of female inflorescence or of emasculated flowers, the e.g., in maize, bajra etc.


2. Mature anthers are collected from the flowers of male parent. The pollen is liberated and applied to the stigma with the help of a camel hair brush, pieces of paper, tooth pick or forceps.


3. Anthers are collected and allowed to burst directly over the stigma. In rice, oats, wheat and barley, one anther is generally inserted in each floret where it dehisces and covers the stigma with pollen grains.


4. The spike of male inflorescence is shaken over the emasculated inflorescence just when the anthers are about to dehisce. The lemma and palea of the spike of male parent are also clipped of to expose the anthers, which are used as the source of pollen.


5. In species like maize, the male inflorescence may be detached and enclosed in the bag covering the female inflorescence. In case of bajra and jawar, panicles from the male parent may be enclosed in the same bags that enclose the panicles of female parent.