Hybridization

(S. Y.  B. Pharm Sem IV) UNIT-II

AS per PCI Syllabus


Hybridization


It is mating or crossing of two genetically dissimilar plants having desired genes or genotypes and bringing them together into one individual called hybrid. The process through which hybrids are produced is called hybridization.        
                             
                                         OR

It is natural and Artificial  process that result in the formation of hybrid.
Hybrid plant result from crossing of two species or varieties differing at least in one set of characters that is called hybridization .



Purpose of Hybridization (Objectives):

1.Development of productive varieties:

One of the objectives is to create new variability which is further used for
development of improved varieties with respect to existing varieties.

2. Removal of Bottle-Neck Genes:

The appearance of new problems (pathogen, insect) to yield can be protected through the creation of new recombinants which will help to come out from this kind of situation.

3. Development of Heterotic F1 Hybrids:


Hybridization between two chosen inbred parents can be resorted to produce F1 hybrids. In case of self-pollinated crops two methods.

(a) Combination Breeding:

The main objective of this method is to transfer one or more characters into a single variety from other varieties. In this breeding the genetic divergence between the two parents is not the major consideration, here among the two parents one must have the character to be transferred & another parent is the popular variety.

(b) Transgressive Breeding:

This method aims at improving yield or contributing characters through transgressive segregation. It is the production of plants in the F2 generation which will be superior to both the parents for one or more characters.

   Fig. General Technique of Artificially Hybridization

The following steps are involved in hybridization of plant.

1. Choice of Parents

The two parents to be selected, at least one should be as well adopted and proven variety in the area The other variety should have the characters that are absent in the first chosen variety.


2. Emasculation: 

In case of hybridization programme for self-pollinated crops, it is very much essential to emasculate the plant to avoid the self-pollination. 

Removal of male sex organs or killing that part of the flower without any damage or disturbances to female reproductive organ is known as emasculation.

Before going to hybridization programme the efficiency of emasculation technique may be tested by bagging the emasculated flowers without pollination. The amount of seed thus set would indicate the frequency of self fertilisation occurring during emasculation.

Emasculation is essential in bisexual flowers only.

There are various techniques of emasculation:

(i)Hand Emasculation:
(a) Emasculation is done before the anthers mature and the stigma has become receptive to minimise self-pollination.
(b) The corolla of selected flowers is opened with the help of fine tip forceps and the anthers are removed.
(c) In case of epipetalous stamen the corolla lobes are removed carefully keeping the gynoecium not injured at all.



(ii) Suction Method:

This method is useful in case of small flowers where hand emasculation is not possible. The petals are generally removed with forceps exposing the anthers and stigma. A thin rubber tube or glass tube attached to a suction hose is used to suck the pollen grains from its surface.

The suction may be produced by an aspirator or by a small suction pump. Care must be taken that suction should be enough to suck the stamen and pollen grains but not the gynoecium. But this method is not very efficient, as 15% of self-pol­lination takes place. Washing the stigma with water may also reduce self-pollination.

(iii) Hot Water Emasculation:

Pollen grains are more sensitive to hot water than the female reproductive organs. So treatment with hot water at particular temp. & for fixed time period is helpful for killing the pollen grains without damaging the female organ.

Treatment with water at 42-48°C for 10 minutes is effective in jowar, treatment at 40-44°C for 10 minutes is effective in rice; in both the cases the whole spike is immersed in thermo-flask containing hot water.

(iv) Alcohol Treatment: 

It is not very popular method; a particular concentration of alcohol is used for a fixed time period to kill the pollen grains. But a little bit more exposure, i.e., few seconds more than the recommended time period will reduce the female receptivity, i.e., seed set, as female organs would also be killed by this treatment.

(v) Cold Treatment:

Like hot water treatment, cold treatment can also kill pollen grains without damaging the gynoecium. Keeping rice plant at 0-6°C kills the pollen grains, and also wheat plants at 0-2°C for 15-24 hrs. kills the pollen grains. But cold treatment is less effective than hot treatment.
(vi) Genetic Emasculation: 

Genetic or cytoplasmic male sterility may be used to eliminate the necessity of emasculation, i.e., male sterile plants are naturally emasculated. For self-incompatible species emasculation is not necessary, but in certain genotypes male sterility can be induced by gene manipulation in cytoplasmic or nuclear genome.

3. Bagging and Tagging

Immediately after emasculation, the flowers or inflorescences are enclosed in bags of suitable sizes 10 prevent random cross-pollination.


Tagging:

The emasculated flower or inflorescence is tagged after bagging. The tags are made up of light weight tin-plate and are written in carbon pencil.

The tag should bear the information:

(a) Date of emasculation;
(b) Date of pollination;
(c) Names of female and male parents, consecutively in the cross.



4. Pollination

In pollination, mature, fertile and viable pollens are placed on a receptive stigma.
The procedure consists of collecting pollens from freshly dehisced anthers and dusting them on the stigmas of emasculated flowers.


5. Raising F1 Plants

Pollination is naturally followed by fertilization. It results in the formation of seeds. Mature seeds of F1 generation are harvested dried and stored these seeds are grown to produce F1 hybrid. 

Hybrids of cinchona yield more amount of quinine. A hybrid developed by crossing Cinchona succirubra with Cinchona ledgering yields a bark, which contains 11.3% of alkaloids.

The parent species produced 3.4% and 5.1% of alkaloids, respectively.Pyrethrum hybrids have been used for Pyrethrum production, these hybrids are produced either by crossing two clones assumed to be self- sterile or planting a number of desirable clones together and bulking the seed.

The hybridization of plant to increase the Pyrethrin contents.



Green House Effect
Normal conditions sun rays reach the earth and heat is radiated back into space. However, when carbon dioxide concentration increases in the atmosphere, it forms a thick cover and prevents the heat from being re-radiated. Consequently, the atmosphere gets heated and the temperature increases. 
This is called green house effect. 

In recent past, amount of carbon dioxide has increased from 290 ppm to 330 ppm due to cutting of forests and excessive burning of fossil fuels.

The rate at which the amount of carbon dioxide in the atmosphere is increasing, it is expected to cause rise in global temperature.

The global warming by two or three degrees would cause polar ice caps to melt, floods in coastal areas, change in hydrologic cycle and islands would get submerged. The following gases produce green house effect like CO2, SO2, oxide of nitrogen, chloro fluoro carbons, etc.


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