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🔀 Hybridization Objectives and Types

Aims of hybridization and the major cross combinations used in plant breeding.

This lesson covers core principles and exam-focused points from this topic in plant breeding.

Objective of hybridization

The chief objective of hybridization is to create variation. When two genotypically

different plants are crossed, the genes from both the parents are brought together in Fl.

Segregation and recombination produce many new gene combinations in F2 and subsequent

generations.

The degree of variation produced depends on the number of heterozygous genes in Fl.

The number of heterozygous genes in F1 in turn depends on number of genes for which the two

parents differ. If the parents are not related they may differ for several genes.

Combination breeding

The main aim of combination breeding is the transfer of one or more characters into a

single variety from other varieties. These characters may be governed by oligogenes or

polygenes. In this approach, increase in yield is obtained by correcting the weaknesses in the

yield contributing traits like tiller number, grains per panicle, seed weight of the concerned

variety. Example for combination breeding is disease resistance achieved by backcross breeding.

Pedigree method is also another example.

Transgressive breeding

Transgressive segregation is the production of plants in F2 generation that are superior to

both the parents for one or more characters. Such plants are produced by the accumulation of

favourable genes from both the parents as a consequence of recombination. In this case the

parents involved in hybridization must combine well with each other and preferably be

genetically diverse. This way, each parent expected to contribute different plus genes which

when brought together by recombination gives rise to transgresive segregation. The pedigree

method as well as population approach are designed to produce transgresive segregants.

Procedure of hybridization

  1. Set up your objective.

  2. Selection of parents.

  3. Evaluation of parents.

  4. Sowing plan.

s. Emasculation and dusting.

  1. Labelling and bagging.

  2. Harvesting and storage of seeds.

Objective

Based on the requirement, set your objective. Because based on the objective only the

selection of parents is done. If it is resistance breeding one of the parents must be a donor.

Selection of parents

Normal practice is, the female parent will be a locally adapted one in which we can bring

in the plus genes. In case of intervarietal hybridization geographically diverse parents will be

selected so as to get superior segregants.

Evaluation of parents

In case of parents which are new to the region they must be evaluated for their

adaptability. Further to ensure homozygosity, they must be evaluated.

Sowing plan

If the flowering duration is same, simultaneous sowing of both the parents can be done.

Otherwise staggered sowing is to be followed. The normal practice is to raise the ovule parent in

the centre of the plot in rows and on the border pollen parent for each combination.

Emasculation and dusting

Emasculation is the removal of immature anthers from a bisexual flower. Depending on

the crop the emasculation practice differs. Normal practice of hand emasculation and dusting of

pollen is done. Depending on the time of anthesis the time of emasculation differs. For E.g. in

rice the anthesis at Coimbatore takes place between 7.00 to 10.00 A.M. So the emasculation is

done at around 6.30 A.M. and dusting of pollen is done immediately.

Labelling and bagging

Immediately after hybridization put a label indicating the parents and date of crossing.

Put appropriate cover to prevent foreign pollen, contamination.

Harvesting and storage of seeds

Normally 15-20 days after crossing the seeds will be set. In the case of pulses the crossed

pods can be easily identified by the shrunken nature of pod and seed set will be reduced. Harvest

of crossed seeds must be done on individual plant basis. Seeds collected from individual plants

are to be stored in appropriate containers with proper label and stored.

Distant Hybridization

When crosses are made between two different species or between two different

genera, they are generally termed as distant hybridization (or) wide hybridization

History

Thomas Fairchild 1717 was the first man to do distant hybridization. He produced an

hybrid between two species of Dianthus

Dianthus caryophyllus ( Carnation ) x D. barbatus ( Sweet william)

Inter generic hybrid produced by Karpechenko, a Russian Scientist in 1928. Raphano brassica

is the amphidiploid from a cross between Radish (Raphanus sativus) and cabbage (Brassica

oleraceae). Triticale was produced by Rimpau in 1890 itself. Triticale is an amphidiploid

obtained from cross between wheat and rye. Another example is Saccharum nobilisation

involving three species.

Hybrids in self-pollinated crops - problems and prospects

Exploitation of heterosis through F I hybrids has hitherto been the prerogative of

cross- pollinated crops, chiefly due to their breeding systems favouring allogamy. However,

possibilities of working for such a proposition have recently been realized in self-pollinated

corps also. Indeed, exploitation of hybrid vigour in autogamous crops is easy and less time

consuming in that homozygous inbreds are already available. There is practically no difference

with regard to hybrid breeding between self and cross-pollinated crops. But the prospects of

hybrids in seIfers is dependant on three major considerations.

  1. How high a heterotic effect can be gained under optimal production conditions.

  2. In fact, a breeder's main concern is the magnitude rather than the frequency of occurrence

of heterosis in crops. Thus the consideration is whether or not it is possible to obtain

economically viable heterosis.

  1. How much of the yield surplus due to high heterosis can offset the extra seed cost? In

major self-pollinated crops like wheat, barley, rice, etc., the seed rate per unit area is

exorbitant and hence the hybrid seed requirement is also more.

  1. How efficient and effective is the mechanism of cross-pollination in seIfers? By nature,

self-pollinated crops are shy pollinators with very poor pollen maneuverability (or

movability to effect allogamy). Therefore, the efficiency (degree of allogamy) with which

cross pollination can take place on a commercial scale is the true determinant of the

success of a hybrid programme in seIfers.

  1. Among self-pollinated crops, FI hybrids have been graduated into the farmer's field in

barely, tomato, Sorghum (often-cross-pollinated) and wheat. Briggle (1963) presented a

vivid account of heterosis in wheat. Work in rice is also most encouraging (IRRI, 1972).

Methods of handling of segregating generations – pedigree method, bulk method, back

cross method and various modified methods

Pedigree method

In this method, individual plants are selected from F2 and subsequent generations and

their progenies are tested. During this process details about the plants selected in each generation

is recorded in Pedigree Record. By looking into Pedigree record we can know about the ancestry

of the selected plants.

For maintenance of pedigree record the basic thing required is Crossing Ledger. This

Ledger gives the details about parentage, Season in which the cross is made.

Sl.No. Cross Number Parentage

  1. XS 9801 Co2 x MS 9804

  2. X S 9802 . Co4 x C152

  3. X S 9803 Co 1 x Co4

There are several ways to maintain the pedigree Record. The selection of plants starts from F2

onwards. The details about selected plants can be recorded as follows. E.g. F2 X S 9801 - 7. Here

the 7 denotes seventh plant selected.

In F3 if selection is made from the 7th plant of cross X S 9801 it can be recorded as F3 X

S 9801 - 7 - 4. The number four indicates that fourth plant of 7th plant of F2 is selected. This can

be followed till F4 or F5 generations. After F4 or F5 the selected plants are bulked to form a

family.

In the pedigree record all the biometerical data like plant height, number of branches, No.

of pods / plant, pod length, seeds / pod, pod weight, seed weight are recorded.

Merits of Pedigree Method

  1. Gives maximum opportunity to the breeder to use his skill and judgement for the

selection of plants.

  1. Well-suited for characters which are simply inherited

  2. Transgressive segregants can be easily identified thro' records.

  3. Information about inheritance is precisely obtained.

Demerits

  1. Maintenance of pedigree record is time consuming and limits handling of larger

population.

  1. The success in this method is largely dependent on skill of the breeder. There is no

opportunity for natural selection.

  1. Selection for yield in F2 and F3 is ineffective. If care is not taken to maintain larger

population, valuable materials may be lost.

Pedigree Method Procedure

F1 Generation

The F1 seeds are space planted so that full expression of F1 can be had. It is advisable to

raise the parents involved in the cross to raise as border rows so that dominance and other

characters can be studied. The F1s are harvested as single plants.

F2 generation

In F2, 2000 to 10,000 plants per cross are planted. About 100 - 500 plants are selected

and harvested on single plant basis. The selection in F 2 depends upon the skill of the breeder.

The selection intensity may be 5 to 10%.

F3 generation

Individual plant progenies are space planted. Again desirable plants are selected. From F3

onwards the term family is introduced. The line selected from each cross is termed as family.

F4 generation

Similar to F3.

F5 generation

Many families would have attained homozygosity and may be harvested as row bulk.

F6 generation

The row bulk may be assessed in multi row trial. The families exhibiting segregation

may be isolated and studied separately.

F7 generation

RRYT

F8 generation

PYT

CYT 3 seasons.

Basis of selection

Depending upon the objective, selection is to be made in segregating generation. For insect

and disease resistance part of the seeds may be reserved in segregating generation and the rest

may be subjected to epiphytotic conditions. The families exhibiting resistance may be identified

and the reserve seeds may be used for further selection and testing.

Early generation testing

If superior families are identified in F3 or F4, they can be tested for desirable characters

and this is known as early generation testing.

Shuttle breeding

This is followed especially in disease or insect resistance breeding. For e.g. at

Coimbatore YMV in blackgram is in epidemic form during summer season only. Whereas at

Vamban (Pudukkottai) the YMV is epidemic during kharif season. So instead of waiting for

next summer at Coimbatore the materials can be tested at Vamban during kharif and thus one

season is saved.

Off season nursery

Some crops may be season bound. But it may be non - season bound in certain agro

climatic zone. For e.g. Thalai virichan cholam. (Sroxburghii) is season bound at Coimbatore. It

has to be sown during July - August and harvested during December January. But this

Sroxbughii is non - season bound in Yercaud. So to save one season, the segregating material can

be raised during Rabi summer at Yercaud. This method is otherwise known as rapid generation

advancement (RGA).

Bulk Method

In this method F2 and subsequent generations are harvested as bulk to grow the next

generation. The duration of bulking may be 6 - 7 generations. Selection can be made in each

generation but harvest is done as bulk. This is similar to mass selection. At the end of bulking

period single plant selection is made and tested for yielding ability. If bulking period is long say

20 - 30 seasons, then natural selection acts on the homozygous lines. In this method the breeder

uses his skill for selecting the plants and at the same time there is no pedigree record. This saves

much time and labour.

Merits of bulk method

  1. Simple, convenient and inexpensive

  2. By inducing artificial epiphytotic conditions undesirable or weaker genotypes can be

eliminated.

  1. If bulking period is longer natural selection operates and desirable genotypes are selected.

  2. No pedigree record is maintained.

  3. Since large population is grown there is chance for appearance of transgressive

segregants which will be superior than parents or F2

Demerits

  1. Takes much longer time to develop a new variety.

  2. In short term bulk there is no chance for natural selection.

  3. A large number of progenies are to be selected in each generation which requires much labour,

time and space.

  1. We cannot get information on inheritance.

Single Seed - Descent Method

It is the modification of the bulk method. In this method a single seed from each of the F2

plants is collected and bulked to raise F3 generation. Similarly single seed from each F3 plant is

collected and carried forward to F4. This procedure is followed till F6 or F7. After wards single

plant selection is made and studied in progeny rows.

In this Scheme the main features are:

  1. Lack of selection till F6 or F7 when the population becomes homozygous.

  2. Each F2 plant is represented till F6 or F7 generation.

  3. In this method there are chances for reduction in population size due to pest, disease or

poor germination.

  1. Rapid generation advancement (RGA) can be made with the use of glass house or off

season nursery.

Modified bulk method

Here selection can be practiced in F2 and F3 and subsequent generations. There will not be

any pedigree record but superior plants are selected bulked and carried forward. In F4 superior

plants are selected and harvested on single plant basis. In F5 these single plants are studied in

progeny rows and best progenies are selected and harvested. In F6 PYT can be conducted to

select best families. In subsequent generations regular trials can be conducted.

This modification of the bulk method provides an opportunity for the breeder to exercise his

skill and judgement in selection. Further there is no maintenance of pedigree record which is

another advantage.

Mass pedigree method

This was proposed by Harrington. It is a solution to one of the deficiencies in the

pedigree method of breeding. For e.g. if the population is to be subjected to disease resistance

screening like YMV and if there is no method to create artificial epiphytotic conditions, it is

wasteful to study the population in pedigree method. Instead we can carry the population as a

mass and test them when there is occurrence of the disease. When conditions are favourable for

the disease, we can terminate the bulking and resort to single plant selection.

Comparison between Pedigree and Bulk Methods

S.
No.		 Pedigree method Bulk method
1. Individual plants are selected in F2 and
the
subsequent
generations
and
individual plant progenies are grown.		 F2 and the subsequent generations are
maintained as bulks.
2. Artificial selection, artificial disease
epidemics etc., are an integral part of the
method		 Artificial selection, artificial disease
epiphytotics etc., may be used to
assist natural selection. In certain
cases, artificial selection may be
essential
3. Natural selection does not play any role
in the method.		 Natural
selection determines
the
composition of the populations at the
end of the bulking period.
4. Pedigree records have to be maintained
which is often time consuming and
laborious		 No pedigree record is maintained.
5. It generally takes 14-15 years to develop
a new variety and to release it for
cultivation.		 It
takes
much
longer
for
the
development and release of a variety.
The bulk population has to be
Col1 Col2 maintained for more than 10 years for
natural selection to act.
6. Most widely used breeding method. Used only to a limited extent.
7. It demands close attention from the
breeder from F2 onwards as individual
plant selections have to be made and
pedigree records have to be maintained.		 It
is
simple,
convenient
and
inexpensive and does not require
much attention from the breeder
during the period of bulking
8. The segregating generations are space -
planted to permit individual plant
selection.		 The bulk populations are generally
planted at commercial planting rates.
9. The size of population is usually smaller
than that in the case of bulk method.
 Large populations are grown. This
and natural selection are expected to
increase the chances of the recovery
of transgressive segregants.

Summary Cheat Sheet

Quick Recall Points

  • This lesson focuses on key plant breeding concepts, terminology, and exam-relevant applications.
  • Review major definitions, classifications, and method-wise distinctions from the sections above.
  • Revise tables and examples from this lesson for fast pre-exam recall.

Exam Traps

  • Do not confuse similarly named breeding methods without checking their core selection logic.
  • Pay attention to crop-specific examples because the same principle can behave differently by species.

References

1 source • [1]

[1]

Standard Plant Breeding Class Notes (GPBR211)

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