agriculture

1. Unit 4
Hybridization:-
• The mating or crossing of two plants or lines of
dissimilar genotype are known as hybridization. The
seeds as well as the progeny resulting from
hybridization are known as hybrid or F1. In plants
crossing is done by placing pollen grains from one
genotype to called the male parent, on to the
stigma of flowers of the other genotype referred to
as the female parent.
1

2. • Thomas Fairchild developd first interspecific hybrid
by crossing sweet willram ( Dianthus Barbatus)
With carnation ( dianthus caryophyllus)
• Today, Hybridization is the most common method
of crop improvement, and the vast majority of crop
varieties have resulted from hybridization.
• The progeny of F1, Obtained by selfing or
intermating F1 plants, and the subsequent
generations are termed as segregating generations.
2

3. Types of hybridization:-
Based on the taxonomic relationships of the parents
involved, hybridization may be classified in to two
broad groups.
1 Interivarietal.
2 Distant hybridization.
1 Intervarietal hybridization.:-
• The parents involved in intervarietal 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.
• Intervarietal crosses are called simple or complex
depending on the number of parents involved.
3

4. Simple cross:- In simple cross two parents are crossed
to produce the F1
e.g. A×B
• It is also known as single cross
Complex Cross: - In complex cross more than two
parents are crossed to produce the F1 hybrid. When
F1 from a simple cross is crossed to a third parents,
It is called three way cross or F1- top cross and
when two F1 from two single crosses are crossed
together the cross is known as double cross.
4

5. 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 often called interspecific hybridization
but when the species belong to two different
genera. It is termed as intergeneric hybridization.
Objectives of hybridization:-
• To transfer one or few qualitative characters.
• To improve one or more quantitative characters.
• To use the F1 as a hybrid variety.
5

6. • Rye (Secale cereale L.) is crossed
of wheat (Triticum L. species). It is thus possible
for rye to be cross-bred with wheat. The result of
this combination is the man-made cereal triticale.
• Thomas Fairchild developd first interspecific
hybrid by crossing sweet willram ( Dianthus
Barbatus) With carnation ( dianthus
caryophyllus)
6

7. Objectives of hybridization:-
1. Combination breeding:-
• The aim of combination breeding is to transfer one
or more oligogenic / polygenic characters into a
single variety from another variety or other
varieties. In this approach, the increase in the yield
of few variety is obtained by correcting the
weakness in yield contributing traits e.g. tiller
number, grains per spike, test weight, disease
resistance etc. of the concerned parent variety. The
backcross method of breeding was designed for
combination breeding.
7

8. 2. Transgressive breeding :-
• Transgressive breeding aims at improving yield or its
contributing characters through transgressive
segregation, appearance of such plants in F2
generation that are superior to both the parents for
one or more characters.
• Emasculation of the flower:- To prevent self-
pollination in the flowers of female parent.
• Bagging of the flowers:- To prevent pollination of
the flowers of female parent by pollen from
undefined sources.
• Hand pollination ( Bagging of male in inflorescence
):- To ensure pollination by the selected male
parent. 8

9. • Procedure of Hybridization.
Hybridization can be done in following seven steps:
1. Choice of parents
2. Evaluation of parents
3. Emasculation
4. Bagging
5. Tagging
6. Pollination
7. Harvesting and storage of F1 seed.
9

10. 1. Choice of parents: -
The choice of parents mainly depends on the
objectives of breeding programme. To increase
yield is always an objective of the breeder.
Therefore at least one of the parents involved in a
cross should be a well adapted and proven variety
in the target area, the area for which the new
variety is being developed. It is essential that all
the characters sought to be improved are present
in one or the other parent. If necessary three or
more parents may be included in a complex cross.
10

11. 2. Evaluation of parents:-
The performance of parents in the area where breeding is
to be done should be known. Particularly for the
characters that are expected to contribute and for
disease and pest resistance.
3. Emasculation:-
Removal of stamens or anthers or 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- pollination
in the flowers of the variety to be used as the female
parent. An efficient emasculation technique should
prevent self- pollination and result in a high percentage
of seed set on pollination. 11

12. a) Hand emasculation:-
• In species with relatively large flowers, stamens or
anthers are removed with the help of forceps.
Emasculation is done before the anthers are mature
and the stigma has become receptive, this minimizes
'accidental self- pollination. Emasculation is generally
done in the evening eg between 4 and 6 p.m. one day
before anthers of the flowers one to dehisce or mature
and stigma is likely to become fully receptive.
• A generalized procedure for hand emasculation is
corolla of the selected flower buds is opened and
anthers are carefully removed with the help of fine-tip
forceps. Care must be taken to remove all the anthers
from the flowers without breaking them and the most
important; the gynoecium must not be injured.
12

13. • In species with relatively larger flowers, hand
emasculation may be adequate in most
hybridization programmes. But in species with
small flowers hand emasculation is generally
difficult, tiring and time consuming.
• In crops like tobacco about 2,000 seeds per
fruit, tomato, brinjal etc hand emasculation is
fruitful.
13

14. b) 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. Petals are
generally removed with forceps exposing anthers
and stigma. A thin rubber or glass tube attached to
a suction hose is used to suck the anthers, the tube
is also passed over the stigmas to suck any pollen
grains present on their surface. With suctions
method, considerable amount of self- pollination
(upto 15%) is likely to occur.
14

15. c) Hot water emasculation :-
• Pollen grains are more sensitive than female
reproductive organs to both genetic and
environmental disturbances. This property is used
to kill pollen grains with hot water or other agents
like alcohol or cold water treatment. In the case of
hot water emasculation, temperature of water and
duration of treatment vary from crop to crop .eg for
sorghum- 42- 480 c for 10 min,
• Rice- 40- 440 c for 10 min
• Hot water treatment is given before anther dehisce
and prior to opening of flowers.
15

16. d) Alcohol treatment :-
• This rarely used method consists of immersing the
flower or the inflorescence in alcohol of a suitable
concentration for a brief period followed by rinsing it
with water.
• E.g. In sweet clover, immersion of the inflorescence in
57 per cent alcohol for 10 seconds was highly effective,
and the percentage of selfing was only 0.89.
e) Cold treatment :-
• Cold treatment, like hot water treatment kills pollen
grains without damaging gynoecium eg. In rice- 0-60 c
cold water treatment kills pollen grains without
affecting gynoecium.
• In wheat 0-20 c for 15-24 hrs kills the pollen grains 16

17. f) 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.
g) Chemical emasculation:-
• Many chemicals eg: sodium methyl arsenate (MG2
), Ethephon, GA3 hybrex etc. induce artificial non-
genetic male sterility. These chemicals are called
chemical hybridizing agent.
•
17

18. h) Pollination without emasculation:-
• Some crops have protogynous flowers and the
stigmas of their flowers become receptive one
or two days before anthesis. In such crops,
such flowers buds that would open the next
day may be selected and directly pollinated
without emasculation because their anthers
will not yet be fully mature.
18

19. 3. Bagging :-
• Immediately after emasculation the flowers or the
inflorescences are enclosed in suitable bags of
appropriate size to prevent and random cross-
pollination. In cross- pollinated crops like maize
male flowers are also bagged to maintain the purity
of pollen used for pollination. The bags are usually
made of paper, butter poper or fine cloth. The bags
are tied on the base of inflorescence or to the stalk
of flower with the help of thread, wire or pins
designed for the purpose.
19

20. 4. Tagging: -
• The emasculated flowers are tagged just after bagging.
Tags are available in different size. According to the size
of plant small bigger tags are used. The following
information is recorded on the tags with.
– carbon pencil
– date of emasculation
– date of pollination
– Name of the female and male parents. The name of female
parent is written first and later that of male parents.
A×B
Female male
20

21. • Pollination:-
• The two most important operations that
determine the amount of seed set in hybridization
are emasculation and pollination. During
emasculation, damage to the female reproductive
organs must be avoided, while during pollination,
mature fertile and viable pollen from the male
parent should be placed on receptive stigmas of
emasculated flowers to bring about fertilization
freshly collected pollen from mature anthers should
be used for pollination.
21

22. 6. Pollination can be done by various ways.
• Pollen grains are collected in a bag, and used for
dusting the stigmas of female inflorescence. e.g.
maize, bajra etc.
• Mature anthers are collected from the flowers of
male parent. Pollen is librated and applied to
stigmas with the help of camel hair brush piece of
paper tooth pick or forceps.
• Anthers are collected directly and allowed to brust
directly over stigma.
• The spike of male inflorescence is shaken over the
emasculated inflorescence just when the anthers
are about to dehisce. 22

23. 7. Harvesting and storing the F1 seeds:
The crossed heads or pods should be harvested,
threshed and the seeds should be dried and
properly stored to protect them from storage pests.
Proper care should be taken to avoid contamination
of the hybrid seed with other seeds. The seeds from
each cross should be kept separately and preferably
the seeds should be kept along with the original
tags to avoid confusion.
23

24. • In self- pollinated species, it is the easiest to permit
self- pollination in F1 to produce in F2 generation.
Segregation and recombination of genes would
produce several new genotypes, in addition to the
two parental types in F2. The number of genotypes
and phenotypes produced in F2 increases rapidly
with the number of segregating genes.
Consequences of hybridization:-
24

25. • It is, therefore, impractical to raise a prefect F2
population for any cross and to hope to recover rare
recombinants from segregating generations. This
would be extremely difficult even if the phenotypic.
However, in the case of quantitative characters the
effects of gene are masked by those of the
environment as well as by gene interactions making
the identification of rare recombinants even more
difficult.
• If F2, a vast majority of plants would be
heterozygous for one or more genes. Heterozygosity
reduces the effectivenes of selection because such
plants segregate to produce variable progeny.
25

26. Heterosis:
• The term heterosis was first used by Shull in 1914.
• Heterosis may be defined as the superiority of an F1
hybrid over both its parents in terms of field or
some other character. Generally heterosis is
manifested as an increase in vigor, size, growth rate,
yield or some other characteristics. But in some
cases, the hybrid may be inferior to the weaker
parent; this is also regarded as heterosis.
•
26

27. Type of heterosis :
1. Average heterosis or relative heterosis:
• When the heterosis is estimated over the mid
parent i.e. mean value or average value of the two
parents is known as average heteroris / mid plant
heterosis/ relative heterosis.
• Mid parent heterosis: x 100
MP= mean of two parents.
27

28. 2. Heterobeltiosis/better parent heterosis:
• When the heterosis is estimated over the
better parent is known as better parent
heterosis/ heterobeltiosis.
• Calculated by using formula,
• Heterobeltiosis= x 100
• Where BP= mean of better parents.
• The term heterobeltiosis was used by Bitzer et
al. (1968)
28

29. 3. Standard/ Economic/ Useful heterosis:
• It refers to the superiority of F1 over the
standard commercial check variety. Calculated
by using formula.
• Standard heterosis= x 100
29

30. Inbreeding depression:
• Inbreeding is mating between individuals related by
descent or ancestry. ( inbreeding depression results
due to fixation of unfavourable recessive genes in
F2)
• Inbreeding depression may be defined as the
reduction or loss in vigour and fertility as a result of
inbreeding. The chief effect of inbreeding is an
increase in homozygosity in the progeny.
30

31. Effect of Inbreeding:
• Reduction in vigour.
• Reduction in reproductive ability.
• Separation of the population into distinct
lines.
• Increase in homozygosity.
• Reduction in yield.
31

32. Degree of inbreeding depression/ types:
1. High inbreeding depression:
• A large proportion of plants produced by selfing
show lethal (harmful) characteristics and do not
survive. The loss in vigour and fertility is so great
that very few lines can be maintained after 3 to 4
generations of inbreeding. Eg. Alfalfa, carrot etc.
The lines that do show greatly reduced yields,
generally 25% of the yield of open-pollinated
varieties.
32

33. 2. Moderate inbreeding depression:
• Many lethal and sub-lethal types appears in the
selfed progeny, but a substantial proportion of lines
can be maintained under self-pollination. There is
appreciable reduction in fertility and many lines
reproduce so poorly that they are lost. However, a
large number of inbred lines can be obtained, which
yield up to 50% of the open pollinated varieties.
Example, maize, sorghum, bajra etc.
33

34. 3. Low inbreeding depression:
• Only a small proportion of the plants produced by
inbreeding show lethal characteristics. The loss in
vigour and fertility is small and rarely a line cannot
be maintained due to poor fertility. Some inbred
lines may yield as much as the open pollinated
varieties from which they were developed.
Example, onion, many cucurbits, rye and sunflower
etc.
4. Lack of inbreeding depression:
• Some pollinated species do not show inbreeding
depression rather they do show heterosis.
34

35. Inbred lines or inbreds:
• The lines which are almost homozygous due to
continued inbreeding and are maintained through close
inbreeding, preferably selfing are known as inbred lines
or inbreds.
Genetics basis of heterosis and inbreeding depression:
• Heterosis and inbreeding depression are closely related
phenomena, and they may be regarded as opposite
sides of the same coin. Therefore, genetic theories that
explain heterosis also explain inbreeding depression.
There are two main theories to explain heterosis and
consequently, inbreeding depression.
• Dominance
• Over dominance
35

36. 1. Dominance hypothesis:
• This theory was proposed by Davenport in 1908 and
explained by Bruce and by Keelde and Pellew in 1910.
• This is the most widely accepted hypothesis of
heterosis.
• This hypothesis explains that heterosis occurs due to
the superiority of dominant alleles over recessive
alleles. Heterosis occurs due to complementing action
of superior dominant alleles from both parental inbred
lines at multiple loci over the corresponding
unfavourable alleles leading to the improved vigour of
hybrid plant.
• Heterosis is directly proportional to the number of
dominant genes contributed by each parent. 36

37. 37

38. 2. Over dominance hypothesis:
• The idea of over dominance was initially put forth by
Fisher in 1903 and it was elaborated independently by
East and Shull in 1908;
• This hypothesis is based on the assumption that there
are loci at which heterozygous (Aa) is superior to both
of the homozygous (AA or aa).
• Model would be like Aa > AA or aa. Superiority of
heterozygotes may exit at the molecular level if the
products of two alleles have different properties.
• Vigour increases proportionate to increase in amount
of heterozygosity.
• Different names are given to this hypothesis such a
single-gene heterosis, cumulative action of divergen
alleles and stimulation of divergent alleles. 38

39. 39

40. 40

41. Effect of inbeeeding:
Following effects are seen due to inbreeding.
1. Appearance of lethal and sub-lethal alleles:
• Lethal, sub-lethal and sub-vital characteristics
appear in the offspring produced through
inbreeding. It is due to accumulation of harmful
recessive alleles after selfing.
• Appearance of lethal characteristics leads to death
while subvital characteristics reduce survival and
reproduction rate.
41

42. 2. Reduction in Vigour:
• Inbreeding causes general reproduction in vigour of
the population. Reduction of various plant parts
caused by inbreeding caused shorter and weaker
plants.
3. Reduction in reproductive ability:
• Inbreeding also causes decrease in reproductive
ability of the offspring. This causes production of
less number of offspring by the plant progenies
produced through inbreeding. These less produced
offspring cannot be maintained for next generation.
42

43. 4. Separation of population into distinct line:
• Inbreeding increases homozygosity in the offspring and
is proportionate to the degree of inbreeding. Higher
proportionate of inbreeding increase higher
homozygosity. This causes rapid separation of offspring
into phenotypically distinct lines. Increase in
homozygosity causes random fixation of various alleles
of different lines. So, lines differ in genotype and also
accordingly in phenotype.
5. Increase in homozygosity:
• Homozygosity is directly proportionate to the
inbreeding. Increasing in inbreeding means increase in
homozygosity. Inbreeding for 7 to 8 generation produce
almost uniform offsprings as they become complete
homogenous i.e. more than 99% homozygosity.
43

44. 6. Reduction in yield:
• Inbreed leads to the reduction of yield.
Offspring produced by inbreeding produces
very less as compare to the yield from open
pollinated varieties. For example, the best
yielding inbred yields half of the open
pollinated varieties.
7. Increase More susceptible to disease:
• in homozygosity causes the rapid loss in disease
resistance property.
44

45. Effect of Heterosis:
Heterosis causes following effect on offspring:
1. Increase yield:
• Yield of hybrid exhibiting heterosis increases.
Increase in yield is measured in terms of grain,
seed, leaf, tubers etc. this character is the most
important for commercial farming.
2. Increase reproductive ability:
• Hybrid exhibiting heterosis has increased fertility or
reproductive ability. Increase in reproductive ability
also increase yield of seeds or fruit etc. This causes
increase in yield of crops in which reproductive part
is consider as yield. 45

46. 3. Increase in size and general vigour:
• Hybrid exhibiting heterosis are generally more
vigour, healthier, and faster growing than their
parents. This is due to increase in cell number and
cell size. Increase in head size of cabbage, cob in
maize, fruit size in tomato are the example of
increase in size and general vigour.
4. Better quality:
• Heterosis shows improved quality in many cases but
not the yield such as onion keeping quality
increases in hybrid onion but the yield.
46

47. 5. Earlier flowering and maturity:
• Hybrid flower mature earlier than their parents in many
cases. For example many hybrids mature earlier than
their parents. But sometimes these may be associated
with lower production. But in vegetable production,
earlier maturity is more preferable than higher yield.
6. Greater resistance to disease and pest:
• Some hybrids show a greater resistance to insect and
disease than parents.
7. Greater adaptability:
• Hybrid are more adapted to environment change than
inbreds.. this is the physiological explanation of
heterosis. Greater adaptability is due to the
significantly smaller variance of hybrid than inbreds.
47

48. 8. Faster growth rate:
• Some variety show faster growth rate than their
parents but the total size grained may be
comparable to that of the parents.
9. Increase in number of plant parts:
• In some cases like bean, the number of nodes,
leaves and other plant parts increases than the
parents but the total size may not be different.
48

49. Physiological basis of Heterosis:
Following are the some of the physiological basis of
Heterosis:
1. Greater initial capital Hypothesis:
• Abhy (1930) conducted a study on the inbreds and
hybrid of maize and tomato and concluded that the
embryo size of F1 is greater in all heterotic crosses.
Because of this, it is expected to have greater
growth rate of seedlings of hybrids, but these are
not detectable in many cases where contradiction.
49

50. 2. Mitochondrial complementation:
• Sarkissian and Srivastva (1967) conducted study
on mitochondrial complementation in five
inbreds and single and double crosses in maize.
They found that heterotic activity was possible
only when mitochondrial from parents were
mixed. Complementation at mitochondrial level
makes energy transfer in heterotic hybrid more
efficient as mitochondrial are the powerhouse of
the cell.
50

51. 3. Net assimilation rate (NAR):
• Sinha and Khanna (1975) found that hybrid shows
heterosis for photosynthesis at the seedling stage. Net
assimilation rate and leaf area index are the major
components of total biomass. Heterosis for
phototsynthesis was found in hybrid rice and CO2
exchange in wheat.
4. Leaf area index (LAI):
• Leaf area index is the leaf area produced per square
meter of a crop. Higher leaf area index was found in
different crops mainly in earlier stage of crop life. In
cotton and rice, heterosis in leaf area index in seedling
stage shows significant advantages than later stage of
crop growth. However, increase in leaf area index only
without the sink capabilities has no value as it does not
increase production.
51

52. 5. Root growth:
• Root growth depends upon the shoot system. May
be due to heterosis in leaf area index, heterosis can
be also seen in root growth. Root serves as the sink
when root is of economical value.
6. Hormone balance:
• Hormonal balance is the major cause of heterosis in
many cases. In maize, the concentration of
endogenous GA3 is higher in hybrid as compare to
the inbreds. This study is based on the seedling but
not the mature plant. So, it cannot be correlated
with final yield.
52

53. 7. Metabolic Concept:
• Yield is the end product of series of reactions
and each reaction requires specific enzyme. It
has been suggested that inbreds contains
certain enzymes in rate limiting concentration
causing unbalanced metabolic system. When
two inbreds with rate limiting enzymes and
that complement any to each other are
crossed, a heterotic F1 is obtained.
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54. 54