seminar topic

3. Heterosis: New insight into old topic by revisiting the
magic of heterosis for crop improvement
B. Sushmita
Sr. M.Sc.
PALB 6244
Dept. of GPB

4. History
Introduction to heterosis
Models of heterosis
Concept of heterotic pools
Case studies
Conclusion
Flow of Seminar

5. HISTORY
‘‘I raised close together two large beds of self-fertilized
and crossed seedlings from the same plant of Linaria
vulgaris. To my surprise, the crossed plants when fully
grown were plainly taller and more vigorous than the
self-fertilized ones.’’ - Charles Darwin
(The Effects of Cross and Self Fertilization in the Vegetable Kingdom, 1876).
Documented growth, development, and seed fertility of cross-pollinated
plants compared with the parents for more than 60 different species of plants.
Results: Inbreeding was generally deleterious & cross-fertilization was
generally beneficial.
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6.  1763: Koelreuter - hybrid vigour in tobacco.
 1875: Wilson A S reported first hybrid between Wheat and Rye in Scotland,
decade later W. Rimpau produced first doubled fertile hybrid- Triticale.
 1908: George H. Shull published ‘The composition of a field of maize’ which
marked the rediscovery of hybrid vigor or heterois and the beginning of
applying heterosis in plant breeding.
 Selfing maize plants led to reduction of overall growth vigor and yield
 1914: Term coined by “SHULL” as “stimulation of heterozygosis ”
 1920s maize yield was increased by 6 fold.
 1927: Karpchenko developed new species from hybrid between Raphanus and
Brassica
 1964, Yuan, Long Ping, initiated research on hybrid rice and heterosis in China
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7. What is heterosis?
Heterosis is the phenomenon in which hybrid progeny of two inbred
varieties exhibits enhanced growth or agronomic performance.
 Term was coined by “SHULL” in 1914
According to Shull heterosis is The increased vigour, speed of
development, resistance to disease and insect pests, or to climatic rigours
of any kind, manifested by crossbred organisms as compared with
corresponding inbreds as the specific result of unlikeness in the
constitutions of the uniting parental gametes .
 Converse of heterosis is inbreeding depression: Different aspect of the
same phenomena.
Fu et al., 201508-02-2019 7

8. Heterosis refers to the phenomenon in which hybrid
offspring exhibit characteristics that lie outside the range
of the parents
Mo17 F1 B73
F1
Mo17 B73
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9. MID-PARENT HETEROSIS:
Hybrid performs
significantly better than the
average(mid-parent) valve.
BEST-PARENT HETEROSIS:
BEST-PARENT HETEROSIS:
Hybrid performs
significantly better than the
better of two homozygous
parents.AA BB AB(case1) AB(case2)
OffspringParents
Mid –parent
heterosis
Best- parent
heterosis
Performance
Best-parent
Mid-parent MPH = [ (F1- MP)/ MP ] x 100
BPH = [ (F1- BP)/ BP ] x 100
QUANTITATIVE DEFINITION
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10. MANIFESTATION OFHETEROSIS
 Increase yield
 Increased reproductive ability
 Increase in size and general vigor
 Better quality
 Earlier flowering and maturity
 Greater resist to disease and pests
 Greater adaptability
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11.  Degree of heterosis is considerably high in production of hybrid and synthetic
variety from cross pollinated to often cross pollinated crop species.
 It can also be used in some self pollinated variety but chief difficulty in commercial
exploitation of heterosis in production of commercial quantity of hybrid seed.
 Cross pollinated crops:- maize, pearl millet, sorghum, cotton, onion, sunflower,
alfalfa, etc.
 Self pollinated crops:- rice, wheat, etc.
 Vegetable crops:- tomato, chillies, etc.
Commercial Exploitation
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12. MODELS OF HETEROSIS
 Genetic basis of heterosis
1. Dominant hypothesis
2. Over dominant hypothesis
3. Epistatic hypothesis
 Physiological basis of heterosis
 Biochemical basis of heterosis
 Molecular basis of heterosis
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13. Dominance hypothesis
• Davenport (1908) well supported by Bruce(1910) Keeble and Pellow(1910)
• Assumption :Favorable- dominant,
Unfavorable – recessive
• hybrid vigour – because of bringing together maximum number of dominant favorable genes acting in
additive manner in F1 hybrid.
• masking of recessive alleles by dominant alleles and prevention
of expression harmful recessive alleles
- Inbred1 X Inbred 2
Genotype - aaBBccDDee AAbbCCddEE Hybrid (F1)
Unit contribution - (1+2+1+2+1)=7 (2+1+2+1+2)=8
Hybrid (F1) - AaBbCcDdEe
2+2+2+2+2(10)
Model of heterosis- Aa=AA>aa Fu et al., 2015
Parent 1 Parent 2
x
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14. Objections
1. Inability to obtain homozygous dominant inbred as vigorous as hybrid
2. The F2 curve should be skewed towards dominant genes, but the
curve of F2 is found always smooth and symmetrical.
Explanations to dominance
 Dominance of linked gene hypothesis(Jones,1917) provided
explanation for this.
 Collins (1921) suggested that triat like yield is governed by large
number of genes or poly genes, which exhibit continuous variation
resulting in symmetrical distribution of genes.
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15. Overdominance hypothesis
• Shull and East in 1908 and supported by Hull (1945)
• State of being heterozygote in single loci itself is cause of heterosis.
• As diversity between uniting gametes increases, heterosis increase
East(1936)- series of alleles a1,a2,a3,a4 ----- of gradually increasing divergence
in function. Thus a combination of more divergent alleles will exhibit higher
heterosis than less divergent combinations.
Model of heterosis-Aa>AA or aa
Objections :
1. Holds good for qualitative traits but may not hold good for quantitative traits
2. Existence of epistasis reveal OD to be case of pseudo- over dominance
Fu et al., 201508-02-2019 15

16. Epistasis hypothesis
• In 1952, Gowen has suggested that influence of one locus on the expression of another
may be involved in Heterosis.
• Interaction between alleles of two or more different loci also called as non allelic
interaction.
1. Epistasis should be complementary type, i.e. the estimates of h and l have the same
sign so that they do not cancel each other out.
2. The interacting pairs of genes should be dispersed in both the parents.
• Non-allelic interaction is of three type’s viz. additive X additive, additive X dominance,
dominance X dominance.
• Particularly that involves dominance effects (dominance X dominance) may contribute
to heterosis.
Ex: cotton and maize.
Fu et al., 201508-02-2019 16

17. PSEUDO-OVERDOMINANCE
 The genetic intermediate of dominance and ODO is Pseudo-ODO, which is
actually a case of simple dominance complementation, because of tight repulsion
phase linkage and appears to be ODO (Stuber et. al., 1992; Graham et. al.,
1997
Genetic models of heterosis
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18. DOMINANCE MODEL:
Complementary action of superior dominant alleles from both
parental inbred lines lead to improved vigor of hybrid plants.
(Davenport 1908; Bruce 1910; Keeble and Pellow 1910; Jones
1917.)
OVERDOMINANCE MODEL:
Heterosis due to superiority of dominant alleles over both the
homozygotes. (Shull, 1908; East, 1908; Crown, 1948; Stuber,
1994).
EPISTASIS MODEL ::
Interactions between nonallelic genes at two or more loci.
(Powers 1945).
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19. Physiological basis of heterosis
Ashby( 1930 ) hypothesis of greater initial capital
Heterozygosity results from the greater initial weight of embryo.
Hybrid vigour is nothing more than maintenance of initial advantage of embryo size.
Hybrids don’t differ from their parent’s in relative growth rate.
Studied in 3 stages of physiological manifestations of heterosis
1. Seed and embryo development
2. Early seedling growth
3. Later plant growth
(Opposed by Sprague, Kempton , Wholey and others because not all hybrids have larger embryos)
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20. Biochemical basis of heterosis
Bottleneck concept (Manglesdorf)
(i) Excellence of genotype depends not upon its strongest link, but
upon its weakest link.
(ii) Emphasis is laid not on the superiority of the hybrid, but on the
inferiority of its parents.
(iii) Inferiority of the parents are thought to be the bottlenecks represented
by inefficient alleles.
(iv) It supports the dominance hypothesis.
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21. Evidence by Robbins (1952)
In the roots of tomato varieties
Roots of Red currant had bottleneck
for pyridoxin production
Roots of johannesfeuer had
bottleneck for supply of
Nicotinamide
Hybrid was able to produce both the
vitamins.
Nicotinamide added to basal medium
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22. Brewbaker hybrid superiority model
A0A0 A1A1 A1A0
Supplementary action
model
X Y X+Y
(rust resistance in flax)
Alternate pathway X In E1 Y In E2 X (E1) Or Y (E2)
( temperature sensitive alleles
of plants)
Optimal amount 0.1X 2X X
(drosophila lethal alleles)
Hybrid substance X Y X+Y+Z
(hybrid maize produces
entirely new product, esterase
enzyme)
08-02-2019 22

23. Molecular bases of Heterosis
• Classified into 2 parts
(1) Genome structure heterosis
• Hybrids contains higher content of nuclear DNA than parents.
• Crossing leads to complementation of the genome.
• Recovery of the lost segments in the genome of the hybrid cells.
(2) Genome activity heterosis
• Increased replication, transcription,
• Translation
Heterosis results in greater activity of hybrid plant because of surplus
genetic information.
Zachary .B et al., 200608-02-2019 23

24. Differential gene expression in hetrosis, - represents active gene expression, x- no gene expression
08-02-2019 24

25. Molecular changes at epigenetic, genomic, proteomic
And metabolic levels lead to heterosis traits.
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26. Current status of understanding of the moleculer basis of hetrosis
(Baranwal et al., 2012)
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27. about cis,
changes in
differential
trans, and
the hybrid,
expression
chromatin level
which results in
of genes. These
expression patterns, which might represent
additive or non additive modes of gene
action, may affect
pathwayswhich,in
major regulatory
turn, send out
regulatory cues that cumulatively affecta
number of downstream metabolic
pathways in either a positive or a negative
manner. These individual pathways,
whether placed on the input side or the
consumption side of the energy equation,
affect various aspects of growth and
development. The net positivity or
negativity in the system, therefore, would
define the state and extent of heterosis
(Goff et al., 2011)
EnergyBiomass = Energyinput - Energyconsumed
Mixing of two distant genomes brings
Emerging model based on energy use efficiency
08-02-2019 27

28. Explanation about the cause of heterosis phenomenon is based on
the ideas:
1.Every trait of an organism depends on many genes:
The appearance of recessive trait may be caused by the absence
of one of genes that control dominant trait. The so-called recessive
gene may or may not exist as a genetic unit that controls the
recessive trait.
2. Inbreeding depression and heterosis are related to individual
genetic diversity(NGPs).
HETEROSIS AT INDIVIDUAL LEVEL (Huyen, 2016)
Huyen, 201608-02-2019 28

29. Eg. Mendel’s experiments with pea plants
Homozygous tall pea plants were crossed with
homozygous dwarf pea plants
In F1 generation all hybrids were tall plants
F1 plants were self-pollinated then in F2 generation tall
plants and dwarf plants were obtained by the ratio of
3:1,respectively
ABOUT THE SO-CALLED RECESSIVE GENE
Huyen, 2016
08-02-2019 29

30. F1 gamete C c
C CC Cc
c Cc cc
Segregation of genotypes in F2;
1CC: 2Cc: 1cc
Segregation of phenotype in
F2; 3 tall plant : 1 dwarf plant
P tall plant dwarf plant
CC x cc
F1 tall plant
Cc
The first explanation
The common explanation
Homozygous tall plants have CC genotype, Homozygous dwarf
plants have cc genotype. C allele is dominant to c allele.
The segregation of plant height in F2 is explained as
Huyen, 201608-02-2019 30

31. Relation between genes and traits
• C or c is not the sufficient gene but a necessary gene for the
trait of plant height
• For existence of the trait plant height, a plant must exist
• For existence of a plant,
I. Genes coding for some proteins that are vital components of
cells and
II. Genes coding for some vital enzymes must exist
• The existence of these genes must relate to some traits among
them there is trait of plant height
Huyen, 201608-02-2019 31

32. Therefore sufficient genes for trait of plant height are
(X1X1 X2X2... XnXn), CC
(X1X1 X2X2... XnXn), Cc
Tall plant
(X1X1 X2X2... XnXn), cc
Tall plant
Dwarf plant
Exist in both the parental plants(X1X1 X2X2... XnXn), --
Huyen, 201608-02-2019 32

33. For simplifying , it can be written:
CC
Cc
Tall plant
cc
High plant
Dwarf plant
The sufficient condition for any trait must be many genes
Huyen, 201608-02-2019 33

34. The second explanation
Assuming that:
Homozygous tall pea plants have genotype;
(X1X1 X2X2... XnXn), CC
Heterozygous tall pea plants have genotype;
(X1X1 X2X2... XnXn), C
Homozygous dwarf pea plants have genotype;
(X1X1 X2X2... XnXn)
The difference between tall pea and dwarf pea plant is that
the tall plants have C gene, the dwarf pea plants have no C gene i.e
dwarf plant has complete absence of one of the genes that control
dominant trait
Huyen, 201608-02-2019 34

35. P Tall plant Dwarf plant
(X1X1 X2X2... XnXn) CC x (X1X1 X2X2... XnXn)
P gamete (X1X2... Xn) C (X1X2... Xn)
F1 Tall plant
(X1X1 X2X2... XnXn), C
F1 gamete (X1X2... Xn), C (X1X2... Xn)
(X1X2... Xn) C (X1X1 X2X2... XnXn) CC (X1X1 X2X2... XnXn) C
(X1X2... Xn) (X1X1 X2X2... XnXn) C (X1X1 X2X2... XnXn)
Segregation of genotypes in F2: 1CC: 2C: no C
Segregation of phenotype in F2: 3 tall plant : 1 dwarf plant
Huyen, 2016
08-02-2019 35

36. There are two ways of explanation for the segregation of ratio 3:1
One possibility :
The appearance of the
recessive trait may be
caused by the recessive
allele in homozygous state
The other possibility :
The appearance of the recessive
trait may be caused merely by
the absence (complete absence)
of the gene that controls the
dominant trait.
Huyen, 2016
08-02-2019 36

37. The so-called recessive gene localized on the chromosome map based on the
Morgan principle may be a gene coding for a or a protein and may be a gene of
non-sense ( a DNA fragment not to be transcribed, translated during the life of
the organism).
If the latter happens,
It will be unreasonable to assign it (a non sense gene) to the role that controls the
recessive trait.
It should be emphasized that the existence of the so-called recessive gene is not
a must for a recessive trait.
eg. 1: Albino recessive gene is not must for the albino trait.
The albino mutation on plants may be caused by the absence of one of the genes
that take part in process of making chlorophyll.
Lethal recessive gene is not a must for lethal mutation.
eg. 2: Lethal mutation in Drosophila
May be caused by the absence of a vital gene: one of the genes that encode for
enzymes taking part in Krebs cycle.
Huyen, 2016
08-02-2019 37

38. In molecular genetics, gene is a fragment of DNA that codes
for a polypeptide or a protein .
• The gene concept does not include genes of non- sense (not to
be transcribed, translated). By this concept of gene, the gene
products are poly peptides and proteins.
From the concept of gene being limited as above,
The term individual genetic diversity and the number of
genetic properties are proposed herein
Huyen, 201608-02-2019 38

39. Individual Genetic Diversity: It is the diversity of gene products of
an organism. Individual genetic diversity is measured by the number of
genetic properties.
Number of genetic properties (NGPs): is the number of different
genes (coding for different poly peptides, proteins)
Non sense genes and no genes are denoted – ng “we do not count” for
NGPs
For eg. :
NGPs of genotype AA BB CC dd ee is 5
NGPs of genotype Aa Bb Cc Dd Ee is 10
NGPs of genotype Aa Bb CC dd ee is 7
NGPs of genotype Aang Bb Cc Dd Ee is 9
NGPs of genotype Aang Bdng Cc Dd Ee is 8
From here A and a does not mean , a is recessive to A allele but means that A and a
belongs to same locus governing same trait. Same applied to other letters.
THE CAUSE OF THE HETEROSIS PHENOMENON
Huyen, 201608-02-2019 39

40. In natural cross pollinating species,
Subset (1) X1X1 X2X2….. XiXi
Subset (2) D1d1 D2d2…… Djdj
Subset (3) T1tng
1 T2tng
2 …. Tktng
k
Homozygous alleles
Heterozygous
alleles
Pseudo-heterozygous allele
NGPs-i NGPs-k
NGPs-2j
TOTAL NUMBER OF GENETIC PROPERTIES OF THIS PLANT IS
i+2j+k
Huyen, 201608-02-2019 40

41. When this plant(with genes X, D, d ,T and tng ) is self pollinated for many
generations we get purelines
A) Purelines having the largest NGPs = i + j + k
1. subset {X1X1 X2X2 … XiXi } NGPs = i
subset {D1D1 D2D2 … DjDj } NGPs = j
subset {T1 T1 T2T2 … TkTk)} NGPs = k
2. subset {X1X1 X2X2 … XiXi } NGPs = i
subset {d1d1 d2d2 … djdj } NGPs = j
subset {T1 T1 T2T2 … TkTk } NGPs = k
3. subset {X1X1 X2X2 … XiXi } NGPs = i
subset {D1D1 d2d2 … djdj } NGPs = j
subset {T1 T1 T2T2 … TkTk } NGPs = k
and so on, many other sets of genes
The changes in number of genetic properties
Huyen, 201608-02-2019 41

42. B) Purelines having NGPs = i + j + 1
1. subset {X1X1 X2X2 … XiXi } NGPs = i
subset {d1d1 D2D2 … DjDj } NGPs = j
subset {T1T1 tng
2tng
2 … tng
ktng
k } NGPs = 1
2. subset {X1X1 X2X2 … XiXi } NGPs = i
subset {d1d1 D2D2 … DjDj } NGPs = j
subset {tng
1tng
1 T2T2 … tng
ktng
k } NGPs = 1
and so on, many other sets of genes.
Huyen, 201608-02-2019 42

43. C) Purelines having the smallest NGPs = i + j correspond to the set of genes
that includes :
1. subset {X1X1 X2X2 … XiXi } NGPs = i
subset {D1D1 D2D2 … DjDj } NGPs = j
subset {tng
1tng
1 tng
2tng
2 … tng
ktngk } NGPs = 0
2. subset {X1X1 X2X2 … XiXi } NGPs = i
subset {d1d1 d2d2 … djdj } NGPs = j
subset {tng
1tng
1 tng
2tng
2 … tng
ktng
k } NGPs = 0
3. subset {X1X1 X2X2 … XiXi } NGPs = i
subset {d1d1 D2D2 … DjDj }) NGPs = j
subset {tng
1tng
1 tng
2tng
2 … tng
ktng
k } NGPs = 0
and so on, many other sets of genes Huyen, 201608-02-2019 43

44. Using the similar analogy, we find that the number
of genetic properties of purelines change from ( i
+2j+K ) in natural population to ( i + j+ k ).
Compared to the starting plant, the number of
genetic properties of purelines decrease at least by
‘j’and largest by ( j + k ).
 ( i +2j+K )-( i + j+ k )=j
( i +2j+K )- (i + j )= j + k
Huyen, 201608-02-2019 44

45. Crossing Between Two Purelines
S1S1 S2S2….. SiSi = i
D1D1 D2D2…. DjDj = j
T1T1 T2T2…TkTk = k
S1S1 S2S2… SiSi = i
E1E1 E2E2… EjEj = j
R1R1 R2R2… RmRm = m
S1S1 S2S2… SiSi
D1E1 D2E2… DjEj
T1tng T2tng… Tktng
R1rng R2rng… Rmrng
Huyen, 201608-02-2019 45

46. NGPs of hybrid….
Subset NGPS = i
Subset NGPS = 2j
Subset NGPS = k
Subset NGPS = m
S1S1 S2S2… SiSi
D1E1 D2E2… DjEj
T1tng T2tng… Tktng
R1rng R2rng… Rmrng
TOTAL NUMBER OF GENETIC PROPERTIES OF THE HYBRID = i+ 2j + k + m
Huyen, 201608-02-2019 46

47. The number of genetic properties of the
second pureline is
( i + j + m )
The number of genetic properties of the
first pureline
( i + j + k )
i + 2j + k + m
Huyen, 201608-02-2019 47

48. i +2j + k +
m
i + j + k
i + j +
m
Increase in genetic properties
compared to 1st line (i+2j+k+m) – (i+j+k) = j+m
compared to 2nd line (i+2j+k+m) – (i+j+m) = j+k
J+m
J+k
Increase in NGPs in hybrid is the cause of heterosis Huyen, 201608-02-2019 48

49. Development of heterotic group / pools
• Heterotic group: a group of related or unrelated genotypes from the same or different
populations, which display similar combining ability and heterotic response when crossed
with genotypes from other genetically distinct germplasm groups.(Melchinger and
Gumber, 1998)
• Heterotic pattern refers to a specific pair of heterotic groups that express high heterosis
and high hybrid performance in their cross.
• Crossing between heterotic patterns helps to maximize the expected heterosis.
• It avoid the development and evaluation of unnecessary hybrids from these heterotic
patterns.
• Rice, rape seed, rye , maize and cotton
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50. (Melchinger and Gumber, 1998)(Cress, 1967)08-02-2019 50

51. Compact types
Reduced Plant height ,
Early bearing habbit
Less length of Monopodia, Sympodia
Reduced internodal length
Robust type
High Plant height
Thick Stem and Leaf
Internodal length is more
Boll size is more
Late bearing
Stay green
Leaves are lustry green
Greenish remain till bolls are
harvested
Photosynthesis till harvest of bolls
Compact types
Robust type
Stay green Patil S. S
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52. 08-02-2019 52

53. Robust line Compact line
Robust x compact hybrid
Heterotic pattern
Patil S. S08-02-2019 53

54. 08-02-2019 54

55.  First example of a single overdominant gene for yield.
 Heterozygosity for tomato loss-of-function alleles of SINGLE FLOWER
TRUSS (SFT), which is the genetic originator of the flowering hormone
florigen, increases yield by up to 60%.
Kriwger et al, Nature Genetics,2010, 42, 459–463
Objective: To identify overdominant mutations for yield in tomato.
08-02-2019 55

56. • Material - M82 variety (determinant), 33 diverse fertile mutants of M82 with defects in
developmental traits and overall growth, which were tested by progeny test for single-gene
segregation.
• sft/sft plants(e4537) flower later , are large and produce the fewest inflorescences, few
flowers and fruits, more vegetation.
M1
M2
.
e0137
.
.
e4537
.
M33
x M82 To create isogenic mutant heterozygotes and compared their yields.
Six mutant heterozygotes showed heterosis.
08-02-2019 56

57. Sft alleles yield and sft/+ hybrids OD
Brix value
Comparision of mean values for total fruit yields between three
independently derived sft homozygous mutants, inbred M82
control and the F1 sft/+ hybrids
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58. SFT- dependent heterosis arises from multiple phenotypic
changes on component traits that integrate to yield.
Representive inflorescence from M82, sft homozygous mutant and sft heterozygote.08-02-2019 58

59. CONCLUSION
• Heterozygosity for the allele SFT with a mutant copy, which controls the
synthesis of the florigen hormone, leads to an increase in yield by more than
60%, confirming the leading role of heterosis.
• The observed reaction is associated with a shift in the developmental
program of the SFT-heterozygous genotype towards an increase in the
number of flowering inflorescences in comparison with wild type
homozygotes, which are characterized by more powerful vegetative growth
but which forms a small number of inflorescences.
• The overdominant effect of the SFT is due to the effect of the dose on
molecular expression, which leads to the balance of the gene product in the
overdominant genotype.
• This example also highlights potential of fine-tuning the development
program of the organism.
08-02-2019 59

60. Objective: To quantify the heterotic response observed in the physiological processes
contributing to grain yield in maize.
Materials and methods: 12 SCH were developed by crossing 4 female inbred lines
(CG57,CG58,CG59 and CG69) to 3 male inbred lines (CG33,LH61
and LH 145).
The 12 hybrids and their 7 parental inbred lines were planted in adjacent blocks.
The blocks of hybrids and inbreds were treated as 2 separate experiments
08-02-2019 60

61. Physiological basis of heterosis for grain yield
Heterosis for its two
component processes,
DMA at maturity and
harvest index
Heterosis for grain
yield in terms of
heterosis for the
processes underlying
the two component
processes of grain
yield.
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62. 08-02-2019 62

63. 08-02-2019 63

64. 08-02-2019 64

65. 08-02-2019 65

66. 08-02-2019 66
Heterosis for rate of DMA during the life cycle can be examined in terms of heterotic effects on phenological
development, light interception and photosynthesis.

67. conclusion
• The inconsistency in heterotic effects in the physiological parameters
indicates that there is variation between values for inbred lines and
hybrids and consequently physiological processes limiting grain yield
are different for inbred lines and hybrids.
• They concluded that four physiological mechanisms associated with heterosis for
grain yield in maize, three of those are associated with the rate of DMA
(heterosis for LAI, Stay green, rate of photosynthesis during grain filling period)
and one of those is associated with dry matter partitioning (Heterosis for harvest
index)
08-02-2019 67

68. 08-02-2019 68

69. CONCLUSION
Studies carried out in different years have confirmed that all three types of gene action
(additivity, dominance and epistasis) mutually control the final manifestation of the heterotic
effect and heterosis cannot be explained from standpoint of any particular theoretical
concept.
Heterosis should be considered as the over all effect on the phenotypically similar action of
heterogenous genetic processes, and apparently various genetic causes underlie the
manifestations of heterosis.
New perspectives in studying the effect of heterosis are revealed by the contemporary
methods of molecular genetics, which make it possible to study the DNA variability and
investigate the structural and non structural sequences of the genome.