Download From out of old fields comes all this new corn

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Heterosis
Heterosis
“ In my opinion, hybrid corn is the most far
reaching development in applied biology in
this quarter century” Mangelsdorf, 1951
A plant breeding phenomena that we very
successful exploit commercially, the
biological basis of which remains poorly
understood
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Heterosis
‘d’ > ‘a’
Defining a locus in terms of scale
bb
mp
BB
Bb
-a
+a
d
Heterosis
• The converse or complement to inbreeding
depression in which the value ‘d’ ≥ ‘a’
• Inbreeding can result in loss of vigor, size,
etc….
• The restoration of phenotypic performance
by crossing inbred lines to produce a hybrid
is called heterosis.
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History
• Fundamentally about the effects of
inbreeding and outbreeding
• When did man first observe and take
advantage of this phenomena?
– Is this only a modern “scientific”
development?
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Pre- History
• Tantalizing anthropological evidence
– Religious rituals associated with maintenance of
maize lines
– Helentjaris - 700 year old Anasazi cobs from SW
USA observed molecular marker fragment
patterns that more closely resembled F1 hybrid
than an O.P. variety
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Modern - History
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• Koelreuter (1766)
– investigated hybrid vigor in Nicotiana, Dianthus, Datura, et. al.
• Darwin (1876)
–
discussed hybrid vigor in his book “The effects of cross and selffertilization in the vegetable kingdom” He demonstrated that cross
fertilization frequently resulted in increased size, vigor and
productiveness when compared with self-fertilization. He did not
attribute the differences to the uniting of different gametes.
• Mendel (1865)
– Wrote “ in repeated experiments, stems of 1 foot and 6 feet in length
yielded plants with varied in length from 6 to 7.5 ft.”
Modern -History
• Beal - 1880
– Described how he planted in alternate rows to stocks of the
same variety, one row was detasseled and the hybrid seed
was more productive than either parent.
• Shull - (1908-1914)
– Shifted emphasis from the negative effects of inbreeding to
the positive effects of hybridization.
– Coined the word “heterosis” to describe the increased vigor
observed from heterozygosity.
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Modern -History
• East - (1908-1909)
– Studied the effects of inbreeding and outbreeding
– His work led to the formulation of the modern
heterosis concept.
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Yield and types of populations
Forest Troyer - 1991
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Mating system and heterosis
• Heterosis has been reported for a wide range
of crops including both self and cross
pollinated species
• Commercial application is via F1 hybrids
• Commercialization
– Added value > cost of hybrid seed
production
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Estimated percentage of hybrids
for selected vegetables
Crop
% hybrid
Method
Tomato (fresh)
(Processing)
Sweet pepper
Onion
Broccoli
Snap beans
Lettuce
60
75
40
60
100
0
0
Hand
Hand
Hand
CMS
S.I.
-
Attributes of F1 hybrids
 Maximum performance under optimal
conditions
 Stability of performance under stress
 Proprietary control of parents
 Often, reduced time to cultivar development
 Joint improvement of traits
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Hybrid cultivars
• Hybrid cultivars are first generation offspring after
cross between different inbred parent lines
• Major steps in breeding
– develop inbred homozygous lines
– find good F1 combination between inbreds
– produce F1 seed in large scale for growers
• Hybrids are uniform, reproducible and ”protected”
if parents are homozygous.
Inbreeding and hybridisation to
produce desirable hybrids
F1 hybrid population
Parent population
YIELD
Rare desirable
genotypes
Genotypes
cannot be
reproduced
Inbreeding
Inbred line
population
Each hybrid can
be produced large
scale from its two
parental inbreds
Major types of hybrid cultivars
• Single cross hybrids (F1)
A x B = F1
• Three way hybrids
(A x B) x C = Three Way Hybrid
• Double cross (Four way hybrid)
(A x B) x (C x D) = Double Cross
Three way and double cross hybrids are used to
reduce seed costs when parentals are weak
Effects of inbreeding
Selfing, full/half-sib pollination
Reduced height, seed set,
disease resistance, etc
Increased lodging,
Increased homozygosity
Hybrid vigour or heterosis
• Heterosis: the increase in size, vigour or
productivity of a hybrid plant over the
average or mean of its parents.
– Midparent heterosis
– High parent heterosis
– Standard heterosis
Measurement of Heterosis
• Mid-parent heterosis
– Hybrid performance is measured relative to mean
of the parents (MP)
– (F1 - MP) / MP * 100
• High-parent heterosis
– Comparison of hybrid to performance of best
parent (HP)
– (F1 - HP) / HP * 100
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Real data from dry beans
% heterosis above HP
• 9 x 9 Diallel of bean cultivars, evaluated in two locations
A132 A476 B1222 A359 X122 A457 A231 Toche A375
A476 5
*
B122 14
12
Yield of Toche = 2.38 T/Ha.
A359 25
0
Yield of A476 = 2.46 T/Ha.
X122 60
30
F1 = 4.96
A457 10
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(4.96 - 2.46)/ 2.46 *100
A231 30
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!! Told me that favorable combinations do exist!!
Toche 70
102
? How to capture this genetic effect?
A375 25
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Genetic basis of heterosis
• Three possible genetic causes:
– Partial to complete dominance
– Overdominance
– Epistasis
• The issue for plant breeders - What is the
Ideal genotype?
– Partial to complete dominance - Homozygote
– Overdominance - Heterozygote
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Dominance Hypothesis
• Davenport (1908)
– Hybrid vigor is due to action and interaction of
favorable dominant alleles
– Hypothesizes decreased homozygosity for
unfavorable recessive alleles (covering up)
– Conversely, inbreeding depression is due to
exposure of these recessive alleles during
inbreeding
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Dominance Hypothesis
Example
• Model AA = Aa > aa -
Parent 1
aaBBccDDee = 20
AA=10 Aa=10 and aa=0
Parent 2
AAbbCCddEE = 30
F1
AaBbCcDdEe = 50
Also note that AABBCCDDEE = 50
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Discussion of
Dominance hypothesis
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• Theoretically, plants homozygous for all favorable
alleles could be developed (AABBCCDDEE….)
– Why then are there no inbred equal in performance to
hybrids??
– This was considered a until it was recognized that only
1 in 4n individuals in a population would be
homozygous for all loci – For 10 loci that would be 410 = one plant in a million.
Dominance hypothesis
Linkage
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• Recombination among loci could result in plants
homozygous for all favorable alleles, but…
• Repulsion phase linkages, either slow or preclude the
development of such lines
A
b
a
B
• Empirical evidence supports dominance hypothesis, as
inbred line are improving in performance.
Repulsion phase linkage
• In 70’s investigators were interested in the relative
magnitude of s2A and s2D
• In F2 crosses the ratio of s2D / s2A was >1 indicating
large amounts of dominance variance, but once the
populations were random mated for several
generations the ratio of s2D / s2A was became <1,
this was likely due to recombination among repulsion
phase loci
A
b
a
B
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Overdominance
 First proposed by Shull (1908) and late
expanded by Hull (1945)
 It states that the heterozygote (Aa) at one or
more loci is superior to either homozygote
(AA or aa)
 Model would be Aa > aa or AA
 They recognized importance of dominance,
but it alone cannot account for observed
heterosis.
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Overdominance
 Superiority of heterozygotes may exist at the
molecular level, if the products of two alleles
have different properties, e.g. heat stability,
or advantages at different environments or
maturities - thus may result in stability.
 But, “single locus heterosis” difficult to
observe and detect if populations are not in
linkage equilibrium.
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Pseudo- Overdominance
• In which nearby loci which have alleles that
are dominant or partially dominant are in
repulsion phase
• If the populations are not in linkage
equilibrium, this could mimic the effects of
overdominance
A
b
a
B
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Epistasis
• Epistasis - interaction among loci, may also
contribute to heterosis
Generation
Parent 1
Parent 2
Hybrid ( add)
Hybrid ( Dom)
Internode
No. nodes
length
3
1
1
3
2
2
3
3
Height
3
3
4
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Epistasis
• Estimates bases on mating designs to
estimate the relative magnitude of add, dom
and epistatic components of variance
indicate that the magnitude of epistatic
variance is small compared to additive and
dominance components.
• Yet, the magnitude of epistatic variance is
difficult to estimate, and may play a very
important role in heterosis.
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Prediction of
heterosis
• The ability to predict heterosis of “Specific
combining ability” has been an elusive goal of
plant breeders
• Combining ability - Testing of hybrids
• Diallel crosses
n(n-1) / 2
– General (GCA) - Average performance - additive effects
– Specific (SCA) - ability of lines to combine in specific combinations
Due to dominance effects and heterosis.
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Genetic distance and heterosis
Heterosis
• Moll (1965) showed a relationship between genetic
distance and heterosis for yield in maize
Genetic distance
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Relationship between genetic distance
and heterosis
Smith et. Al. TAG 1990
Note, r2 of 0.76
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Relationships between genetic
distance and Heterosis
No relationship
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Heterosis for yield in self pollinated vegetables
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Crop
Mean %
Range
Tomato (fresh)
(solids)
Sweet pepper
Eggplant
Beans (dry)
Peas
Lettuce
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-10
15
80
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6
-59 to 168
-45 to 53
-16 to 52
-29 to 242
-38 to 146
116 to 218
-6 to 119
Hybrid Rice in China
• Hybrid rice yields about 20% more than the
best commercial varieties
• 8.4 Million Ha. was hybrid in 1988
• Based on CMS system
• ? If you believe the dominance hypothesis, is
this the best investment of plant breeding
effort?
• ? What is the ideal genotype in rice?
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