Heterosis

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Heterosis
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• 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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Heterosisd gt a
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• Defining a locus in terms of scale
• bb mp BB Bb

a
-a
d
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Heterosis
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• 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
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• 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
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• Tantalizing anthropological evidence
• Religious rituals associated with maintenece 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 self-fertilization 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.

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Modern -History
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• 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
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• 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 populationsForest 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 gt cost of hybrid seed production

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Estimated percentage of hybrids for selected
vegetables
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• Crop hybrid Method
• Tomato (fresh) 60 Hand
• (Processing) 75 Hand
• Sweet pepper 40 Hand
• Onion 60 CMS
• Broccoli 100 S.I.
• Snap beans 0 -
• Lettuce 0 -

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Attributes of F1 hybrids
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• 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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Measurement of Heterosis
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• 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 24 (4.96 - 2.46)/ 2.46 100
• A231 30 20 !! Told me that favorable combinations
  do exist!!
• Toche 70 102 ? How to capture this genetic
  effect?
• A375 25 33

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Genetic basis of heterosis
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• 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
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• 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 HypothesisExample
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• Model AA Aa gt aa - AA10 Aa10 and aa0
• Parent 1 Parent 2
• aaBBccDDee 20 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.

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Dominance hypothesisLinkage
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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
• Empirical evidence supports dominance hypothesis,
  as inbred line are improving in performance.

A b
a B
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Repulsion phase linkage
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• In 70s investigators were interested in the
  relative magnitude of s2A and s2D
• In F2 crosses the ratio of s2D / s2A was gt1
  indicating large amounts of dominance variance,
  but once the populations were random mated for
  several generations the ratio of s2D / s2A was
  became lt1, this was likely due to recombination
  among repulsion phase loci

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Overdominance
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• 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 gt aa or AA
• They recognized importance of dominance, but it
  alone cannot account for observed heterosis.

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Overdominance
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• 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
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• 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

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Epistasis
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• Epistasis - interaction among loci, may also
  contribute to heterosis
• Internode
• Generation No. nodes length Height
• Parent 1 3 1 3
• Parent 2 1 3 3
• Hybrid ( add) 2 2 4
• Hybrid ( Dom) 3 3 9

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Epistasis
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• 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
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• 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
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• Moll (1965) showed a relationship between genetic
  distance and heterosis for yield in maize

Heterosis
Genetic distance
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Relationship between genetic distance and
heterosisSmith et. Al. TAG 1990
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Note, r2 of 0.76
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Relationships between genetic distance and
Heterosis
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No relationship
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Heterosis for yield in self -pollinated
vegetables
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• Crop Mean Range
• Tomato (fresh) 41 -59 to 168
• (solids) -10 -45 to 53
• Sweet pepper 15 -16 to 52
• Eggplant 80 -29 to 242
• Beans (dry) 29 -38 to 146
• Peas 28 116 to 218
• Lettuce 6 -6 to 119

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Hybrid Rice in China
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• 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?