Breeding cross-pollinated crops

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Breeding cross-pollinated crops
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Cultivar Development in Cross-pollinated Species

• Compared to self-pollinated species,
  cross-pollinated species differ in their gene
  pool structure, and in the extent of genetic
  recombination
• Unselected populations typically consist of a
  heterogeneous mixture of heterozygotes as a
  result of outcrossing, genes are re-shuffled in
  every generation

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• The breeder focuses more on populations, rather
  than individual plants, and on quantitative
  analysis, rather than qualitative traits
• Progeny do not breed true, since the parent plant
  is pollinated by another plant with a different
  complement of alleles

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• Because progeny do not breed true in
  cross-pollinated crops, the usual progeny testing
  that would be employed in self-pollinated
  material is much less informative.
• A more useful way to assess genetic potential is
  to examine combining ability
• General combining ability
• Specific combining ability

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• Combining ability the ability of an inbred line
  to give characteristic performance in hybrid
  combinations with other lines.
• The progenies are tested for performance as
  populations and related back to parental.
• More precision can be obtained by using a
  homozygous inbred line as the pollen donor
  (tester line).

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General combining ability the average or overall
performance of a line in hybrid combinations
(open pollinated) represents additive genetic
variance and additive x additive epistasis
Specific combining ability the performance of a
line as compared to other lines when crossed with
the same pollen donor (specific pollen source)
represents non-additive genetic variance
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Recurrent selection

• Any breeding system designed to increase the
  frequency of desired alleles for particular
  quantitatively inherited characters by repeated
  cycles of selection
• Identify superior genotypes for the trait under
  selection.
• Inter-mate the superior genotypes and select
  improved progeny.

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Population structures

• Self-pollinators
• mixture of homozygous lines
• a single homozygous line
• improve through cross, inbreed, select new
  superior homozygous line
• Cross-pollinators
• mixture of heterozygous plants (population)
• maintain through cross-pollination (OP)
• improve through selection of plants with desired
  genes, avoid too much inbreeding

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Progeny test vs combining ability test

• Self-pollinators
• evaluate pure line offspring (Measure agric.
  value)
• Cross-pollinators
• evaluate selfed offspring (if possible) No C.A.)
• evaluate test cross offspring (Measure combining
  ability)
• homozygous line tester (specific combining
  ability)
• heterogeneous population tester (general
  combining ability

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Recurrent selection principle
1. Select best plants 2. Intercross selected to
form next generation
Phenotypic recurrent sel. Mass
selection Genotypic recurrent sel. Evaluate
offspring
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Mass selection
Very simple population improvement
Efficient only for high heritability traits
Population maintenance Remove off types First
step in breeding programs
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Half-sib selection
1. season
Source population Select good looking plants and
intercross
2. season
Plants in each offspring have female parent in
common. They are half-sibs They reveal
combining ability of selected plant
Progeny test of selected plants in isolation
3. season
A is less efficient than B
A. Composite seed from superior progenies
B. Composite remnant seed from plants
with superior progenies
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Source population Superior plants selected
Half-sib selection with testcross
Tester can be more or less uniform
Composite open-pollinated
Composite selfed
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Source population Cross pairs of selected plants
Full sib selection based on pair crosses
Measures specific combining ability between
selected plants
Composite remnant cross seed from combinations
with superior progenies
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Source population self-pollinate selected plants
Selection from S1 progeny offspring test
Only if selfing is possible
Composite remnant selfed seed from selected
plants with superior progenies
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Formation of synthetic cultivars
Clones
Source population Clone selected plants
The selected base clones are kept to form new
Syn1 seeds regularly
Polycross selected superior clones
Polycross offspring evaluation
The cultivar is propagated until Syn2-Syn5 to
obtain enough seed
Clones with high combining ability
With well combining clones the synthetic can be
more uniform and vigorous than traditional OP
cultivars
Open pollinate to form syn2
Intercross to form Syn1
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Breeding clonally propagated species

• Plants are highly heterozygous
• Often semi-sterile
• Often polyploids
• Two major breeding methods
• Spontaneous or induced mutations (sports)
• Hybridization (often between subspecies / species)

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Hybridization in clonally propagated species
X
Clone A
Clone B
Potato Begonia Orchids
Select best looking seedlings
Evaluate clones 1-2 seasons
Multiply and market superior clones