Hybridization Avise Ch. 7

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Hybridization (Avise Ch. 7)

• Historical background
• Natural hybridization
• Genetic distance and hybridization
• Hybrid zones
• - geography
• - theoretical models
• - examples
• - sexual asymmetries (FA x MB gt FB x MA)
• - cytonuclear disequilibria (mtDNA/nucDNA)
• Evolutionary significance

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Speciation and Hybridization

• The time to divergence for sister species is
  shorter at high latitudes and longer in the
  tropics.

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Mussel Bimodal Hybrid Zone - Prezygotic
- Postzygotic
F1
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Bimodal Hybrid Zones

• Reading for Wed. March 21
• Bimodal hybrid zones and speciation
• Chris D. Jiggins and James Mallet
• http//www.mun.ca/biology/dinnes/B4250/Biol4250.ht
  ml

5
Hybridization

• G. L. Stebbins (1950) Evolutionary role of
  hybridization
• the true situation, at least as far as the
  higher plants are concerned, lies somewhere
  between the extremes.
• interspecific hybridization may not be as
  uncommon in animals as is usually believed.
• while hybridization is certainly less common in
  animals than plants, and is correspondingly less
  important as a factor in evolution, its influence
  in certain groups may be considerable.

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Speciation and Hybridization
http//www.nature.com/nature/journal/v446/n7133/fu
ll/nature05706.html
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Hybridization in Plants
Higher frequency of hybridization in plants than
animals due to Sex
determination Mating system
variation (pollination)
Vegetative reproduction Ecotypes
(genetic-environment associations)
Polyploidy
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Experimental Hybridization
Scarlet Gilia
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Louisiana IrisesNatural Hybridization
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Iris hexagona
Iris fulvia
Iris brevicaulis
Louisiana Irises
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Genetic markers

• Nuclear RAPD, microsatellites,
• allozymes
• Cytoplasmic cpDNA
• Additional information
• Floral morphology, habitat

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I. fulva I. brevicaulis

• absence of intermediate
• hybrids (F1)
• fulva ? forest
• brev. ? pasture

Nuclear and cpDNA
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• Environmental Variables
• Elevation
• Light
• Veg. Comp.

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Hybridization in Plants
Uncoupling of male and female components to gene
flow Pollen nuclear genes
Seed nuclear genes cpDNA
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Iris Hybrid Zone
fulva hexagona
fulva hexagona
Nuclear Markers (pollen flow)
cpDNA (seed dispersal)
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Hybrid Speciation in Plants
Hybrids between species if
fertile ? introgression if sterile ?
allopolyploid Reproductively isolated from parent
species
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Hybrid Speciation (introgression)
F1
Time
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Iris hexagona
Iris fulvia
Iris brevicaulis
Iris nelsonii
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Hybrid Speciation in Irises(introgression)
Example Iris fulvia (Saltwater) Iris
hexagona (Freshwater) Iris brevicaulis (Pasture)
Iris nelsonii (Swamp, Ecotone)
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Hybrid populations Iris nelsonii
Lack of foreign markers in I. nelsonii (genetic
stability) (Homoploid not polyploid)
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Hybrid Speciation in Plants
Example Clarkia spp. (Reticulate
evolution)
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allopolyploids



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Sunflower Reticulate evolution
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Evolutionary Significance of Hybridization

• Introgression source of genetic variation
  for
• adaptation
• Hybrid speciation
• Evolution of reproductive isolation
  (speciation)
• (reinforcement)

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Evolutionary Significance of Hybridization

• Introgression source of genetic variation for
• adaptation
• Requires viable and reproductive hybrids for
  gene flow between species

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Evolutionary Significance of Hybridization

• Are natural hybrids fit or unfit relative to
  their parents? Arnold and Hodges (1995)
• - Viable and fertile F1 hybrids may be rare,
  but repeated opportunities to form F1
• - wide range of hybrid fitness values
• lt , , gt parental taxa
• - importance of defining separate hybrid
  classes to evaluate fitness (based on genetic
  markers)

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Hybrid Speciation

• The same sexual processes that formed the hybrid
  can breakup the hybrid genotype
• Stability of hybrid (limits to gene flow between
  species)
• - genetic incompatibility (gene, chromosome)
• - distinct ecological preference
• - polyploidy
• - agamospermy (asexual) asexual vertebrates

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Speciation

• Bimodal hybrid zones and speciation
• Jiggins and Mallet (2000)
• Several genetic markers (multilocus)
• - classification of multiple hybrid
  genotypes
• Unimodal no assortative mating, hybrid swarm
• Bimodal strong assortative mating
• /- postzygotic incompatibilities

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Bimodal Hybrid Zones

• Reading for Wed. March 21
• Bimodal hybrid zones and speciation
• Chris D. Jiggins and James Mallet
• http//www.mun.ca/biology/dinnes/B4250/Biol4250.ht
  ml

31
Hybrid Zones

• Natural experiments
• - many generation of
  hybridization and
• recombination
• - areas of strong selection
• - ecological context
• - adaptive evolution
• - processes that cause divergent
  evolution
• 
  (speciation)
• Windows on evolutionary process