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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Hybridization

• Artificial Hybridization
• - useful for studying the genetics of species
  differences and reproductive isolating mechanisms
  involved in speciation
• Natural Hybridization
• - a better understanding of speciation
• - interaction of genetics and ecology
• - the role of hybridization in evolution

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Hybridization

• E. Mayer (1963) Evolutionary role of
  hybridization
• Perfection of isolating mechanisms
  (reinforcement)
• Source of new species
• Increased genetic variability (introgression)
• The total weight of available evidence
  contradicts the assumption that hybridization
  plays a major evolutionary role among higher
  animals

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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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Hybridization

• Burke, J. M. and M. L. Arnold (2001) Genetics and
  the fitness of hybrids. ARES 35 31 52.
• Rieseberg L.H. (1998) Molecular ecology of
  hybridization. Pp. 243 265. In Advances in
  Molecular Ecology. G. R.. Carvalho (ed.)
• Dowling, T. E. and C. L. Secor (1997) The role of
  hybridization and introgression in the
  diversification of animals. ARES 28 253-619
• Rieseberg L.H. (1997) Hybrid origins of plant
  species. ARES 2835989
• Harrison, R. G. (ed.) (1992) Hybrid Zones and the
  Evolutionary Process
• Arnold, M. L. (1992) Natural hybridization as an
  evolutionary process. ARES 23237-261
• Barton, N. H. and G. M. Hewitt (1989) Adaptation,
  speciation, and hybrid zones. Nature 314 497-
  503
• Barton, N. H. and G. M. Hewitt (1985) Analysis of
  hybrid zones. ARES 16113-148.
• Moore W.S. (1977) An evaluation of narrow hybrid
  zones in vertebrates. QRB 52 263277

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Hybridization
http//www.genetics.uga.edu/people_bio_arnold_m.ht
ml
1997
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Hybridization

• Definitions
• 1. Systematics (narrow)
• Offspring resulting from a cross
  between species
• 2. Current (broad)
• Crosses between genetically
  differentiated forms
• regardless of their current
  taxonomic status
• Introgression gene movement between species (or
  well differentiated populations) mediated by
  hybridization and backcrossing

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Hybridization
Hybridization among widely genetically separated
species How genetically divergent can species be
and still hybridize? North American and European
Sycamore 20 Myr separation fertile
hybrids Frogs, Birds, Mammals Immunological
Distance (molecular clock)
Frogs and Birds 20 Myrs
Mammals 2 3 Myrs Explanation of
difference chromosome evolution, regulatory
genes????
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Hybridization
X
Plantanus occidentalis
Plantanus orientalis
London Plane Tree
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Hybrid Zones

• The geography of hybridization
• - spatial relationship among parental
  species
• and hybrids
• - sporadic among broadly sympatric
  species
• - contact zone between parapatric
  species
• - linear (hybrid zone width)
• - mosaic (patchy)
• - spatial/temporal stability

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Hybrid Zones
Dr. Stephen A. Karl
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Hybrid Zones

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

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Hybrid Zones

• Regions of secondary contact of formally
  allopatric (or parapatric) taxa.
• Evidence
• - cline of allele frequencies through
  hybrid zone
• - concordance among several genetic
  markers
• - deviation from HWE
• - LD among loci

Due to nonrandom mating
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Bombina
Hybrid Zone
Allele freq. 5 loci
LD
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Hybrid Zone Models

• Models of Hybrid Zone maintenance
• 1. Tension Zone Model
• - balance between dispersal into
  hybrid zone and
• selection against hybrids
• 2. Bounded Hybrid Superiority Model
• - hybrids have high fitness in
  ecological transition
• zones between parental taxa
• 3. Mosaic Model

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Hybrid Zone Models

• Models of Hybrid Zone maintenance
• 1. Tension Zone Model
• - selection against hybrids reflects
  intrinsic hybrid
• inferiority regardless of
  environment
• - hybrid zone maintained by
  interactions among
• genes and are not fixed to any
  position

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Hybrid Zone Models

• Models of Hybrid Zone maintenance
• 2. Bounded Hybrid Superiority Model
• - some hybrid genotypes more fit in
  habitats that
• are novel relative to the habitats
  of the parent
• species
• 3. Mosaic model
• - patchy distribution of habitats.
  Parental species
• and hybrids adapted to different
  environments

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Genetic Markers

• Nuclear
• Cytoplasmic (mtDNA, cpDNA)
• Sexual asymmetries (which species maternal and
  paternal parent)

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Hyla cinerea
Hyla gratiosa
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Example Hybridization

• Tree Frogs
• Hyla cinerea (c)
• Hyla gratiosa (g)
• F x
  M
• From behaviour expect g x c hybrids
• 5 allozyme markers mtDNA

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Example Hybridization

• Genotype Categories
• Pure cinerea, pure gratiosa
• F1 hybrids 5-locus heterozygote
• Backcross cinerea
• Backcross gratiosa
• Later-generation hybrids (F2)

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Example Hybridization

• Two markers
• cc cc x gg gg
• cg cg F1
• Backcross
• cg cg x cc cc cg cg x
  gg gg
• cc cc
  cg cg
• cc cg
  cg gg
• cg cc
  gg cg
• cg cg
  gg gg

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Example Hybridization

• True backcross mistaken for pure species
• (½)n
• n of markers
• 5 markers 0.03

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Example Hybridization

• Table 7.5 mtDNA
• gratiosa
  cinerea
• Pure gratiosa 103
  0
• Pure cinerea 0
  60
• F1 20
  0
• cinerea BC 22
  36
• gratiosa BC 52
  1

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Example Hybridization

• Table 7.5
• Interpretation
• F1 hybrids F g x M c
• cinerea BC mtDNA
• F1 g x c g
• c x F1 c
• gratiosa BC
• F1 g x g g
• g x F1 g

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Example Hybridization

• Genetic structure of Hyla hybridization
• Not all individuals participate in hybrid
  matings (high frequency of both parental species)

Bimodal Hybrid Zone
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Example Hybridization

• Table 7.5 mtDNA
• Allozymes gratiosa cinerea
• Pure gratiosa 103 0
• Pure cinerea 0 60
• F1 20
  0
• cinerea BC 22 36
• gratiosa BC 52 1
• Later generation 9 2
  3

54
7
36
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Bimodal Hybrid Zones

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