Biology 4250 Evolutionary Genetics

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Biology 4250 Evolutionary Genetics

• Dr. David Innes
• Dr. Dawn Marshall
• W 2008

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Outline of
topics 1. Introduction/History of Interest in
Genetic Variation 2. Types of Molecular
Markers 3. Molecular Evolution 4.
Individuality and Relatedness 5. Population
Demography, Structure Phylogeography 6.
Phylogenetic Methods Species Level
Phylogenies 7. Speciation, Hybridization and
Introgression 8. Human Evolutionary
Genetics 9. Conservation Genetics
Background
Applications
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Hybridization

• Topics
• 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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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 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 patchy distribution of
  habitats. Parental species
• and hybrids adapted to different
  environments

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Hyla cinerea
Hyla gratiosa
Sympatric
Allopatric
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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

• 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
11
Avise, 2001
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Example Hybridization

• Genetic structure of Hyla hybridization
• - Not all individuals participate in hybrid
  matings (high frequency of both parental species)
• - No pure species with opposite species
  mtDNA (no mtDNA introgression)

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Secondary Contact
Role of Mate Choice and Hybrid Fitness 1.
Maintained as separate entities (species) 2.
Fusion (single species) Function of
- error in mate choice
- hybrid fitness
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High
Error in Mate Choice
Low
Low
High
Fitness of Hybrid
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Reproductive Character Displacement
Hybrid zone - hybrids unfit (inviable,
sterile) - selection to avoid
interspecific mating - evolution of
reproductive character displacement
by reinforcement Reinforcement evolution of
prezygotic isolation
barriers in response to selection
against hybridization
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cinerea
Reproductive Character Displacement
Allopatric
A
gratiosa
S
Sympatric
Allopatric Sympatric Allopatric
Dr. Stephen A. Karl Department of
BiologyUniversity of South Florida
Call frequency similar in allopatry, displaced in
sympatry (selection to avoid mating)
http//chuma.cas.usf.edu/karl/evolution/chapter_1
2_2.htm
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Bimodal Hybrid Zones

• Bimodal hybrid zones and speciation Chris D.
  Jiggins and James Mallet (2000)
• Contact zones exemplify a series of
  stages in speciation. In unimodal hybrid zones
  intermediates predominate in bimodal zones
  hybrids are rare and parental forms predominate
  and finally, species might overlap, but never
  hybridize. Recent studies show bimodality to be
  associated strongly with assortative mating or
  fertilization, and only weakly with overall
  levels of genetic divergence or intrinsic genomic
  incompatibility.

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Bimodal Hybrid Zones
Bombina unimodal
Cricket flat - bimodal
Heliconius bimodal
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Example
A mussel hybrid zone in Eastern
North America Genetic differentiation
- Enzyme variation - mtDNA
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Koehn et al. (1984)
II
III
I
Fst 0.006 (5 loci)
III
II
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Mytilus Species

• Group I and II Mytilus edulis L.
• Group III Mytilus trossulus Gould
• Morphologically similar (Cryptic species)
• Several partially diagnostic enzyme genes

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Enzyme Genes (partially diagnostic)

• M. edulis M. trossulus
• Pgm allele Frequency
• 93 0.077
  0.020
• 100 0.808
  0.020
• 106 0.115
  0.300
• 108 0.000
  0.060
• 111 0.012
  0.580
• 114 0.000
  0.020

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Mt
Me
4 enzyme loci
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Diagnostic Genetic Markers
Enzyme genes Est Mpi
DNA

• Glu-5
• ITS
• mtDNA

Hy Tr Ed Ed
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F1
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mtDNA inheritance in mussels

• Most species mtDNA maternally inherited
• Mussels
• - females inherit mtDNA from mother
• - males inherit mtDNA from mother and
• father but pass on only paternal mtDNA

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Doubly Uniparental Inheritance
mtDNA
X
Males
Females
M
F
Females
Males
homoplasmic
heteroplasmic
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mtDNA genotypes

• Females Males
• M. edulis F-ed F-ed/M-ed
• M. trossulus F-tr F-tr/M-tr

4 types of mtDNA
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Hybrids

• Genes Male x
  Female
• Nuclear t/t
  e/e
• mtDNA F-tr, M-tr
  F-ed
• F1 Hybrid
• Nuclear e/t
  e/t
• mtDNA F-ed, M-tr F-ed

Heterospecific mtDNA
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mtDNA
Pure Species

• M. edulis M.
  trossulus
• female male female
  male
• F-ed 56 - -
  -
• F-ed/M-ed - 69 -
  -
• F-tr - -
  69 -
• F-tr/M-tr - - -
  87

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mtDNA in Hybrids

• F1 Males (nuclear e/t e/t e/t
  e/t)
• F-ed/M-tr 2
• F-tr/M-ed 3
• TOTAL 5

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mtDNA in Male Backcross Hybrids
Expected
Observed Homospecific 22
39 Heterospecific 22
5 Total 44
44 X2 26.3,
df 1, p lt 0.001
Homospecific M and F mtDNA from same species (e
e t t) Heterospecific M and F mtDNA from
different species (e t t e)
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mtDNA in Female Backcross Hybrids

• Nuclear Genes
• Bc ed Bc tr
  
• F-ed 14 (12.5) 0 (12.5)
  obs (exp)
• F-tr 0 (12.5) 36 (12.5)
• X2 8.33, df 1, p lt 0.005

mtDNA
No individuals with a discordance between mtDNA
and majority of their nuclear genes
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Mussel Hybridization

• mtDNA introgression blocked
• - nuclear-cytoplasmic genetic
• incompatibility ?
• Frequency of hybrids 25 for gt 15 mm
• What factors involved ?

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Early Life History
Glu ITS
(mm)
0.214 8.1
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F1
0.214 mm
Glu ITS 1 e/e 2 t/t 3 e/t 9
genotypes
Percent
8.1 mm
Genotype
tr
ed
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Frequency of Hybrids due to

• Prezygotic gamete incompatibility/sperm
• choice?
• Postzygotic genetic incompatibility during
  early
• embryonic development ?

Laboratory crosses E x E, E x T, T x E, T x T
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Laboratory Experiments
Proportion of eggs fertilized
Fert. 0.5 Surv. 0.5 Proportion of hybrid larvae
Expected 0.25 Observed 0.25
Larval survival to day 10 ()
M. Miranda, PhD
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Summary

• Mussel Hybrid Zone
• - bimodal due to a combination of prezygotic
• barriers (fertilization) and postzygotic
  genetic
• incompatibility
• Hybrid Zone model
• - tension zone?
• - evidence for reinforcement?
• - evolution of barriers to interspecific
  fertilization?