Hybridization Avise Ch. 7

1
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

2
Bimodal Hybrid Zones

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

3
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

4
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

5
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

6
Hyla cinerea
Hyla gratiosa
7
Example Hybridization

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

8
Example Hybridization

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

9
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
10
Avise, 2001
11
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)

12
Secondary Contact
Mate Choice and Hybrid Fitness 1. Maintained as
separate entities (species) 2. Fusion (single
species) Function of - error in
mate choice -
hybrid fitness
13
High
Error in Mate Choice
Low
Low
High
Fitness of Hybrid
14
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
15
cinerea
Reproductive Character Displacement
Allopatric
A
gratiosa
S
Sympatric
Dr. Stephen A. Karl Department of
BiologyUniversity of South Florida
http//chuma.cas.usf.edu/karl/evolution/chapter_1
2_2.htm
16
A mussel hybrid zone inEastern North America

• Genetic differentiation
• - Enzyme variation
• - mtDNA

17
Koehn et al. (1984)
II
III
I
Fst 0.006 (5 loci)
III
II
18
Mytilus Species

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

19
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

20
Bates and Innes, 1995
Mt
Me
4 loci
21
Diagnostic Genetic Markers
Enzyme genes Est Mpi
DNA

• Glu-5
• ITS
• mtDNA

Hy Tr Ed Ed
22
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

23
Doubly Uniparental Inheritance
mtDNA
X
Males
Females
M
F
Females
Males
homoplasmic
heteroplasmic
24
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
25
F1
26
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

27
Mussel Hybridization

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

28
Early Life History
Glu ITS
(mm)
0.214 8.1
29
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
30
Laboratory Experiments
Proportion of eggs fertilized
Fert. 0.5 Surv. 0.5 Proportion of hybrid larvae
expected 0.25
Larval survival to day 10 ()
M. Miranda, PhD
31
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
32
Experimental Hybridization
Scarlet Gilia
33
Louisiana IrisesNatural Hybridization
34
Iris hexagona
Iris fulvia
Iris brevicaulis
Louisiana Irises
35
Genetic markers

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

36
I. fulva I. brevicaulis

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

Nuclear and cpDNA
37

• Environmental Variables
• Elevation
• Light
• Veg. Comp.

38
Hybridization in Plants
Uncoupling of male and female components to gene
flow Pollen nuclear genes
Seed nuclear genes cpDNA
39
Iris Hybrid Zone
fulva hexagona
fulva hexagona
Nuclear Markers (pollen flow)
cpDNA (seed dispersal)
40
Hybrid Speciation in Plants
Hybrids between species if
fertile ? introgression if sterile ?
allopolyploid Reproductively isolated from parent
species
41
Hybrid Speciation (introgression)
F1
Time
42
Iris hexagona
Iris fulvia
Iris brevicaulis
Iris nelsonii
43
Hybrid Speciation in Irises(introgression)
Example Iris fulvia (Saltwater) Iris
hexagona (Freshwater) Iris brevicaulis (Pasture)
Iris nelsonii (Swamp, Ecotone)
44
Hybrid populations Iris nelsonii
Lack of foreign markers in I. nelsonii (genetic
stability)
45
Hybrid Speciation in Plants
Example Clarkia spp. (Reticulate
evolution)
46


allopolyploids



47
(No Transcript)
48
Sunflower Reticulate evolution
49
Evolutionary Significance of Hybridization

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

50
Evolutionary Significance of Hybridization

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

51
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

52
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

53
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

54
Speciation

• Bimodal Hybrid Zones
• Likelihood of reinforcement greatly enhanced
• But bimodal hybrid zones may have evolved
• through reinforcement

55
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