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The population genetics of
hybridization
Jonathan Degner
[email protected]
20 November, 2014
Overview
• What is hybridization?
• Hybridization at a single locus
• Hybridization at multiple loci
• Quantitative traits
• Epistatis
• Hybridization and fitness
• Additive effects
• Transgressive effects
• Summary
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Population genetics of hybridization
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What is hybridization?
“The crossing of individuals belonging to two unlike natural
populations that have secondarily come into contact”
-Ernst Mayr, 1970
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Population genetics of hybridization
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What is hybridization?
Grammostola rosea
Tamias striatus
x Tamiastola horrifadorablis
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Population genetics of hybridization
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What is hybridization?
• Important in understanding many aspects of speciation
• Reproductive isolation
• Hybrid speciation
• Interspecific gene flow
• Hybridization may refer to one of several different processes.
• May refer to first generation (F1) or advanced-generation (Fn)
Jonathan Degner
Population genetics of hybridization
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What is hybridization?
“The crossing of individuals belonging to two unlike natural
populations that have secondarily come into contact”
-Ernst Mayr, 1970
Jonathan Degner
Population genetics of hybridization
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What is hybridization?
“The crossing of individuals belonging to two unlike natural
populations that have secondarily come into contact”
-Ernst Mayr, 1970
Jonathan Degner
Population genetics of hybridization
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What is hybridization?
Temperature
• Secondary contact
Time
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Population genetics of hybridization
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What is hybridization?
• Secondary contact
Ancestral population
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Isolation
Divergence
Population genetics of hybridization
Secondary contact
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What is hybridization?
• Intraspecific hybridization
•
•
•
•
Gene flow between genetically distinct populations
Increases heterozygosity
Natural hybrids generally show intermediate phenotypes
Artificial hybrids may show transgressive phenotypes (e.g. maize)
• Interspecific hybridization
• Gene flow between diverged species
• Increases heterozygosity and can generate new polymorphisms
• Hybrids may show intermediate, transgressive, or novel phenotypes
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Population genetics of hybridization
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What is hybridization?
• Homoploid hybridization
• Does not result in a change in ploidy (e.g. 2N to 4N)
• Generally less phenotypically pronounced than polyploidy hybridization
• Hybrids may be infertile or unfit due to differing chromosome numbers between
parents or epistatic interactions
• Polyploid hybridization
• Ploidy in hybrids is higher than parents
• Caused by fusion of non-haploid gametes
• Hybrids may be infertile or unfit due to uneven ploidy or unusual allelic effects
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Population genetics of hybridization
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Hybridization at a single locus
Single locus
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Population genetics of hybridization
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Hybridization at a single locus
Single locus
• For first-generation (F1) hybrids, genotype frequencies do not occur in
Hardy-Weinberg equilibrium
• If we are considering only hybrids, we are observing non-random mating i.e. matings
within populations are not being considered
Parent
AA1
Aa1
aa1
AA2
Aa2
aa2
AA1
0
0.5
1
Aa1
0.5
0.5
0.5
aa1
1
0.5
0
AA2
0
0.5
1
Aa2
0.5
0.5
0.5
aa2
1
0.5
0
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Hybridization at a single locus
Single locus
• If allele frequencies favor different
alleles in two populations, hybrids
will have “excess” heterozygosity
i.e. > 0.5
• Taken to an extreme, populations
with fixed differences will create
fully heterozygous hybrids
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Population genetics of hybridization
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Hybridization at a single locus
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Population genetics of hybridization
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Hybridization at multiple loci
Quantitative traits
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Population genetics of hybridization
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Hybridization at multiple loci
Quantitative traits
Frequency
Trait value
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Offspring population
Frequency
Parent populations
Trait value
Population genetics of hybridization
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Hybridization at multiple loci
Quantitative traits
Parent populations
Low variance
Offspring population
High variance
Brennan et al. 2009
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Population genetics of hybridization
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Hybridization at multiple loci
Epistasis
• Hybridization may cause combinations of alleles across loci that have never
been tested by selection, and may be deleterious.
• Dobzhansky-Müller-Bateson incompatibilities
• Alleles that are co-adapted for local conditions can be broken up by gene flow and
recombination
• Outbreeding depression
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Population genetics of hybridization
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Hybridization at multiple loci
Epistasis
• Dobzhansky-Müller-Bateson Incompatabilities
Isolation
Neutral mutation
at separate loci
Secondary contact
Ancestral genotype
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Hybridization at multiple loci
Epistasis
• Can result in hybrid sterility or low fitness
• Thought to be responsible for many
speciation events
• Orr and Turelli 2001
Bomblies et al. 2007
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Hybridization and fitness
• Increased heterozygosity emphasizes selection on heterozygous
genotypes over the short term
• Additive phenotypes may be more fit in intermediate habitats (hybrid
superiority) or universally less-fit (hybrid inferiority)
• Hybrids may exhibit transgressive phenotypes
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Hybridization and fitness
Additive effects
• Hybrids are phenotypically
intermediate between parents
• Most common outcome of
hybridization due to the large
number of genes typically
involved in quantitative traits
Keim et al. 1989
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Population genetics of hybridization
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Hybridization and fitness
Additive effects
• In the habitat of parent population 1, we expect
𝑤parent1 > 𝑤hybrid > 𝑤parent2
• In the hybrid habitat, one of two scenarios can occur
𝑤parent1 < 𝑤hybrid > 𝑤parent2
𝑤parent1 > 𝑤hybrid < 𝑤parent2
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Hybridization and fitness
Additive effects
Hybrid superiority
𝑤 parent1 < 𝑤 hybrid > 𝑤 parent2
• Common outcome of
hybridization between
populations with low to
moderate divergence
• Not enough time for high
levels of reproductive
isolation to form
•
Usually environmentdependent
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Wang et al. 1997
Population genetics of hybridization
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Hybridization and fitness
Additive effects
Hybrid superiority
𝑤 parent1 < 𝑤 hybrid > 𝑤 parent2
• Can result in the
formation of stable,
extensive hybrid zones
• If hybrids are more fit
over a large area, can
result in the formation
of “hybrid swarms”
De La Torre et al. 2014
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Hybridization and fitness
Additive effects
Hybrid superiority
𝑤parent1 < 𝑤hybrid > 𝑤parent2
• Allows species to colonize habitats that would otherwise be
unavailable to them
• If hybrid populations become physically or reproductively isolated
from parent populations, they may form a new species.
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Hybridization and fitness
Hybrid inferiority
𝑤 parent1 > 𝑤 hybrid < 𝑤 parent2
• Common outcome of
hybridization between
populations with high
divergence
Pollen viability
Additive effects
• Usually environmentindependent
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Hybrids
Species identity
Population genetics of hybridization
Rushton 1978
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Hybridization and fitness
Additive effects
Hybrid inferiority
𝑤 parent1 > 𝑤 hybrid < 𝑤 parent2
• Typically caused by epistatic interactions or the loss of phenotypes
important for survival e.g. disease resistance
• Stable hybrid zones can still occur at an equilibrium between gene
flow promoting hybridization and selection against it
• Stable hybrid zones often appear as narrow bands between two species’
range margins
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Hybridization and fitness
Transgressive effects
Parental sizes
•
Phenotype in hybrids is nonadditive between parents
•
Hybrids more likely to have
higher or lower fitness than
either parent
•
May allow colonization of new
habitats unavailable to either
parent
Offspring sizes
Facon et al. 2005
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Summary
• Hybridization has profound effects at allelic and
phenotypic levels
• Excess heterozygosity
• New polymorphism
• Increased genetic variance
• The overall outcome of hybridization on fitness is
complex, difficult to predict, and often
environment-specific
• Additive fitness effects
• Hybrid superiority/inferiority
• Transgressive effects
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References
Bomblies, K., Lempe, J., Epple, P., Warthmann, N., Lanz, C., Dangl, J., and Weigel, D. 2007. Autoimmune response as a
mechanism for a Dobzhansky-Muller-type incompatibility syndrome in plants. PloS Biology 5(9): e236.
Brennan, A., Bridle, J., Wang, A., Hiscock, S., and Abbott, R. 2009. Adaptation and selection in the Senecio (Asteraceae) hybrid
zone on Mount Etna, Sicily. New Phytologist 183(3): 702-717.
De La Torre, A., Wang, T., Jaquish, B. and Aitken, S. 2014. Adaptation and exogenous selection in a Picea glauca x Picea
engelmannii hybrid zone: Implications for forest management under climate change. New Phytologist 201(2): 687-699.
Facon, B., Jarne, P., Pointier, J., and David, P. 2005. Hybridization and invasiveness in the freshwater snail Melanoides tubercula:
hybrid vigour is more important than increase in genetic variance. Journal of Evolutionary Biology 18(3): 524-535.
Keim, P., Paige, K., Whitham, T., and Lark, K. 1989. Genetic analysis of an interspecific hybrid swarm of Populus: Occurrence of
unidirectional introgression. Genetics 123: 557-565.
Mayr, E. 1970. Populations, Species, and Evolution: An Abridgement to “Animal Species and Evolution”. Harvard University Press.
Orr, H. and Turelli, M. 2001. The evolution of postzygotic isolation: Accumulating Dobzhansky-Muller incompatibilities. Evolution
55(6): 1085-1094.
Rushton, B. 1978. Quercus robur L. and Quercus petraea (Matt) Liebl: A multivariate approach to the hybrid problem. 1. Data
acquisition, analysis and interpretation. Watsonia 21: 81-101.
Wang, H., McArthur, E., Sanderson, S., Graham, J., and Freeman, D. 1997. Narrow hybrid zone between two subspecies of big
sagebrush (Artemesia tridentata: Asteraceae). IV. Reciprocal transplant experiments. Evolution 51(1): 95-102.
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