Download Document

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project

Document related concepts

Heritability of IQ wikipedia , lookup

Human genetic variation wikipedia , lookup

Genetic drift wikipedia , lookup

Molecular Inversion Probe wikipedia , lookup

Quantitative trait locus wikipedia , lookup

Behavioural genetics wikipedia , lookup

Polymorphism (biology) wikipedia , lookup

Epistasis wikipedia , lookup

Polyploid wikipedia , lookup

Koinophilia wikipedia , lookup

Human–animal hybrid wikipedia , lookup

Medical genetics wikipedia , lookup

Population genetics wikipedia , lookup

Microevolution wikipedia , lookup

Hybrid (biology) wikipedia , lookup

Transcript
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
Jonathan Degner
Population genetics of hybridization
2
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
3
What is hybridization?
Grammostola rosea
Tamias striatus
x Tamiastola horrifadorablis
Jonathan Degner
Population genetics of hybridization
4
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
5
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
6
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
7
What is hybridization?
Temperature
• Secondary contact
Time
Jonathan Degner
Population genetics of hybridization
8
What is hybridization?
• Secondary contact
Ancestral population
Jonathan Degner
Isolation
Divergence
Population genetics of hybridization
Secondary contact
9
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
Jonathan Degner
Population genetics of hybridization
10
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
Jonathan Degner
Population genetics of hybridization
11
Hybridization at a single locus
Single locus
Jonathan Degner
Population genetics of hybridization
12
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
Jonathan Degner
Population genetics of hybridization
13
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
Jonathan Degner
Population genetics of hybridization
14
Hybridization at a single locus
Jonathan Degner
Population genetics of hybridization
15
Hybridization at multiple loci
Quantitative traits
Jonathan Degner
Population genetics of hybridization
16
Hybridization at multiple loci
Quantitative traits
Frequency
Trait value
Jonathan Degner
Offspring population
Frequency
Parent populations
Trait value
Population genetics of hybridization
17
Hybridization at multiple loci
Quantitative traits
Parent populations
Low variance
Offspring population
High variance
Brennan et al. 2009
Jonathan Degner
Population genetics of hybridization
18
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
Jonathan Degner
Population genetics of hybridization
19
Hybridization at multiple loci
Epistasis
• Dobzhansky-Müller-Bateson Incompatabilities
Isolation
Neutral mutation
at separate loci
Secondary contact
Ancestral genotype
Jonathan Degner
Population genetics of hybridization
20
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
Jonathan Degner
Population genetics of hybridization
21
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
Jonathan Degner
Population genetics of hybridization
22
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
Jonathan Degner
Population genetics of hybridization
23
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
Jonathan Degner
Population genetics of hybridization
24
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
Jonathan Degner
Wang et al. 1997
Population genetics of hybridization
25
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
Jonathan Degner
Population genetics of hybridization
26
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.
Jonathan Degner
Population genetics of hybridization
27
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
Jonathan Degner
Hybrids
Species identity
Population genetics of hybridization
Rushton 1978
28
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
Jonathan Degner
Population genetics of hybridization
29
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
Jonathan Degner
Population genetics of hybridization
30
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
Jonathan Degner
Population genetics of hybridization
31
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.
Jonathan Degner
Population genetics of hybridization
32
Thank you for listening!