Upload
kylan-merriman
View
216
Download
0
Tags:
Embed Size (px)
Citation preview
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 DegnerPopulation genetics of
hybridization2
What is hybridization?
“The crossing of individuals belonging to two unlike natural populations that have secondarily
come into contact”-Ernst Mayr, 1970
Jonathan DegnerPopulation genetics of
hybridization3
What is hybridization?
Jonathan DegnerPopulation genetics of
hybridization4
Grammostola rosea Tamias striatus
x Tamiastola horrifadorablis
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 DegnerPopulation genetics of
hybridization5
What is hybridization?
“The crossing of individuals belonging to two unlike natural populations that have secondarily
come into contact”-Ernst Mayr, 1970
Jonathan DegnerPopulation genetics of
hybridization6
What is hybridization?
“The crossing of individuals belonging to two unlike natural populations that have secondarily
come into contact”-Ernst Mayr, 1970
Jonathan DegnerPopulation genetics of
hybridization7
What is hybridization?• Secondary contact
Jonathan DegnerPopulation genetics of
hybridization8
Time
Tem
pera
ture
What is hybridization?• Secondary contact
Jonathan DegnerPopulation genetics of
hybridization9
Ancestral population Isolation Divergence Secondary contact
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 DegnerPopulation genetics of
hybridization10
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 DegnerPopulation genetics of
hybridization11
Hybridization at a single locus
Jonathan DegnerPopulation genetics of
hybridization12
Single locus
Hybridization at a 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
Jonathan DegnerPopulation genetics of
hybridization13
Single locus
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
Hybridization at a 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 DegnerPopulation genetics of
hybridization14
Single locus
F 1 h
eter
ozyg
osity
p1
p 2
Hybridization at a single locus
Jonathan DegnerPopulation genetics of
hybridization15
F 1 h
eter
ozyg
osity
p1
p 2
Hybridization at multiple loci
Jonathan DegnerPopulation genetics of
hybridization16
Quantitative traits
Hybridization at multiple loci
Jonathan DegnerPopulation genetics of
hybridization17
Quantitative traits
Trait value
Freq
uenc
y
Freq
uenc
yTrait value
Parent populations Offspring population
Hybridization at multiple loci
Jonathan DegnerPopulation genetics of
hybridization18
Quantitative traits
Brennan et al. 2009
Parent populations Offspring population
Low variance High variance
• 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 DegnerPopulation genetics of
hybridization19
Hybridization at multiple lociEpistasis
• Dobzhansky-Müller-Bateson Incompatabilities
Jonathan DegnerPopulation genetics of
hybridization20
Hybridization at multiple lociEpistasis
IsolationNeutral mutation at separate loci
Secondary contactAncestral genotype
• Can result in hybrid sterility or low fitness
• Thought to be responsible for many speciation events• Orr and Turelli 2001
Jonathan DegnerPopulation genetics of
hybridization21
Hybridization at multiple lociEpistasis
Bomblies et al. 2007
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 DegnerPopulation genetics of
hybridization22
Hybridization and fitness
Jonathan DegnerPopulation genetics of
hybridization23
Additive effects
Keim et al. 1989
• Hybrids are phenotypically intermediate between parents
• Most common outcome of hybridization due to the large number of genes typically involved in quantitative traits
Hybridization and fitness
Jonathan DegnerPopulation genetics of
hybridization24
Additive effects
• In the habitat of parent population 1, we expect
parent1 > hybrid > parent2
• In the hybrid habitat, one of two scenarios can occur
parent1 < hybrid > parent2
parent1 > hybrid < parent2
Hybridization and fitness
Jonathan DegnerPopulation genetics of
hybridization25
Hybrid superiority
parent1 < hybrid > parent2
• Common outcome of hybridization between populations with low to moderate divergence• Not enough time for
high levels of reproductive isolation to form
• Usually environment-dependent
Additive effects
Wang et al. 1997
Hybridization and fitness
Jonathan DegnerPopulation genetics of
hybridization26
Hybrid superiority
parent1 < hybrid > 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”
Additive effects
De La Torre et al. 2014
Hybridization and fitness
Jonathan DegnerPopulation genetics of
hybridization27
Hybrid superiority
parent1 < hybrid > parent2
• 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.
Additive effects
Hybridization and fitness
Jonathan DegnerPopulation genetics of
hybridization28
Additive effects
Rushton 1978Po
llen
viab
ility
Species identity
Hybrids
Hybrid inferiority
parent1 > hybrid < parent2
• Common outcome of hybridization between populations with high divergence
• Usually environment-independent
Hybridization and fitness
Jonathan DegnerPopulation genetics of
hybridization29
Additive effects
Hybrid inferiority
parent1 > hybrid < 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
Hybridization and fitness
Jonathan DegnerPopulation genetics of
hybridization30
Parental sizes Offspring sizes
Facon et al. 2005
Transgressive effects
• Phenotype in hybrids is non-additive between parents
• Hybrids more likely to have higher or lower fitness than either parent
• May allow colonization of new habitats unavailable to either parent
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 DegnerPopulation genetics of
hybridization31
ReferencesBomblies, 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 DegnerPopulation genetics of
hybridization32
Thank you for listening!