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The population genetics of hybridization

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Presentation on theme: "The population genetics of hybridization"— Presentation transcript:

1 The population genetics of hybridization
Jonathan Degner 20 November, 2014

2 Population genetics of hybridization
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

3 Population genetics of hybridization
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

4 Population genetics of hybridization
What is hybridization? Grammostola rosea Tamias striatus x Tamiastola horrifadorablis Jonathan Degner Population genetics of hybridization

5 Population genetics of hybridization
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

6 Population genetics of hybridization
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 Population genetics of hybridization
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

8 Population genetics of hybridization
What is hybridization? Secondary contact Time Temperature Jonathan Degner Population genetics of hybridization

9 Population genetics of hybridization
What is hybridization? Secondary contact Ancestral population Isolation Divergence Secondary contact Jonathan Degner Population genetics of hybridization

10 Population genetics of hybridization
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

11 Population genetics of hybridization
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

12 Hybridization at a single locus
Jonathan Degner Population genetics of hybridization

13 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 Parent AA1 Aa1 aa1 AA2 Aa2 aa2 0.5 1 Jonathan Degner Population genetics of hybridization

14 Hybridization at a single locus
F1 heterozygosity p1 p2 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

15 Hybridization at a single locus
F1 heterozygosity p1 p2 Jonathan Degner Population genetics of hybridization

16 Hybridization at multiple loci
Quantitative traits Jonathan Degner Population genetics of hybridization

17 Hybridization at multiple loci
Quantitative traits Frequency Parent populations Frequency Offspring population Trait value Trait value Jonathan Degner Population genetics of hybridization

18 Hybridization at multiple loci
Quantitative traits Parent populations Offspring population Low variance High variance Brennan et al. 2009 Jonathan Degner Population genetics of hybridization

19 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

20 Hybridization at multiple loci
Epistasis Dobzhansky-Müller-Bateson Incompatabilities Neutral mutation at separate loci Isolation Ancestral genotype Secondary contact Jonathan Degner Population genetics of hybridization

21 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

22 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

23 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

24 Hybridization and fitness
Additive effects In the habitat of parent population 1, we expect 𝑤 parent1 > 𝑤 hybrid > 𝑤 parent2 In the hybrid habitat, one of two scenarios can occur 𝑤 parent1 < 𝑤 hybrid > 𝑤 parent2 𝑤 parent1 > 𝑤 hybrid < 𝑤 parent2 Jonathan Degner Population genetics of hybridization

25 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 environment- dependent Wang et al. 1997 Jonathan Degner Population genetics of hybridization

26 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

27 Hybridization and fitness
Additive effects Hybrid superiority 𝑤 parent1 < 𝑤 hybrid > 𝑤 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. Jonathan Degner Population genetics of hybridization

28 Hybridization and fitness
Additive effects Hybrid inferiority 𝑤 parent1 > 𝑤 hybrid < 𝑤 parent2 Common outcome of hybridization between populations with high divergence Usually environment- independent Pollen viability Hybrids Species identity Rushton 1978 Jonathan Degner Population genetics of hybridization

29 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

30 Hybridization and fitness
Transgressive effects Parental sizes Offspring sizes Facon et al. 2005 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 Jonathan Degner Population genetics of hybridization

31 Population genetics of hybridization
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

32 Population genetics of hybridization
References Bomblies, K., Lempe, J., Epple, P., Warthmann, N., Lanz, C., Dangl, J., and Weigel, D 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 Adaptation and selection in the Senecio (Asteraceae) hybrid zone on Mount Etna, Sicily. New Phytologist 183(3): De La Torre, A., Wang, T., Jaquish, B. and Aitken, S Adaptation and exogenous selection in a Picea glauca x Picea engelmannii hybrid zone: Implications for forest management under climate change. New Phytologist 201(2): Facon, B., Jarne, P., Pointier, J., and David, P 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): Keim, P., Paige, K., Whitham, T., and Lark, K Genetic analysis of an interspecific hybrid swarm of Populus: Occurrence of unidirectional introgression. Genetics 123: Mayr, E Populations, Species, and Evolution: An Abridgement to “Animal Species and Evolution”. Harvard University Press. Orr, H. and Turelli, M The evolution of postzygotic isolation: Accumulating Dobzhansky-Muller incompatibilities. Evolution 55(6): Rushton, B Quercus robur L. and Quercus petraea (Matt) Liebl: A multivariate approach to the hybrid problem. 1. Data acquisition, analysis and interpretation. Watsonia 21: Wang, H., McArthur, E., Sanderson, S., Graham, J., and Freeman, D Narrow hybrid zone between two subspecies of big sagebrush (Artemesia tridentata: Asteraceae). IV. Reciprocal transplant experiments. Evolution 51(1): Jonathan Degner Population genetics of hybridization

33 Thank you for listening!


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