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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 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!