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

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Presentation on theme: "The population genetics of hybridization Jonathan Degner 20 November, 2014."— Presentation transcript:

1 The population genetics of hybridization Jonathan Degner jcdegner@alumni.ubc.ca 20 November, 2014

2 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

3 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

4 What is hybridization? Jonathan DegnerPopulation genetics of hybridization4 Grammostola rosea Tamias striatus x Tamiastola horrifadorablis

5 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 (F 1 ) or advanced-generation (F n ) Jonathan DegnerPopulation genetics of hybridization5

6 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

7 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

8 What is hybridization? Secondary contact Jonathan DegnerPopulation genetics of hybridization8 Time Temperature

9 What is hybridization? Secondary contact Jonathan DegnerPopulation genetics of hybridization9 Ancestral population Isolation DivergenceSecondary contact

10 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

11 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

12 Hybridization at a single locus Jonathan DegnerPopulation genetics of hybridization12 Single locus

13 Hybridization at a single locus For first-generation (F 1 ) 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 ParentAA 1 Aa 1 aa1AA 2 Aa 2 aa 2 AA 1 00.51 Aa 1 0.5 aa 1 10.50 AA 2 00.51 Aa 2 0.5 aa 2 10.50

14 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 heterozygosity p1p1 p2p2

15 Hybridization at a single locus Jonathan DegnerPopulation genetics of hybridization15 F 1 heterozygosity p1p1 p2p2

16 Hybridization at multiple loci Jonathan DegnerPopulation genetics of hybridization16 Quantitative traits

17 Hybridization at multiple loci Jonathan DegnerPopulation genetics of hybridization17 Quantitative traits Trait value Frequency Trait value Parent populationsOffspring population

18 Hybridization at multiple loci Jonathan DegnerPopulation genetics of hybridization18 Quantitative traits Brennan et al. 2009 Parent populationsOffspring population Low variance High variance

19 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 loci Epistasis

20 Dobzhansky-Müller-Bateson Incompatabilities Jonathan DegnerPopulation genetics of hybridization20 Hybridization at multiple loci Epistasis Isolation Neutral mutation at separate loci Secondary contact Ancestral genotype

21 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 loci Epistasis Bomblies et al. 2007

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 DegnerPopulation genetics of hybridization22

23 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

24 Hybridization and fitness Jonathan DegnerPopulation genetics of hybridization24 Additive effects

25 Hybridization and fitness Jonathan DegnerPopulation genetics of hybridization25 Additive effects Wang et al. 1997

26 Hybridization and fitness Jonathan DegnerPopulation genetics of hybridization26 Additive effects De La Torre et al. 2014

27 Hybridization and fitness Jonathan DegnerPopulation genetics of hybridization27 Additive effects

28 Hybridization and fitness Jonathan DegnerPopulation genetics of hybridization28 Additive effects Rushton 1978 Pollen viability Species identity Hybrids

29 Hybridization and fitness Jonathan DegnerPopulation genetics of hybridization29 Additive effects

30 Hybridization and fitness Jonathan DegnerPopulation genetics of hybridization30 Parental sizesOffspring 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

31 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

32 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 DegnerPopulation genetics of hybridization32

33 Thank you for listening!


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