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Open Access Highly Accessed Research article

Rapid fixation of non-native alleles revealed by genome-wide SNP analysis of hybrid tiger salamanders

Benjamin M Fitzpatrick1*, Jarrett R Johnson2, D Kevin Kump3, H Bradley Shaffer2, Jeramiah J Smith34 and S Randal Voss3

Author Affiliations

1 Ecology & Evolutionary Biology, University of Tennessee, Knoxville, TN 37996, USA

2 Department of Evolution and Ecology and Center for Population Biology, University of California, Davis, CA 95616, USA

3 Department of Biology, University of Kentucky, Lexington, KY 40506, USA

4 Benaroya Research Institute at Virginia Mason & Department of Genome Sciences, University of Washington, Seattle, WA 98101, USA

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BMC Evolutionary Biology 2009, 9:176  doi:10.1186/1471-2148-9-176

Published: 24 July 2009

Abstract

Background

Hybrid zones represent valuable opportunities to observe evolution in systems that are unusually dynamic and where the potential for the origin of novelty and rapid adaptation co-occur with the potential for dysfunction. Recently initiated hybrid zones are particularly exciting evolutionary experiments because ongoing natural selection on novel genetic combinations can be studied in ecological time. Moreover, when hybrid zones involve native and introduced species, complex genetic patterns present important challenges for conservation policy. To assess variation of admixture dynamics, we scored a large panel of markers in five wild hybrid populations formed when Barred Tiger Salamanders were introduced into the range of California Tiger Salamanders.

Results

At three of 64 markers, introduced alleles have largely displaced native alleles within the hybrid populations. Another marker (GNAT1) showed consistent heterozygote deficits in the wild, and this marker was associated with embryonic mortality in laboratory F2's. Other deviations from equilibrium expectations were idiosyncratic among breeding ponds, consistent with highly stochastic demographic effects.

Conclusion

While most markers retain native and introduced alleles in expected proportions, strong selection appears to be eliminating native alleles at a smaller set of loci. Such rapid fixation of alleles is detectable only in recently formed hybrid zones, though it might be representative of dynamics that frequently occur in nature. These results underscore the variable and mosaic nature of hybrid genomes and illustrate the potency of recombination and selection in promoting variable, and often unpredictable genetic outcomes. Introgression of a few, strongly selected introduced alleles should not necessarily affect the conservation status of California Tiger Salamanders, but suggests that genetically pure populations of this endangered species will be difficult to maintain.