Genomic variation in cline shape across a hybrid zone

Authors

  • Sarah E. Kingston,

    Corresponding author
    1. Vertebrate Zoology, Smithsonian Institution, National Museum of Natural History, Washington, District of Columbia, USA
    • Program in Behavior, Ecology, Evolution, and Systematics, University of Maryland, College Park, Maryland, USA
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  • Robert W. Jernigan,

    1. Department of Mathematics and Statistics, American University, Washington, District of Columbia, USA
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  • William F. Fagan,

    1. Department of Biology, University of Maryland, College Park, Maryland, USA
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  • David Braun,

    1. Plateau Land and Wildlife Management, Dripping Springs, Texas, USA
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  • Michael J. Braun

    1. Program in Behavior, Ecology, Evolution, and Systematics, University of Maryland, College Park, Maryland, USA
    2. Vertebrate Zoology, Smithsonian Institution, National Museum of Natural History, Washington, District of Columbia, USA
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  • Funding provided by University of Maryland Program in Behavior, Ecology, Evolution, and Systematics, Smithsonian Ornithology, and Smithsonian Institution Science Endowments Program. NSF grants DEB0228675 and DEB0733029 and the NMNH Frontiers in Phylogenetics Program provided research assistantships.

Correspondence

Sarah E Kingston, Smithsonian Institution, National Museum of Natural History, Vertebrate Zoology, MRC 534, 4210 Silver Hill Rd., Suitland 20746, Maryland USA. Tel: +(301) 238-1103; +(301) 238-3059 E-mail: kingstons@si.edu

Abstract

Hybrid zones are unique biological interfaces that reveal both population level and species level evolutionary processes. A genome-scale approach to assess gene flow across hybrid zones is vital, and now possible. In Mexican towhees (genus Pipilo), several morphological hybrid gradients exist. We completed a genome survey across one such gradient (9 populations, 140 birds) using mitochondrial DNA, 28 isozyme, and 377 AFLP markers. To assess variation in introgression among loci, cline parameters (i.e., width, center) for the 61 clinally varying loci were estimated and compiled into genomic distributions for tests against three empirical models spanning the range of observed cline shape. No single model accounts for observed variation in cline shape among loci. Numerous backcross individuals near the gradient center confirm a hybrid origin for these populations, contrary to a previous hypothesis based on social mimicry and character displacement. In addition, the observed variation does not bin into well-defined categories of locus types (e.g., neutral vs. highly selected). Our multi-locus analysis reveals cross-genomic variation in selective constraints on gene flow and locus-specific flexibility in the permeability of the interspecies membrane.

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