title: Differential introgression and the maintenance of species boundaries in an advanced generation avian hybrid zone.
abstract: BACKGROUND
  Evolutionary processes, including selection and differential fitness, shape the introgression of genetic material across a hybrid zone, resulting in the exchange of some genes but not others. Differential introgression of molecular or phenotypic markers can thus provide insight into factors contributing to reproductive isolation. We characterized patterns of genetic variation across a hybrid zone between two tidal marsh birds, Saltmarsh (Ammodramus caudacutus) and Nelson's (A. nelsoni) sparrows (n = 286), and compared patterns of introgression among multiple genetic markers and phenotypic traits.
  
  
  RESULTS
  Geographic and genomic cline analyses revealed variable patterns of introgression among marker types. Most markers exhibited gradual clines and indicated that introgression exceeds the spatial extent of the previously documented hybrid zone. We found steeper clines, indicating strong selection for loci associated with traits related to tidal marsh adaptations, including for a marker linked to a gene region associated with metabolic functions, including an osmotic regulatory pathway, as well as for a marker related to melanin-based pigmentation, supporting an adaptive role of darker plumage (salt marsh melanism) in tidal marshes. Narrow clines at mitochondrial and sex-linked markers also offer support for Haldane's rule. We detected patterns of asymmetrical introgression toward A. caudacutus, which may be driven by differences in mating strategy or differences in population density between the two species.
  
  
  CONCLUSIONS
  Our findings offer insight into the dynamics of a hybrid zone traversing a unique environmental gradient and provide evidence for a role of ecological divergence in the maintenance of pure species boundaries despite ongoing gene flow.
authors.affiliation: Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH, USA. jennifer.walsh.emond@gmail.com.|Cornell Lab of Ornithology, Cornell University, Ithaca, NY, USA. jennifer.walsh.emond@gmail.com.
authors.forename: Jennifer
authors.identifier: http://orcid.org/0000-0002-8146-7029
authors.initials: J
authors.lastname: Walsh
authors.affiliation: Department of Entomology and Wildlife Ecology, University of Delaware, Newark, DE, USA.
authors.forename: W Gregory
authors.initials: WG
authors.lastname: Shriver
authors.affiliation: School of Biology and Ecology, University of Maine, Orono, ME, USA.
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Olsen
authors.affiliation: Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH, USA.
authors.forename: Adrienne I
authors.initials: AI
authors.lastname: Kovach
mesh_terms: D000818:Animals; D005075:Biological Evolution; D005260:Female; D051456:Gene Flow; D014644:Genetic Variation; D006824:Hybridization, Genetic; D008297:Male; D018895:Microsatellite Repeats; D009511:New England; D046109:Passeriformes / Q000145:classification* / Q000235:genetics*; D010858:Pigmentation; D011156:Population Density; D060047:Reproductive Isolation; D053833:Wetlands
keywords: Ammodramus caudacutus; Ammodramus nelsoni; Genomic clines; Geographic clines; Haldane’s rule; Hybrid zones; Introgression