title: No evidence for assortative mating within a willow warbler migratory divide.
abstract: BACKGROUND
  In contact zones, genetic mixing of two taxa can be restricted by prezygotic (e.g. assortative mating) or postzygotic (lower fitness of hybrid offspring) barriers, or a combination of the two. A hybrid zone between two willow warbler subspecies (Phylloscopus trochilus trochilus, P. t. acredula) with distinctive migratory strategies occurs in central Sweden. These subspecies exhibit differences in migratory direction and distance, resulting in geographically distinct wintering areas in Africa. The subspecies may have diverged from a common refuge after the last ice age, and neutral genetic markers are homogeneous across their range. By contrast, several phenotypic traits and genetic markers of two chromosomal regions previously identified show steep clines across the divide. The evolutionary forces that maintain this migratory divide remain unknown. Here we use plumage colour, morphology, genetic markers and feather stable nitrogen-isotopes (δ (15)N) to assess if assortative mating between migratory phenotypes could be acting as a possible mechanism for keeping the two forms genetically separate and maintaining the migratory divide. We colour-ringed a willow warbler breeding population in the central part of the hybrid zone and observed the breeding population to assess phenotypic and genotypic traits of social pairs.
  
  
  RESULTS
  Our data suggest that wintering area and genetic ancestry had an effect on male arrival time to the breeding grounds which could contribute to assortment. However, evidence for assortative mating could not be detected based on a comparison of plumage colour, morphology and δ (15)N between social mates.
  
  
  CONCLUSIONS
  This finding was strengthened by analyses of subspecies-specific genetic markers, which allowed us to identify the presence of a large proportion of potential hybrids and backcrosses at the study site. Our results supported the hypothesis that pre-mating isolation in willow warblers is weak, resulting in extensive hybridisation across the migratory divide.
authors: Center for Animal Movement Research (CAnMove), Department of Biology, Lund University, Lund, 22362, Sweden ; Wildlife Ecology and Management, Freiburg University, Tennenbacher Str. 4, Freiburg, 79106, Germany.
authors: Miriam
authors: M
authors: Liedvogel
authors: Center for Animal Movement Research (CAnMove), Department of Biology, Lund University, Lund, 22362, Sweden.
authors: Keith W
authors: KW
authors: Larson
authors: Center for Animal Movement Research (CAnMove), Department of Biology, Lund University, Lund, 22362, Sweden.
authors: Max
authors: M
authors: Lundberg
authors: Center for Animal Movement Research (CAnMove), Department of Biology, Lund University, Lund, 22362, Sweden ; Department of Biology, Faculty of Arts and Sciences, University of Ondokuz Mayis, Samsun, Turkey.
authors: Arzu
authors: A
authors: Gursoy
authors: Environment Canada, 11 Innovation Blvd., Saskatoon S7N 3H5, SK, Canada.
authors: Leonard I
authors: LI
authors: Wassenaar
authors: Environment Canada, 11 Innovation Blvd., Saskatoon S7N 3H5, SK, Canada.
authors: Keith A
authors: KA
authors: Hobson
authors: Center for Animal Movement Research (CAnMove), Department of Biology, Lund University, Lund, 22362, Sweden.
authors: Staffan
authors: S
authors: Bensch
authors: Center for Animal Movement Research (CAnMove), Department of Biology, Lund University, Lund, 22362, Sweden.
authors: Susanne
authors: S
authors: Akesson
keywords: Hybrid zone; Nitrogen-15; Phylloscopus trochilus; Postzygotic selection; Prezygotic selection; Reproductive isolation; Willow warbler