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Free keywords:
hybrid zone; prezygotic selection; postzygotic selection; reproductive isolation; Willow warbler; Phylloscopus trochilus; nitrogen-15
Abstract:
Introduction: 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 (δ15N) 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 δ15N between social mates.
Conclusion: 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.
