No evidence for assortative mating within a willow warbler migratory divide

Area of Science:

• Evolutionary biology
• Ornithology
• Population genetics

Background:

• Hybrid zones can be maintained by prezygotic or postzygotic barriers.
• A hybrid zone exists between two willow warbler subspecies (Phylloscopus trochilus trochilus and P. t. acredula) in Sweden, differing in migratory strategies.
• While neutral genetic markers are homogeneous, phenotypic traits and specific genetic markers show clines, suggesting ongoing divergence.

Purpose of the Study:

• To investigate if assortative mating between migratory phenotypes maintains the genetic and migratory divide in willow warblers.
• To assess the role of pre-mating isolation in preventing gene flow across the hybrid zone.

Main Methods:

• Color-ringing and observation of a willow warbler breeding population in the hybrid zone.
• Assessment of phenotypic traits (plumage, morphology) and feather stable nitrogen isotopes (δ15N) in social pairs.
• Analysis of subspecies-specific genetic markers to identify hybrids and backcrosses.

Main Results:

• Male arrival time to breeding grounds was influenced by wintering area and genetic ancestry, suggesting potential for temporal assortment.
• No evidence of assortative mating was found based on plumage color, morphology, or feather stable nitrogen isotopes between social mates.
• Genetic analyses revealed a high proportion of hybrids and backcrosses, indicating significant gene flow.

Conclusions:

• Pre-mating isolation between willow warbler subspecies is weak.
• Extensive hybridization occurs across the migratory divide, challenging the hypothesis that assortative mating maintains subspecies separation.
• The migratory divide is not maintained by assortative mating in this hybrid zone.