No evidence for assortative mating within a willow warbler migratory divide

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 (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.

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.

Keywords: Hybrid zone; Nitrogen-15; Phylloscopus trochilus; Postzygotic selection; Prezygotic selection; Reproductive isolation; Willow warbler.

Figure 1

Genetic ancestry based on two subspecies-specific markers. Bayesian assignment probability (genetic ancestry) to a northern (acredula) subspecies population cluster for all individuals computed in STRUCTURE (n = 181; details listed in Additional file 1: Table S2). Reference samples for the southern (P. t. trochilus, n = 18, open bars) and northern (P .t. acredula, n = 15, black bars) subspecies were plotted separately from the samples collected in the hybrid zone (n = 148, grey bars). Each bar represented one individual; the y-axis represented the probability of genetic ancestry to the northern subspecies cluster. The distribution within the study population in the hybrid zone was skewed towards southern genotypes. Dashed lines in the hybrid zone population indicated mean ancestry estimates for individuals from the northern and southern reference sets, respectively.

Figure 2

Male phenotype (A) and genotype (B) in relation to capture date (for males captured between 10 May and 10 June) (n = 56).Aδ¹⁵N values (dashed lines at 7‰ and 9‰ represented cut-offs for individuals likely wintering in West Africa and East to South Africa, respectively), B Bayesian assignment probabilities (genetic ancestry) to the northern subspecies cluster (dashed lines represent mean assignment probabilities in the southern and northern reference sets, respectively). Time for capture dates was plotted as the number of days after 1 May.

Figure 3

Male–female trait comparison for all pairs (n = 40).Aδ¹⁵N values; dashed lines at 7‰ and 9‰ represented cut-offs for birds likely wintering in West Africa and East to South Africa, respectively; B plumage colour scores; C PC1 for body size; and D genetic ancestry to the northern subspecies. Pair counts for colour and genetic ancestry (values rounded to 2 significant digits) were represented by the size of the circle.