Blue‐winged (Vermivora pinus) and golden‐winged warblers (Vermivora chrysoptera) have an extensive mosaic hybrid zone in eastern North America. Over the past century, the general trajectory has been a rapid replacement of chrysoptera by pinus in a broad, northwardly moving area of contact. Previous mtDNA‐based studies on these species’ hybridization dynamics have yielded variable results: asymmetric and rapid introgression from pinus into chrysoptera in some areas and bidirectional maternal gene flow in others. To further explore the hybridization genetics of this otherwise well‐studied complex, we surveyed variation in three nuclear DNA marker types — microsatellites, introns, and a panel of amplified fragment length polymorphisms (AFLPs) — with the goal of generating a multilocus assay of hybrid introgression. All markers were first tested on birds from phenotypically and mitochondrially pure parental‐type populations from outside the hybrid zone. Searches for private alleles and assignment test approaches found no combination of microsatellite or intron markers that could separate the parental populations, but seven AFLP characters exhibited significant frequency differences among them. We then used the AFLP markers to examine the extent and pattern of introgression in a population where pinus‐phenotype individuals have recently invaded a region that previously supported only a chrysoptera‐phenotype population. Despite the low frequency of phenotypic hybrids at this location, the AFLP data suggest that almost a third of the phenotypically pure chrysoptera have introgressed genotypes, indicating the presence of substantial cryptic hybridization in the history of this species. The evidence for extensive cryptic introgression, combined with the lack of differentiation at other nuclear loci, cautions against hybrid assessments based on single markers or on phenotypic traits that are likely to be determined by a small number of loci. Considered in concert, these results from four classes of molecular markers indicate that pinus and chrysoptera are surprisingly weakly differentiated and that far fewer genetically ‘pure’ populations of chrysoptera may exist than previously assumed, two findings with broad implications for the conservation of this rapidly declining taxon.
We assayed a total of 127 individuals using a variety of molecular markers. Nineteen of these individuals came from an allopatric population of chrysoptera from Manitoba (MB), Canada (50°46′N, 99°30′W) and 35 came from an allopatric population of pinus from Kentucky (KY) and Tennessee, USA (36°32′N, 87°22′W). In both cases, the samples were collected from regions with both contemporary and historical isolation from the other taxon, and which are currently separated from populations of the other species by approximately 675 and 300 miles, respectively. These populations served as our ‘control’ samples of pure parental genotypes. An important assumption in our subsequent analyses is that these samples from phenotypically pure, allopatric populations are not genetically introgressed. Likewise, an additional assumption is that any differences uncovered are species‐specific and not population‐specific differences. This can readily be verified in future work when samples are obtained from other allopatric, phenotypically pure populations.
The remaining 73 samples (N = 21 female, N = 52 male) were collected between 2001 and 2004 from a population of chrysoptera at Queen's University Biological Station near Elgin, Ontario (ON), Canada (44°34′N, 76°19′W). This population has been the target of intensive monitoring efforts since 1997 (R. J. Robertson and T. Demmons, unpublished data), and has only recently been invaded by hybrid‐ and pinus‐phenotype individuals. During the 2001–2004 period, the composition of the ON population was approximately 82% phenotypic chrysoptera , 17% hybrids, and 1%pinus . We classified the phenotype of each adult (male and female) sampled as pure chysoptera , pure pinus , classic ‘Brewster's’ warbler, classic ‘Lawrence's’ warbler (Parkes 1951) or phenotypically introgressed (Parkes 1951, Short 1963). An individual was characterized as introgressed if plumage characteristics did not match any of the stereotypical parental or hybrid phenotypes described by Parkes (1951). For example, an individual with chrysoptera facial and body patterns but a band of bright yellow feathers under the throat patch would be considered introgressed as it displays neither a classic chrysoptera pattern nor a ‘Lawrence's’ warbler pattern, but appears to be a variant of the two.