Reciprocal Introgression between Golden-Winged Warblers (Vermivora chrysoptera) and Blue-Winged Warblers (V. pinus) in Eastern North America

Abstract

ABSTRACT.• Golden-winged Warblers (Vermivora chrysoptera) and Blue-winged Warblers (V: pinus) are small, brightly colored Neotropical migrant birds that breed in eastern North America. Wherever the two species occur together, they hybridize to a limited degree, producing distinctive hybrid phenotypes. In recent decades, chrysoptera has experienced dramatic population declines across much of its range. Those declines have often been correlated with establishment and increase of pinus in the same areas, but it remains uncertain what, if any, role pinus has played in driving the decline of chrysoptera. In a first attempt at molecular genetic analysis of chrysoptera-pinus population dynamics. Gill (1997) reported cryptic, completely asymmetric, and possibly very rapid introgression of pinus mitochondrial DNA (mtDNA) into chrysoptera, causing what he termed "local cytonuclear extinction" of chrysoptera. As Gill (1997) noted, however, those results were based on relatively small samples from a single area in Pennsylvania. To begin to investigate the generality of Gill's findings and to establish a baseline for long-term genetic and ecological studies, we intensively sampled one new study area (in southern West Virginia) and also sampled more broadly across two other areas (in Michigan and Ohio) that have experienced pinus invasions and chrysoptera declines. In southern West Virginia, introgression of mtDNA appeared to be roughly symmetrical: 15% (11 of 72) of pinus phenotypes possessed chrysoptera mtDNA, and 12% (17 of 137) of chrysoptera phenotypes possessed pinus mtDNA. Results from much smaller samples from Michigan and Ohio also failed to show any evidence of asymmetric mitochondrial introgression. The results we report here, based on mtDNA and plumage phenotype information for 337 birds representing much of the range of the two species, indicate that previous genetic results and inferences from Pennsylvania may not be broadly applicable to the many areas of contact between chrysoptera and pinus in eastern North America.

Geographic variability in mitochondrial introgression among hybridizing populations of Golden-winged (Vermivora chrysoptera) and Blue-winged (V.opinus) Warblers

“…A more comprehensive recent survey (Shapiro et al 2004) examined phenotype-haplotype associations in a greatly expanded sample of individual warblers from additional study populations. In contrast to the rapid and unidirectional introgression seen at Gill's sites, Shapiro et al (2004) found bi-directional introgression at all locations where both species were well represented, with no tendency for disproportionate introgression as indicated by Blue-winged haplotypes in Goldenwinged phenotype individuals.…”

Do the Golden-winged Warbler and Blue-winged Warbler Exhibit Species-specific Differences in their Breeding Habitat Use?

“…Much of the research related to the decline of the Golden-winged Warbler has focused on hybridization with the Blue-winged Warbler (Gill 1997, Shapiro et al 2004, Dabrowski et al 2005. Although understanding the genetic challenges that face the species is important, improving habitat conditions for the Goldenwinged Warbler in newly colonized areas such as those found in eastern Kentucky is also valuable, particularly where the Blue-winged Warbler is absent.…”

Fire created habitat partitioning and isolation between hybridizing warblers

“…The breakdown of reproductive barriers and subsequent hybridization between GWWA and BWWA populations is well documented (Shapiro et al 2004, Dabrowski et al 2005, Vallender et al 2009) and is considered a threat to many GWWA populations (Parkes 1951, Gill 1980, Buehler et al 2007. We provide evidence fire may be creating conditions leading to habitat partitioning and isolation of GWWA and BWWA populations in central Wisconsin.…”

“…Hybridization between the two species has received extensive phenotypic (Parkes 1951, Gill 1980, 1987, mitochondrial DNA (mtDNA) (Gill 1997, Shapiro et al 2004, Dabrowski et al 2005, and nuclear DNA ) study indicating genetic introgression is bi-directional (Shapiro et al 2004, Dabrowski et al 2005) and extensive (Vallender et al 2009). Based on the mtDNA studies, the species' genomes overlap by 95.1-97.0% (Gill 1997, Shapiro et al 2004, Dabrowski et al 2005 and split from an ancestral species approximately 1.5 million years before present (Gill 2004). Based on genetic similarity and time since separation, relatively easy genetic exchange between the sister species is predictable (Price and Bouvier 2002).…”