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

Abstract

The rapidly declining Golden-winged Warbler (Vermivora chrysoptera) is of conservation concern owing in part to hybridization with the closely related Blue-winged Warbler (V. pinus). These species hybridize extensively in eastern North America and over the past century the Blue-winged Warbler has displaced the Golden-winged Warbler from substantial regions of its historic breeding range. A previous study suggested that these genetic interactions result in rapid and asymmetric introgression of Blue-winged Warbler mitochondrial DNA (mtDNA) into Golden-winged phenotype populations within the zones of contact, but more recent and extensive surveys have documented a more complex pattern of genetic interchange between these taxa. We surveyed mtDNA/phenotype associations in 104 individuals of known phenotype drawn from two locations with different histories of contact and found substantial variation between sites in the extent of introgression. Where both species have co-existed for more than a century, we found evidence of bi-directional introgression and the long-term persistence of Golden-winged mtDNA haplotypes. At the leading edge of the northward expansion of Blue-winged Warblers, we found predominantly Golden-winged Warbler mtDNA haplotypes in both Golden-winged and hybrid-phenotype individuals. Across both sites, genetic swamping does not appear to be occurring via the early immigration of Blue-winged Warbler females into populations dominated by Golden-winged Warbler phenotypes. Instead, the differing patterns of mitochondrial introgression may be driven by the relative local population sizes of the parental species coupled with subtle between-species differences in mate choice and habitat preferences.

Complex hybridization dynamics between golden‐winged and blue‐winged warblers (Vermivora chrysoptera and Vermivora pinus) revealed by AFLP, microsatellite, intron and mtDNA markers

“…Smaller samples from Michigan and Ohio also failed to show evidence of asymmetric introgression. Likewise, similar analyses by Dabrowski et al . (2005) determined bidirectional gene flow and long‐term persistence of chrysoptera mtDNA haplotypes in a sympatric population in New York with an extensive history of contact between the species.…”

Empirical study of hybrid zone movement

“…Introgression between newts Triturus cristatus and Triturus marmoratus is found in the area of recent species replacement, but is absent in other areas (Arntzen and Wallis, 1991). At a hybrid zone between two warbler species, predominantly Vermivora chrysoptera mtDNA haplotypes are found in hybrids at the leading edge of a Vermivora pinus expansion (Dabrowski et al, 2005) and cytonuclear disequilibrium is also found at a crayfish Orconectes rustica-Orconectes propinquus hybrid zone. Two studies of deer, Cervus elaphus and alien Cervus nippon nippon, show introgression in both taxa (Abernethy, 1994;Goodman et al, 1999), but the expanding alien taxon shows higher levels of introgression which are uniform across its range, while introgression is lower in C. elaphus and more common near the region of overlap (Goodman et al, 1999).…”

Genetic Introgression: An Integral but neglected component of speciation in birds

“…128 the expanding Blue-winged Warbler. However, follow-up studies in this system have been more optimistic: mtDNA introgression seems to be more or less symmetrical within most contact regions (Shapiro et al 2004, Dabrowski et al 2005, whereas foreign nDNA may predominantly move into the blue-winged invaders (Vallender et al 2007), not into the golden-winged invadees. Although Vallender et al's (2007) hybrid zone samples were biased toward phenotypically pure Golden-winged Warblers (which were, on average, composed of 30% introgressed nDNA), the one phenotypic Blue-winged Warbler they examined was made up of 85% introgressed nDNA, which suggests much higher introgression rates into the invaders.…”