Extensive Rangewide Mitochondrial Introgression Indicates Substantial Cryptic Hybridization in the Golden-winged Warbler (Vermivora chrysoptera)

Widespread population declines of the Golden-winged Warbler (Vermivora chrysoptera) are thought to be due in part to hybridization with the expanding Blue-winged Warbler (V. pinus), which predictably replaces Golden-winged Warblers at breeding sites in which the two species come into contact. However, the mechanism by which this replacement occurs remains unresolved. Recent genetic work has indicated that, even in areas where the two species have been in contact for a short period, introgression of Blue-winged mitochondrial (mtDNA) and nuclear genes into Golden-winged individuals is common. To explore this process on a broader scale, we screened more than 750 individuals from nine U.S. states and three provinces to examine geographic patterns of mtDNA introgression. The only population in which all phenotypic Golden-winged Warblers had Golden-winged mtDNA haplotypes, and in which there are no breeding Blue-winged or hybrid individuals, was in the province of Manitoba, near the northwestern edge of the species' breeding distribution. The near ubiquity of mitochondrial introgression suggests that there are far fewer genetically pure populations of Golden-winged Warblers than previously believed, a finding with important implications for this threatened species.Original Abstract: On pense que le declin a grande echelle des populations de Paruline a ailes dorees (Vermivora chrysoptera) est en partie attribuable a son hybridation avec la Paruline a ailes bleues (V. pinus), laquelle est en expansion et remplace, comme on peut s'y attendre, la Paruline a ailes dorees aux sites de nidification ou les deux especes entrent en contact. Toutefois, le processus par lequel survient ce remplacement demeure inconnu. Des etudes genetiques recentes ont montre que, meme aux endroits ou les deux especes n'ont ete en contact que sur une courte periode, l'introgression des genes mitochondriaux (ADNmt) et nucleaires de la Paruline a ailes bleues chez les individus de Paruline a ailes dorees est commune. Pour explorer ce processus a plus grande echelle, nous avons procede au criblage de plus de 750 individus provenant de neufs Etats des Etats-Unis et de trois provinces afin d'examiner les variations geographiques de l'introgression mitochondriale. La seule population dans laquelle toutes les Parulines a ailes dorees phenotypiques presentaient des haplotypes d'ADNmt de Paruline a ailes dorees, et dans laquelle il n'y avait pas d'individus nicheurs de Paruline a ailes bleues ni d'h