Evidence of genetic introgression found in the last remaining "pure" population of Golden-winged Warblers

Background/Question/Methods

Extinction due to hybridization and genetic introgression has been implicated across a wide range of taxa. Habitat modification, climate change, and species introductions have increased the rates of hybridization worldwide. The Golden- and Blue-winged Warbler hybridization complex is a well-studied system in which genetic introgression results in the replacement and subsequent local extirpation of Golden-winged Warblers by Blue-winged Warblers within 50 years of secondary contact. The causal mechanism(s) resulting in this genetic swamping remains unknown. Manitoba, Canada was recently declared to be the last refuge of genetically “pure” Golden-winged Warblers; the purpose of our study was to monitor this population for evidence of genetic introgression as Blue-winged Warbler populations continue to expand their range northward. Genetic techniques can now be used to identify levels of introgression that are phenotypically undetectable. A 3.0% (cytochrome-b) to 4.5% (NDII) mitochondrial sequence divergence has been documented between the ancestral mitochondrial genomes of these species. We used a combination of polymerase chain reaction (PCR) and fragment digest with a restriction endonuclease to assign individuals to their ancestral haplotype group. A mismatching phenotype and mitochondrial genotype indicates a past female-line hybridization events in the ancestry of an individual.

Results/Conclusions

We screened a total of 57 individuals from SE Manitoba and 11 individuals from W Manitoba, all sampled in 2011. One individual from SE Manitoba was a Brewster’s Warbler, therefore by definition was introgressed. In addition, one individual from W Manitoba showed mtDNA introgression due to a mismatch between the phenotype and mitochondrial haplotype classification. A second Brewster’s Warbler was also observed in W. Manitoba near the capture site of the cryptic hybrid. All sampled birds from Manitoba were found to be “pure” as recently as 2010, but our results show that genetic introgression may have begun in these populations. These may, in fact, be underestimates because mtDNA can only identify female-mediated introgression. Blue-winged Warblers currently do not nest farther north than southern Minnesota, but as their range continues to expand, we expect to find more introgressed individuals. It remains to be seen if and how quickly a hybrid swarm will develop, but if the same pattern of rapid Golden-winged Warbler replacement that has been observed in many other locations also occurs here, this species may be facing genetic extinction in Canada.