House Wren. Photo: John Harris
William Anderson, one of four recipients of 2022 WFO research grants, made exciting discoveries analyzing the genome data of Eastern and Western House Wrens.
The genetic basis of cryptic divergence in the House Wren (Troglodytes aedon)
Every spring, the bubbly song of the House Wren meets the ears of many birders eagerly awaiting their return. Known (and named) for its affinity for human-occupied areas, the House Wren is one of the most well-known and well-studied species in North America. Almost every aspect of this bird’s ecology has been studied, from diet to promiscuous breeding behavior (4 in 10 nests generally contain young that don’t belong to one of the parents!).
What many people don’t know is that this bird is part of a much larger species complex, and that their beloved backyard wrens are only two of 32 different described subspecies of House Wren. The House Wren complex ranges from British Columbia east to the Canadian Maritimes and south to Tierra Del Fuego, covering almost the entirety of North and South America. The majority of the United States and Canada are home to only two subspecies, known as the “Eastern” and “Western” subspecies. They are the only migratory subspecies within the complex and have very few plumage differences and no vocal differences. But, these extremely similar subspecies hold some interesting patterns of divergence within their genomes.
Genetic differences between the subspecies were recently documented by Klicka et al. (2023) in the mitochondrial genome, which has as many genetic differences as the mitochondrial genomes of Black-capped and Carolina Chickadees. Importantly, Klicka et al. (2023) found that both mitochondrial types, Eastern and Western, were present along the Front Range of Colorado. This discovery led to work I conducted as an undergraduate honors student at University of Colorado Boulder in the Taylor Lab on the wren contact zone in Colorado. Using a genetic dataset that spanned the entire wren genome, I found evidence of extensive hybridization between both subspecies.
Following the findings of my undergraduate honors thesis work that hybridization between Eastern and Western House Wrens was occurring along the Front Range, I, along with my advisor Dr. Scott Taylor, collaborator Dr. Garth Spellman, and other members of the Taylor Lab, wanted to explore the exact genetic regions that differ between the Eastern and Western subspecies. To do this, we sequenced the entire genomes of the Eastern and Western subspecies, as well as some of the birds from within the Front Range contact zone with admixed genomes. To obtain samples, we collaborated with both the Burke Museum at the University of Washington and the Cornell Museum of Vertebrates. They provided us with Eastern and Western House Wrens far from the zone of contact and unlikely to have experienced hybridization, and we included samples we obtained from along a nest box transect in Boulder County, Colorado. This higher-resolution whole genome data allowed us to determine the exact genetic regions that differ between Eastern and Western House Wrens, and to observe, for the first time, how they are distributed along the genome.
Our results were very exciting and not what we expected. We found that there are quite a few more differences between Eastern and Western House Wren genomes than we previously expected, and that those differences are concentrated on the Z chromosome. The differences between the two subspecies are captured within two large probable chromosomal inversions on one of the wren sex chromosomes, the “Z” chromosome (which is analogous to the X chromosome in humans). Within these regions, there are many more genes related to metabolic processes than we would expect by chance, and one that was previously implicated in migratory direction in Golden-winged and Blue-winged Warblers. The overrepresentation of metabolic genes could indicate a role for metabolic genes in the differences between House Wren subspecies. The rest of the wren genome is rather homogenous.
Excitingly, something within the chromosomal inversions may play a role in reproductive isolation between Eastern and Western House Wrens. The whole genome data also supported our previous results based on lower resolution genomic data. We confirmed that Eastern and Western subspecies were indeed quite differentiated, and that the birds found along the Front Range are hybrids. Moving forward, we are going to work to rigorously define the wren hybrid zone, and work to understand wren divergence across the entire species complex.
This project would not have been possible without the generous contribution by the WFO. Whole genome sequencing is a very expensive endeavor, and without WFO support we would not have been able to do it. I am very appreciative of WFO for this contribution, as it has let me pursue my scientific passions and explore both House Wren genomes and, more broadly, the generation of cryptic genetic diversity.
—William Anderson, University of Colorado Boulder
William Anderson