Museum specimens reveal phenotypic introgression in a Scrub-Jay contact zone

By Devon DeRaad

The Blue Jay (Cyanocitta cristata) is a classic charismatic backyard bird of Eastern North America, but it’s lesser known cousins have recently stolen the show, at least scientifically. Closely related to the well-known Blue Jay, the genus Aphelocoma is comprised of the Scrub-Jays, Mexican Jays, and Unicolored Jays, which inhabit Western North America and Mexico. While not the most flashy “blue jays” in North America, this genus has received extensive scientific attention over the past decade, and has quickly become a model for understanding the processes of allopatric lineage diversification, local environmental adaptation, and hybridization upon secondary contact.

 

Three Distinct Groups

The former Western Scrub-Jay was a classic example of a polytypic species, with close to a dozen subspecies recognized across Western North America and Mexico. However, DNA sequence data indicated that among this multitude of locally adapted forms, there were three genetically distinct groups, corresponding with the previously described California Scrub-Jay, Woodhouse’s Scrub-Jay, and Sumichrast’s Scrub-Jay. The California Scrub-Jay was thusly described as a unique species, while Woodhouse’s and Sumichrast’s were lumped as Woodhouse’s Scrub-Jay, due to a lack of information about the degree of phenotypic differentiation and genetic introgression between the two forms.

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A Woodhouse’s Scrub-Jay perched on Joshua tree, Red Rock Canyon, southern Nevada © Stan Shebs | Wikimedia Commons

 

Contact Zone

The Moore Laboratory of Zoology, where I worked as an Occidental College undergraduate researcher, has an extensive collection of Woodhouse’s Scrub-Jay specimens from throughout Mexico. Using this valuable series of specimens, and extra specimens from near the contact zone, which we loaned from other natural history museums from around the country, we set out to discover exactly what was going on between Woodhouse’s and Sumichrast’s Scrub-Jays. We used digital calipers to take careful measurements of the tail, wing chord, tarsus, bill length, bill width, and bill depth, of 133 specimens from throughout Mexico, and used light spectroscopy to quantify the color of the back feathers of each specimen.

Our results show that Sumichrast’s Scrub-Jay is significantly larger in overall body size, and has brown back feathers, as opposed to the blue back feathers of the Woodhouse’s Scrub-Jays in northern Mexico. Our results align with Pitelka’s description of Sumichrast’s Scrub-Jay from his seminal 1951 work on the genus ‘Speciation and ecologic distribution in American jays of the genus Aphelocoma’. Additionally, our results shed new light on the contact zone between Woodhouse’s and Sumichrast’s Scrub-Jays, where Pitelka described intermediate specimens as ‘scant’. By accessing specimens unavailable to Pitelka and analyzing them with modern methods, we reveal phenotypic introgression between the groups, with clinal transitions in body size and back color centered concurrently near Mexico City. These results indicate that the transition between these forms is likely a case of introgression upon secondary contact, following a period of divergence in geographic and genetic isolation.

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The clines across the phenotypic hybrid zone between subspecies of the Woodhouse’s Scrub-Jay (from DeRaad et al. 2019 The Auk)

 

Speciation in Action

This new discovery is exciting because the existence of a contact zone between these two divergent forms creates a natural experiment, where we can study speciation in action. In the future we hope to collect modern specimens from the contact zone and use genetic sequencing to identify whether hybridization is occurring and if so, at what frequency. These future genetic studies will help us understand the genomic architecture of introgression between divergent populations that have come back into contact, and will help us make more informed taxonomic decisions about how to recognize Woodhouse’s and Sumichrast’s Scrub-Jays.

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Woodhouse’s and Sumichrast’s Scrub-Jays © Maggie Schedl

 

References

DeRaad, D. A., Maley, J. M., Tsai, W. L. E., & McCormack, J. E. (2019). Phenotypic clines across an unstudied hybrid zone in Woodhouse’s Scrub-Jay (Aphelocoma woodhouseii). The Auk, 136(2).

Gowen, F. C., Maley, J. M., Cicero, C., Peterson, A. T., Faircloth, B. C., Warr, T. C., & McCormack, J. E. (2014). Speciation in Western Scrub-Jays, Haldane’s rule, and genetic clines in secondary contact. BMC evolutionary biology, 14(1), 135.

Pitelka, F. A. (1951). Speciation and ecologic distribution in American jays of the genus Aphelocoma. University of California Press. Berkeley and Los Angeles, CA.

 

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