Is the Red-backed Shrike a panmictic population?

Genomic analyses fail to find population genetic structure.

“Population genomics uses technology to increase the number of genetic markers by orders of magnitude, thereby offering the potential for fine-scale resolution of population structure and determination of population boundaries and population membership.” This statement comes from a book chapter that I wrote with several colleagues a few years ago. At first sight, we seem to suggest that genomic data will mostly be able to detect subtle genetic population structure. Just add more data and you are bound to find some differences. This is, however, not always the case. In the book chapter, we mentioned some genomic studies that failed to reveal population structure, even for species with broad geographic distributions, such as the European Turtle Dove (Streptopelia turtur) or the Mountain Chickadee (Poecile gambeli).

This lack of genetic population structure is known as panmixia. And a recent study in the journal Diversity adds another example to this list of panmictic bird species: the Red-backed Shrike (Lanius collurio).

mtDNA and Genomics

In 2019, Liviu Pârâu and his colleagues analyzed the mitochondrial DNA (mtDNA) of 132 breeding Red-backed Shrikes across their entire breeding range. They found two distinct groups with no clear geographical pattern, pointing to a panmictic population. But perhaps the mtDNA does not tell the complete story? That is why the researchers recently revisited this species with genomic data. They sequenced the whole genomes of 88 Red-backed Shrikes from 11 countries. The genetic analyses showed … the same patterns as the mtDNA. No sign of genetic population structure. The Red-backed Shrike does seem to be a panmictic bird species.

The Principal Component Analyses (PCA) does not reveal any population structure in the Red-backed Shrike. From: Pârâu et al. (2022) Diversity.

An Ice Age Legacy

But what evolutionary processes resulted in this panmictic population? The researchers argue that this panmixia is a “genetic legacy of the widespread and continuous distribution of the species, high locomotion capacities, and, most importantly, the numerous ice ages from the past few million years, which forced various populations to retract to refugia and expand their ranges several times, and to interbreed both in the glacial refugia and during warm periods in Eurasia.”

However, these ice ages might have left some subtle genetic signatures in the genomes of these birds. Particular genomic regions might reveal in which refugia the ancestors of present-day birds survived. Finding these genomic regions – if they even exist – will require more detailed genomic analyses (see for example this study on bean geese). The genetic population structure in the Red-backed Shrike might be hidden deep within its genome.

References

Pârâu, L. G., Wang, E., & Wink, M. (2022). Red-Backed Shrike Lanius collurio Whole-Genome Sequencing Reveals Population Genetic Admixture. Diversity, 14(3), 216.

Featured image: Red-Backed Shrike (Lanius collurio) © Antonios Tsaknakis | Wikimedia Commons