Interview with the authors: Latitudinal divergence in a wide-spread amphibian: contrasting patterns of neutral and adaptive genomic variation

It is difficult to parse the effects of demography and historic processes and the effects of selection, particularly in species that are widespread over heterogeneous environments. In this paper, Patrik Rödin‐Mörch and colleagues use reduced-representation genomic data to investigate the demographic and selective forces driving patterns of genetic diversity in the moor frog. They find evidence of two refugial linages with support for gene flow between lineages, and they find striking differences between neutral and putatively adaptive markers. Read the full article here, and see below for an interview with the authors.

What led to your interest in this topic / what was the motivation for this study? 
We are generally interested in how amphibian populations diverge along environmental gradients, in particular relating to latitude. We have previously focused on adaptive divergence in phenotypic traits relating to growth and development in this system. Amphibians occurring at higher latitudes are very constrained by seasonality and differences in thermal regimes as well as other aspects of the environment, and this should result in strong selection to cope with these constraints. In northern Europe, populations also have a history of glacially mediated range expansions and we are very interested in how this influences divergence along the gradient. Amphibians are very good organisms to study local adaptation as a number of species have quite wide distributions where they occur in different habitat types and thermal regimes with large differences in season length. We wanted to build on previous research by taking a more genome-wide approach that would enable us to detect signatures of divergent selection, explore the distribution of genetic variation along the gradient and model the post-glacial demographic history of the populations.

What difficulties did you run into along the way?
Applying a custom ddRAD library prep protocol on R.arvalis for the first time was a bit challenging in the beginning, as the protocol was put together in another lab for another organism. Because of the large genome of this species, it was challenging settling on which restriction enzyme combination to use and how many fragments that would result in, as we wanted to multiplex ~150 individuals and had limited funds for sequencing. We also wanted to sample populations over the contact zone to get a more comprehensive look at what’s going on there, but finding populations in between the two edge regions of the contact zone was ultimately unsuccessful.

What is the biggest or most surprising finding from this study? 
The findings that intrigued us the most was the contrasting way neutral and putatively adaptive  genetic variation is distributed along the gradient. Particularly so over the post-glacial contact zone, both in terms of nucleotide diversity and based on hybrid index estimation. We were also very pleased that we obtained good support for a model that describes what we initially thought was the correct post-glacial demographic scenario, involving two lineages diverging before the last glacial maximum. After divergence they colonized Scandinavia from two different directions, with gene flow occurring over a contact zone that we could place further south than previously proposed.

Moving forward, what are the next steps for this research? 
In order to continue this work, the next step will be to replicate the latitudinal gradient on the eastern side of the Baltic sea, as well as obtaining samples across the contact zone. The plan is also to move away from ddRAD seq to RNA-seq, and eventually whole genome sequencing. We are currently planning to look at how gene expression as well as SNP variation differs with latitude and combine that information with common garden experiments on larval life-history variation. Ultimately we want to understand the genetic basis of local adaptation based on larval life-history variation and how the demographic effects of post-glacial range expansion has influenced that.

What would your message be for students about to start their first research projects in this topic? 
Make sure you know the literature. Many previous studies have investigated adaptive divergence along various environmental gradients for a number of species, including amphibians in different settings. Also, be prepared to conduct extensive field work, common garden experiments, lab work and bioinformatics, and make sure you have collaborators that can help you out.

What have you learned about science over the course of this project? 
That things usually never work out like you first planned, and sometimes you need to adjust your conceptual and methodological approach as you go along. Another important lesson is the value of collaboration and relying on other people’s expertise and skills.

Describe the significance of this research for the general scientific community in one sentence.
Amphibian populations extending their distribution range northwards after the last ice age have adapted to the environmental constraints experienced at higher latitudes and this has influenced the distribution of genetic variation along the gradient.

Describe the significance of this research for your scientific community in one sentence.
We find neutral and putatively adaptive gene flow over a post-glacial contact zone within a single species and together with strong environmental constraints and historical range dynamics this has shaped patterns of contrasting genetic variation and adaptive divergence along the gradient.

Full article:

Rödin‐Mörch P, Luquet E, Meyer‐Lucht Y, Richter‐Boix A, Höglund J, Laurila A. Latitudinal divergence in a widespread amphibian: Contrasting patterns of neutral and adaptive genomic variation. Mol Ecol. 2019;28:2996–3011.

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