This is a guest post by Lukas Tietgen

Fur colour in the Arctic fox: genetic architecture and consequences for fitness (2021) Tietgen et al., Proceedings B, https://doi.org/10.1098/rspb.2021.1452

The Crux

Researchers who try to understand the dynamics of wild populations often look at how different traits affect the survival and reproduction of different individuals within those populations. Usually, the investigated traits are visible and easy to observe, like an animal’s size or their colour. However, there may be cases where the important traits are not as conspicuous or even hidden behind more striking features.

The arctic fox (Vulpes lagopus) occurs with two distinct fur colours, often called morphs. The two most common are the white morph and the blue morph. Which of these morphs is more common depends on the population. In Norway, the white morph is more common but in recent years an apparent increase in foxes of the blue morph has been observed. Previous research has shown that blue arctic foxes are usually fitter, but until now there hasn’t been a good explanation of why.

We wanted to dive a little bit deeper into the differences between the two colour morphs, explore the genetics behind this trait and seeing whether we could find any “hidden” traits connected to fur colour that could explain the difference in fitness between the two morphs.

What We Did

The first thing we did in our study was to perform a genome-wide association study (GWAS). A GWAS makes use of known portions of genetic code that are distributed across the genome of the study species. This allowed us to find associations between different regions of the genome and the specific trait we were interested in, fur colour. In other words, if a trait has a genetic basis and isn’t a product of the environment, the GWAS helps us pinpoint the exact gene controlling the trait.

We also investigated which genes lie in the same genomic region as the fur colour gene, to see whether those genes could help explain any differences in fitness.

After using the GWAS to identify the gene underlying arctic fox fur colour, we had a closer look at how selection might affect arctic foxes with different fur colour genotypes. We used the Norwegian arctic fox monitoring programme to compile a data set containing data on how different arctic fox individuals fared in terms of fitness. This dataset included genotypes, meaning we could find differences between arctic foxes with different fur colour genotypes.

Did You Know: A Species On The Mend

Due to extensive hunting, the arctic fox was close to extinction across Fennoscandia (Norway, Sweden and Finlad) in the beginning of the 20^th century. The species was then protected across the region in the 1920s and 30s. In both Sweden and Norway, major conservation projects have been in place since the early 2000s in an effort to protect the arctic fox from extinction and increase its population size in Fennoscandia. The Norwegian Institute for Nature Research (NINA) started the Norwegian Arctic Fox Captive Breeding Programme in 2000 and since 2006 over 400 captive bred arctic fox pups have successfully been released into the wild. In 2020, the Norwegian population was estimated to approximately 300 adult individuals.

What We Found

We were able to identify a gene called MC1R (melanocortin-1-receptor) as the basis for arctic fox fur colours. MC1R comes in two versions, each with different alleles, C and T. A fox with two C alleles (a CC genotype) has the white colour morph, whereas a fox with one or two T alleles (TC or TT genotype) has the blue colour morph.

We also found that foxes with the TC genotype (blue foxes) had higher fitness than foxes with the CC genotype (white foxes). Specifically, theyhad a higher survival probability and produced more offspring. Interestingly, this advantage in fitness was more pronounced in female foxes.

Lastly, we identified a wide range of genes in the same region of MC1R and many of these are known to play a role in vital functions such as immune response, stress resistance and metabolism.

Problems

Unfortunately, we had only a handful of foxes with the other blue morph genotype (TT genotype) in our data set and could therefore not include these in the analyses. It would have been interesting to see whether there are any fitness differences between the TC and TT genotypes as this would have been a strong indicator that these differences are not solely due to the actual fur colour.

So What?

With our study we confirmed previous findings that arctic foxes of the blue genotype have a fitness advantage in the Norwegian population and were able to point to both survival and reproductive advantages. Information like this is especially important in conservation efforts, especially given the arctic foxes status (see Did You Know). It allows us to plan for which families or populations are more likely to flourish in different environments.

Not only did we learn more about the dynamics of the endangered arctic fox population in Norway, but we also showed how knowledge about the genetic basis of the traits that drive fitness can be used to look beyond the trait. This approach will also help unravel the details of how other species’ wild populations work, and why some individuals do better than other. Many previous studies have failed to point out single genes lying behind fitness-altering traits, and it was exciting to show that this was possible.

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Lukas Tietgen finished his master’s degree in ecology last year and has recently published the work of his thesis as an article. Since finishing studying, Lukas has contributed to different projects of the Norwegian Institute for Nature Research, including the Arctic fox project.