Are crows on islands experiencing a mutational meltdown?

A recent study quantified the amount of deleterious mutations in different crow species.

In theory, deleterious mutations accumulate more easily in small populations. Here is how it works: imagine a bucket of 9 green balls and one red ball (i.e. a deleterious mutation). The chance that you will randomly select the red ball is then 1 in 10. If you would take a bigger bucket with 99 green balls and one red ball. The chance of selecting the red ball drops to 1 in 100. The same principle applies to populations. The chances of passing on a deleterious mutation to the next generation are higher in small populations compared to big ones. Over time, deleterious mutations will more easily spread through small populations. This process – called genetic drift – results in a higher mutational load and consequently a higher extinction risk for small populations.

Surprisingly, few studies have tested this prediction with genome-wide data. A recent paper in the journal Molecular Biology and Evolution focuses on several crow species to see whether smaller populations carry more deleterious mutations.

Corvus_splendens

The House Crow (Corvus splendens) © Vinayaraj | Wikimedia Commons

 

Estimating the DFE

Before we can assess the accumulation of deleterious mutations, we first need to know which mutations are deleterious. Mutations can be divided into three broad categories: deleterious, neutral and advantageous. In reality, however, there is a continuum which ranges from strongly deleterious mutations to highly advantageous ones. The relative frequencies of these mutations form the distribution of fitness effects (DFE).

The DFE can be constructed using mutation experiments. First, you mutate one site in the genome (e.g., turn an A into a C). Then you compare the fitness of an individual with this mutation to the fitness of an individual with the original DNA sequence. And you do this for several locations in the genome. This approach has been performed on the vesicular stomatis virus and showed that most mutations are deleterious (see graph below). Understandably, this method is not feasible for crows.

DFE_in_VSV

The distribution of fitness effects (DFE) for the vesicular stomatis virus. Most mutations are deleterious (smaller than 1). However, there are some advantegeous ones (bigger than 1). © Fiona126 | Wikimedia Commons

 

Selection and Effective Population Size

Luckily, there are several methods to estimate the DFE from DNA sequence data. These methods rely on two parameters, namely the strength of selection (s) and the effective population size (N). First, the strength of selection determines how easily a mutation can spread through a population. The more deleterious a mutation, the less likely it will conquer a population. Second, the efficiency of selection depends on the effective population size. As I explained above, small populations are vulnerable to the random fluctuations of genetic drift. The larger the population, the less genetic drift plays a role and the more efficient selection becomes.

By combining these two features – selection (s) and the effective population size (N) – we can create several mutation-categories. When the product Ns is bigger than 10, we consider the mutation strongly deleterious. When Ns is between 1 and 10, the mutation is deleterious. And when Ns is equal to 1 or smaller, the mutation is slightly deleterious (or effectively neutral). Applying this approach to human data revealed that about 43% of mutations are strongly deleterious and will be unlikely to spread through the population.

DFE_Humans

Distribution of fitness effects for human data (based on 230 genes). Adapted from Eyre-Walker & Keightley (2007) Nature Reviews Genetics

 

Crows

With this thorough understanding of the DFE we can finally turn to the crows. Verena Kutschera and her colleagues estimated the DFE for seven crow species. Five species can be found on the mainland, namely the American crow (C. brachyrhynchos), the Carrion Crow (C. corone), the Daurian Jackdaw (C. dauuricus), the Eurasian Jackdaw (C. monedula) and the House Crow (C. splendens). The other two species reside on islands: the New Caledonian Crow (C. moneduloides) is native to – you guessed it – New Caledonia and the White-billed Crow (C. woodfordi) lives on the Solomon Islands.

Analyses of the DFEs from these seven species revealed that island species harbor more deleterious mutations compared to mainland species. Moreover, there was a significant relationship between the amount of deleterious mutations and the geographic range of the species. The smaller the geographic range, the more deleterious mutations. The authors conclude that “species living on islands accumulate mildly deleterious mutations more readily than more widely distributed species.”

crows_graphs

Islands populations have more deleterious mutations compared to mainland populations (left). And there is a significant correlation between the number of deleterious mutations and geographic range (right). Adapted from Kutschera et al. (2019) Molecular Biology and Evolution

 

Mutational Meltdown?

Does this mean that crows on islands are experiencing a mutational meltdown (as I wrote in the click-bait title above)? Not necessarily. They are certainly more vulnerable to mutational meltdown and consequent extinction, but there might be an unexpected byproduct of this rapid accumulation of mutations. An experiment with the bacteriophage φX174 showed that small populations contained 15% advantageous mutations whereas large populations had none. Moreover, small populations might be able hold on to slightly deleterious or neutral mutations that turn advantageous when the environment changes. However, it is dangerous to extrapolate experimental results from bacteriophages to wild bird populations. Instead, we can monitor the genetic health of these island populations and try to protect them.

American_Crows

A couple of American Crows in Victoria, British Columbia, Canada. © Michal Klajban | Wikimedia Commons

 

References

Eyre-Walker, A., Woolfit, M., & Phelps, T. (2006). The distribution of fitness effects of new deleterious amino acid mutations in humans. Genetics173(2), 891-900.

Eyre-Walker, A., & Keightley, P. D. (2007). The distribution of fitness effects of new mutations. Nature Reviews Genetics8(8), 610.

Kutschera, V. E., Poelstra, J. W., Botero-Castro, F., Dussex, N., Gemmell, N., Hunt, G. R., Ritchie, M. G., Rutz, C., Wiberg, R. A. W. & Wolf, J. B. (2019). Purifying Selection in Corvids Is Less Efficient on Islands. Molecular biology and evolution.

Sanjuán, R., Moya, A., & Elena, S. F. (2004). The distribution of fitness effects caused by single-nucleotide substitutions in an RNA virus. Proceedings of the National Academy of Sciences101(22), 8396-8401.

Silander, O. K., Tenaillon, O., & Chao, L. (2007). Understanding the evolutionary fate of finite populations: the dynamics of mutational effects. PLoS biology5(4), e94.

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