Convergent evolution of color morphs in Skuas

How far do the similarities reach on the genetic level?

Convergent evolution refers to different species that independently evolved similar traits. An obvious example concerns the streamlined shapes of sharks and dolphins. The common ancestor of these aquatic creatures lived about 300 million years ago and dolphins descended from land mammals. Hence, it is easy to see that sharks and dolphins have independently evolved their similar shapes.

However, when it comes to closely related species, detecting convergent evolution is more tricky. The more closely related two species are, the more likely that their similarities were inherited from a common ancestor (and are thus not the result of convergent evolution). But looks can be deceiving. Perhaps the similarities are only skin deep. When you start exploring deeper levels – compare it to peeling away the layers of an onion – you might discover genetic differences. The shared traits might be encoded by different genes, or they might be the outcome of different mutations in the same genes. There is only one way to find: start peeling that genetic onion.

Three Skuas

A nice example of possible convergent evolution among closely related species can be found in skuas (genus Stercorarius). All three species in this group of seabirds show several color morphs, based on their ventral plumage. In the Arctic Skua (S. parasiticus) and the Pomarine Skua (S. pomarinus), this plumage polymorphism has been linked to the gene MC1R (melanocortin-1 receptor). Interestingly, the two species experienced different mutations in the MC1R-gene, thus showing convergent evolution at a deep genetic level. But what about the third species, the South Polar Skua (S. maccormicki)?

Kirstin Janssen and her colleagues explored the MC1R-gene for 25 individuals, representing the three color morphs (8 pale, 10 dark and 7 intermediate birds). The DNA sequences revealed two variable sites within the gene: one synonymous mutations (no change in amino acid) and one mutation replacing a glutamate with a lysine. However, these genetic variants did not line up with the color morphs. These patterns suggest that the MC1R-gene is not associated with plumage polymorphism in the South Polar Skua.

Plumage variation in the South Polar Skua, a pale female and a dark male bird. From: Janssen et al. (2021).

Sex-linked Variation

Finding out that the MC1R-gene is not involved might be disappointing. But don’t despair yet (or start crying, which can happen while peeling onions). The researchers tested another candidate gene: TYRP1 (tyrosinase-related protein 1). This particular gene can be found on the Z-chromosome, one of the sex chromosomes in birds. Because the plumage variation in the South Polar Skua seems sex-specific, with males being darker than females, it makes sense to focus on a sex-linked gene.

Unfortunately, this approach was also unsuccessful. The researchers did not find any variation in this gene. We can thus conclude that TYRP1 is not involved in the plumage polymorphism of the South Polar Skua. The search will continue and there are still plenty of options to explore. Perhaps the color morphs are due to differences in gene expression. Or because of mutations in different genes. We will just have to keep peeling away at that onion.

References

Janssen, K., Bustnes, J. O., & Mundy, N. I. (2021). Variation in genetic mechanisms for plumage polymorphism in skuas (Stercorarius). Journal of Heredity112(5), 430-435.

Featured image: South Polar Skua (Stercorarius maccormicki) © Denis Luyten | Wikimedia Commons

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