Evolution under pressure and the adaptation of visual pigment compressibility in deep-sea environments

• Opsin proteins in deep-sea fish and cephalopods are less compressible than those in species living at shallower depths.
• Reduced opsin compressibility is due to positive destabilizing selection at common amino acid sites in both groups.
• Reduced opsin compressibility in each group is due to selection on the amino acid physiochemical properties.
• The selection of specific amino acid properties may play a role in protein evolution in the deep-sea.

Understanding the link between how proteins function in animals that live in extreme environments and selection on specific properties of amino acids has proved extremely challenging. Here we present the discovery of how the compressibility of opsin proteins in two evolutionarily distinct animal groups, teleosts and cephalopods, appears to be adapted to the high-pressure environment of the deep-sea. We report how in both groups, opsins in deeper living species are calculated to be less compressible. This is largely due to a common set of amino acid sites (bovRH# 159, 196, 213, 275) undergoing positive destabilizing selection in six of the twelve amino acid physiochemical properties that determine protein compressibility. This suggests a common evolutionary mechanism to reduce the adiabatic compressibility of opsin proteins. Intriguingly, the sites under selection are on the proteins’ outer faces at locations known to be involved in opsin-opsin dimer interactions.

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