Ancient whale rhodopsin reconstructs dim-light vision over a major evolutionary transition: Implications for...

Area of Science:

• Evolutionary biology
• Molecular evolution
• Marine biology

Background:

• Cetaceans evolved from terrestrial mammals to fully aquatic life, developing specialized diving adaptations.
• Understanding the evolution of underwater vision is key to reconstructing cetacean evolutionary history.
• Rhodopsin, the visual pigment in rod cells, offers insights into visual ecology and light adaptation in aquatic environments.

Purpose of the Study:

• To reconstruct and analyze the function of ancestral cetacean rhodopsin.
• To investigate the spectral sensitivity and light-activation kinetics of rhodopsin across the terrestrial-to-aquatic transition in cetaceans.
• To infer the diving capabilities and visual ecology of early whales.

Main Methods:

• Ancestral sequence reconstruction using a broadly sampled cetartiodactyl species tree.
• Protein resurrection experiments to quantify light-activation metrics of ancestral rhodopsins.
• Comparison of multiple reconstruction methods for robust ancestral sequence estimation.

Main Results:

• A significant blue-shift (14 nm) in spectral sensitivity was observed in ancestral cetacean rhodopsin (λmax = 486 nm), indicating adaptation to dim, deep-sea light.
• Increased decay rates of light-activated rhodopsin were found in ancestral cetaceans, suggesting accelerated dark adaptation.
• These functional shifts suggest early cetaceans could access the mesopelagic zone (>200 m).

Conclusions:

• Early cetaceans possessed rhodopsin adapted for deep-sea vision, enabling mesopelagic diving.
• The observed rhodopsin functional shifts predated the divergence of toothed and baleen whales.
• An evolutionary trade-off between photoprotection and dark adaptation may have influenced rhodopsin evolution in early aquatic cetaceans.