Whales’ Eyes Offer Glimpse Into Their Development From Land to Sea

College of Toronto researchers have clarified the evolutionary transition of whales’ early ancestors from on-shore living to deep-sea foraging, recommending that these ancestors had visual systems that could quickly adapt to the dark.

Their findings show that the common forefather of living whales was already a deep diver, able to observe in the blue twilight zone of the ocean, with eyes that quickly adjusted to dim conditions as the whale hurried down on a deep breath of surface air.

“In the evolvement of whale diving, there’s been a long-standing question of when deep-sea foraging advanced,” states Belinda Chang, a professor in the Faculty of Arts & Science’s departments of ecology and evolutionary biology and cell and systems biology. “And it appears that based on our data, this happened before toothed and baleen whales split. The common forefather of all living cetaceans was deeper diving– and then later species evolved all the varied foraging expertise we see in modern whales and dolphins today.”

Chang collaborated with Sarah Dungan, a former member of Chang’s laboratory who has a Ph.D. in ecology and evolutionary biology from U of T, on a research study defining their experiments, computational analysis, and results in the Procedures of the National Academy of Sciences.

Deep diving by marine creatures is one of the significant evolutionary transitions, along with powered trips and living on land, and reveals much regarding how quickly life can adjust in a transforming world.

Whales developed from mammals that share a common ancestor with hippos and which were partially aquatic. The most powerful secret of their transition to deep-sea foraging was how rapidly this ability developed. Dungan and Chang observed at whale fossils on a molecular degree and focused on the rhodopsin protein, which absorbs light and also sends a signal that travels through the retina to the brain.

“Among the most intriguing aspects of this iconic land-to-sea evolutionary transition is that the qualities of the visual environment completely changed,” states Chang. “This helped to define which genes could be the most fascinating for us to target in our studies.”

Dungan applied robust information science models to rhodopsin proteins from various living whales and related mammals. This computerized examination revealed a gene sequence representing the rhodopsin found in the common forefather of all living whales. She expressed this gene in lab-grown cells to “reanimate” the anticipated protein and experiment on purified samples.

“The fossil record is the gold requirement for understanding transformative biology,” says Dungan. “However, despite what Jurassic Park would have you believe, extracting DNA from fossil specimens is rare because the condition tends to be poor. So, suppose you are interested in how genes and DNA are progressing. In that case, you depend on mathematical modeling and a solid sample of genes from living organisms to complement what we know from the fossil record.”

Dungan and Chang were surprised by the biochemical properties of the resurrected protein compared to land creatures. Early whale rhodopsin was more sensitive to the blue light which penetrates deepest into the ocean, to a degree that went beyond expectations. Its biochemical properties likewise suggested that the retinas of early whales could respond rapidly to changes in light levels.

Early whales eventually evolved into the many types of toothed whales and baleen whales we see today. As separate species of whales developed, they established ecological niches at various sea levels and even in freshwater rivers. Dungan and Chang’s work reveals that there were further evolutionary adjustments as participants of both teams either surfaced from the early deep degrees to hunt closer to the surface or specialized to become even more extreme divers.

“I have always been interested in whales,” states Dungan. The concept that there was a land mammal like me which eventually developed to live underwater blew my mind as a youngster, even though I really did not understand precisely what that meant at the time.

“It is amazing that now we could have this degree of insight into the lifestyle of a long-extinct organism just from doing lab experiments on one protein. Ancestral protein resurrection is an incredibly powerful method for us to interrogate how ancient organisms progressed that most people do not understand about,” she adds.

Next, Dungan and also Chang plan to resurrect the ancestral whale proteins that communicate the rhodopsin light signal from the retina to the brain to offer insights into the neurological adaptations associated with deep diving. They will penetrate ancient evolutionary adjustments associated with new behaviors and want to acquire greater insight into how animals may adjust to a changing world.

Reference:

Sarah Z. Dungan et al, Ancient whale rhodopsin reconstructs dim-light vision over a major evolutionary transition: Implications for ancestral diving behavior, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2118145119

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