Researchers armed with the killer whale genome have discovered the molecular mechanism behind these organisms’ evolutionary adaptation to underwater vision.
The ancestors of modern aquatic mammals such as whales and dolphins moved from land to sea some 50 million years ago. This animal group, called Cetaceans, now occupies all major aquatic habitats and has acquired many evolutionary adaptations to their current lifestyles.
Many of these adaptations, such as skeletal transformations, have been studied and understood for decades. However, the molecular mechanisms driving physiological adaptations, such as underwater vision, have been elusive because of the lack of genetic information for these creatures.
But advances in whole genome sequencing have now allowed for the genome of Orcinus orca (the killer whale) to be completed.
Researchers from the Department of Ecology and Evolutionary Biology at the University of Toronto have used this genome to conduct a detailed comparative study of the visual pigment called rhodopsin in killer whales and common cows.
Cows are related to Cetaceans through their common terrestrial ancestor and offer an excellent model (a terrestrial “control”) to study Cetacean adaptation to an aquatic lifestyle.
Statistical analyses of comparing cow and killer whale rhodopsin show that the rhodopsin gene is not only under natural selection pressure in whales, but also that the selected mutations in the gene confer greater sensitivity towards blue-shifted underwater light.
These findings provide evidence for the hypothesis that natural selection in a blue-shifted light environment (subsurface water) will select for rhodopsin with increased capacity for capturing blue-shifted light. This adaptation likely allowed killer whale ancestors to better forage for food.
The research team used both genetic information and experimental manipulation of the rhodopsin gene to validate their findings.
The study is one of the first Cetacean evolutionary studies to directly link an adaptation (rhodopsin shifted sensitivity) to a measurable genetic change.
The ability to propose hypotheses about genetic adaptations based on environmental conditions and then test these hypotheses with molecular and experimental genetics is emerging as a powerful tool for discovering patterns of evolution.