Some evolutionary losses may not be lost forever

A University of Adelaide study, led by PhD candidate Isaac Rossetto, found the genetic trait may have existed in a common ancestor of the nine species, which all belong to the Hydrophis genus, dating back three million years.

Isaac previously identified one species of sea snake that had re-elaborated the visual function – the fully marine Hydrophis cyanocinctus, which did so in response to its spectrally complex environment.

With the re-elaboration now identified in so many species, Isaac says there is sufficient evidence to suggest evolutionary losses can be somewhat reversed.

“We often think of evolution as a force that moves in just one direction – forward. But really, an organism’s ecological circumstances are continuously dynamic, and sometimes becoming the ‘fittest’ means revisiting traits that were once less beneficial,” Isaac says.

The re-elaboration of Hydrophis cyanocinctus’ visual function was in response to its bright underwater environment – which differed from the low-light habitats of some of its ancestors.

“Snakes descended from lizard-like ancestors which had a full visual opsin complement, which makes sense as they inhabited bright, colourful environments,” says Associate Professor Kate Sanders, who also contributed to the research.

“The earliest snakes underwent a period of dim-light living, and consequently lost two visual opsin genes, which caused them to lose much of their ability to distinguish colours.

“Descendants of these earlier snakes inhabit a diverse variety of light environments today, including bright and colourful marine ecosystems. This opsin expansion showcases how new sensory innovations can more or less re-elaborate visual functions previously thought to be lost.”

Though the species of sea snakes identified in this new research have regained the genetic requirements for advanced colour vision, the functions these expanded visual opsins have conferred upon the snakes is unclear.

“As far as we can tell, the function of these additional visual opsin genes remains the same between these species; however, much more information is required,” Isaac says.

“Behavioural studies will definitively show if this gene expansion has caused enhanced colour sensitivity, and this will bring us closer to answering an important question in evolutionary biology: when a complex trait is lost, how can it be regained?”