Evolution of Bioluminescence May Be Surprisingly Predictable

A longstanding question among scientists is whether evolution is predictable, and they may have finally found their answer in two distinct species of squid. The genetic underpinning of bioluminescence, or the ability to emit light, in these cephalopods may in fact be surprisingly predictable, new research shows.

While the two squid species - the Hawaiian bobtail squid (Euprymna scolopes) and the swordtip squid (Uroteuthis edulis) - evolved separately, they did so in a remarkably similar manner.

"They are much more similar than we expected in terms of their genetic makeup," researcher Todd Oakley, from the University of California, Santa Barbara (UCSB), said in a statement. "Usually when two complicated organs evolve separately we would expect them to take very different evolutionary paths to arrive where they are today. The unexpectedly similar genetic makeup demonstrates that these two squid species took very similar paths to evolve these traits."

The bobtail and swordtip squid are distantly related species, and one of five genera known to have bioluminescent organs called photophores, which contain light-emitting bacteria.

Scientists set out to understand how similar the two species' photophores are in terms of their genetic makeup. To find the answer, they sequenced all of the genes expressed in these light organs. They found not only that the evolutionary paths towards bioluminescence were similar, but that the bioluminescent organs originated repeatedly during squid evolution and so gene expression underlying those organs were strikingly - even predictably - similar.

"I did find some individual genes that were counter to the main pattern, which means we can no longer study just one gene anymore in order to test these questions about the genetic basis of convergence," co-author Sabrina Pankey explained. "We're at the point now where we need to - and can - study all of them."

The researchers believe marine animals like squid use bioluminescence for camouflage, even though that seems counterintuitive. They manage to blend in with the light streaming into the deep ocean without casting a shadow as others would.

The research is described in detail in the journal Proceedings of the National Academy of Science.