The Evolution of Light: Complex Bioluminescence May Evolve Predictably

Is evolution predictable? That's the question that scientists asked themselves in this latest study as they explored bioluminescence. Now, they've taken a closer look at the genetic underpinnings of complex traits in cephalopods, like squid, and have found that being able to generate light could actually be predictable.

In order to better understand how bioluminescent organs evolved, the scientists examined the Hawaiian bobtail squid and the swordtip squid, a Japanese species. These distantly related species have bioluminescent organs called photophores that contain symbiotic, light-emitting bacteria. The squid can control the aperture of their organ in order to influence how much light is produced.

To see how similar these photophores were, the scientists sequenced all of the genes expressed in these light organs. Surprisingly, they found they were more similar than expected.

"Usually when two complicated organs evolve separately we would expect them to take very different evolutionary paths to arrive where they are today," said Todd Oakley, one of the researchers, in a news release. "The unexpectedly similar genetic makeup demonstrates that these two squid species took very similar paths to evolve these traits."

In fact, the scientists found that bioluminescent organs originated repeatedly during squid evolution. In addition, they discovered that the global gene expression profiles underlying these organs are predictably similar.

That said, the scientists did find some individual genes that were counter to the main pattern. This shows that scientists can no longer study just one gene in order to test these questions about the genetic basis of convergence.

Overall, though, the new study reveals that the evolution of overall gene expression underlying convergent complex traits may be predictable. The evolution of convergent phenotypes is associated with the convergent expression of thousands of genes.

"These results have broad implications for workers in the fields of evolution, genetics, genomics/bioinformatics, biomaterials, symbiosis, invertebrate zoology and evolutionary development," said Oakley.

The findings are published in the journal Proceedings of the National Academy of Sciences.