It’s complicated being green

Green frogs tend to be green for the same reason, but a new study suggests they don’t all achieve it in the same way.

A team from the US, Argentina, Brazil and Ecuador says it has found that while most species rely on colour-controlling structures in their skin called chromatophores, others, which have nearly translucent skin and thus few chromatophores, employ a different approach.

Their greenness, which can be found in their lymphatic fluid, soft tissues and even bones, comes from “a clever biochemical workaround”.

Many of these frogs are known to contain very high levels of bile pigment called biliverdin – a by-product of breaking apart old red blood cells that is responsible for the greenish colour sometimes seen in bruises.

It is normally considered a toxin to be filtered out in the liver and quickly excreted, not stored in large quantities (as much as 200 times the levels in chromatophore-equipped frogs).

However, when Carlos Taboada from Duke University, US, and colleagues took a close look at Boana punctata, the polka-dot treefrog of South America, they found that biliverdin exists alongside a protein they call BBS (biliverdin-binding serpin), part of a virus-fighting family.

BBS makes biliverdin less toxic, they discovered, and also fine-tunes its light absorbance, creating just the right shade of green.

“This new protein has the same spectroscopic properties or light absorption properties as some plant pigments,” says Taboada. “The light properties are very similar to what we see, for example, in some plant proteins called phytochromes; but here we have a completely different protein.”

It’s a clever adaptation of existing biochemistry that normally serves other functions in vertebrates, he adds, and has evolved more than 40 times across 11 different families, most of them treefrogs.

“So this is a convergence in evolution. Being arboreal – living in trees – they developed a different way to make their coloration.”

It also shows how natural selection can co-opt proteins for just about any purpose, adds Duke’s Sönke Johnsen, a co-author of the paper.

“Biliverdin is a bile pigment that would normally be excreted from the body because of its potential for harm, but here it is in spectacular concentrations precisely because it’s also useful as a green pigment.”

The findings are described in a paper in the journal Proceedings of the National Academy of Sciences.