Green is more than skin-deep for hundreds of frog species
Biochemical workaround makes pigment that tunes light to the exact shade
of a green leaf

Date:
July 13, 2020
Source:
Duke University
Summary:
The through-and-through greenness of hundreds of frog species that
can be found deep in their lymphatic fluid, soft tissues and even
bones, comes from a clever biochemical workaround that combines a
normally virus- fighting type of protein with a toxic byproduct of
blood breakdown. The camouflage innovation has happened at least
40 times across 11 families of frog and toad.



FULL STORY ========================================================================== Frogs and toads are green for a very good reason -- it makes them harder
to see in their leafy environments. Good camouflage allows them to eat
and not be eaten. But not all frogs have arrived at this life-saving
greenness in the same way.


==========================================================================
Most of these animals rely on color-controlling structures in their skin
called chromatophores that use crystals to bend light to specific colors
and make them appear green. But there are hundreds of species of frogs
and toads that have nearly translucent skin and very few chromatophores.

Their greenness, which can be found deep in their lymphatic fluid, soft
tissues and even bones, comes from a clever biochemical workaround that combines a normally virus-fighting type of protein with a toxic byproduct
of blood breakdown.

The finding, by post-doctoral researcher Carlos Taboada at Duke
University, solves a few longstanding mysteries about these frogs and
shows how the necessity of survival can be very inventive indeed. It
appears the week of July 13 in the Proceedings of the National Academy
of Sciences.

Scientists have long grappled with the fact that many of these frogs
contain very high levels of bile pigment called biliverdin that is
a byproduct of breaking apart old red blood cells. This pigment is
normally considered a toxin to be filtered out in the liver and excreted
as quickly as possible. But these frogs are found to carry four times
as much biliverdin as even the sickest human with liver disease, and
200 times as much as their chromatophore-equipped frog cousins.

To understand the biochemistry better, the researchers focused on one
species, Boana punctata, the polka-dot treefrog of South America. From it,
they isolated a protein they're calling BBS (biliverdin-binding serpin),
which is part of a superfamily of protease inhibitors, the proteins that normally step in the way of viral replication and detoxify enzymes.



==========================================================================
When you see something green, its color really should be called
'everything but green,' because it is soaking up all the colors of
incoming light except for green. The color we see is the frequency of
light it does not absorb that bounces back to our eyes.

Biliverdin by itself would appear to be somewhat greenish, as sometimes
seen in an old bruise, but the researchers found that a bound serpin,
BBS, stretches out biliverdin's helical shape to fine-tune its light absorbance, making it more cyan, a blue-green. Cyan, added to some other
yellow pigments scattered in the skin, bounces back just the right shade
of green. It also makes biliverdin less toxic as well, apparently.

"This new protein has the same spectroscopic properties or light
absorption properties as some plant pigments," said Taboada, who began
the work in Argentina, Ecuador and Brazil and completed it at Duke. "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. Taboada said this innovation has evolved more than 40 times across 11 different families,
most of them treefrogs. The adaptation happened again and again in
far-flung Madagascar, South America and Southeast Asia.

"So this is a convergence in evolution," Taboada said. "Being arboreal
(living in trees), they developed a different way to make their
coloration." Their through-and-through greenness ensures good camouflage
on foliage, even in near- infrared light.

"This shows how natural selection can co-opt proteins for just about
any purpose," said So"nke Johnsen, a professor of biology at Duke and
coauthor on 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." "In other words, Kermit has jaundice," Johnsen said.

Having earlier discovered that many species of frogs reflect fluorescent wavelengths -- essentially glowing in the dark -- Taboada is now working
with engineers at Duke's Pratt School of Engineering to shine precisely
tuned lasers at frogs to learn more about their coloration.

"This paper is a tour de force reach between the worlds of molecular
biology, biochemistry, and ecology," Johnsen said.


========================================================================== Story Source: Materials provided by Duke_University. Original written
by Karl Leif Bates.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Carlos Taboada, Andres E. Brunetti, Mariana L. Lyra, Robert
R. Fitak, Ana
Faigon, Santiago R. Ron, Maria G. Lagorio, Ce'lio F. B. Haddad,
Norberto P. Lopes, So"nke Johnsen, Julian Faivovich, Lucia
B. Chemes, Sara E.

Bari. Multiple Origins of Green Coloration in Frogs Mediated by
a Novel Biliverdin-Binding Serpin. Proceedings of the National
Academy of Sciences, 2020 DOI: 10.1073/pnas.2006771117 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200713165602.htm

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