Shine a blue light on a lancelet and it glows green. Why these fish-like invertebrates, also known as “amphioxus,” have this green glowing protein has both intrigued and puzzled scientists for nearly a decade.
Today, Scripps Institution of Oceanography at the University of California San Diego researchers and colleagues are steps closer to understanding the evolutionary process behind the mysterious green fluorescent protein, referred to as GFP. These findings offer new insights to help improve its use as a medical tool to amplify light in living cells, and in other biomedical applications.
GFPs are found scattered throughout the animal kingdom in forms ranging from those that emit very bright light to ones that absorb light rather than emit it, and are therefore invisible to the human eye. They were first discovered in jellyfish, corals, and sea anemones, and more recently in more complex organisms such as lancelets, which resemble fish but lack the more sophisticated backbone and nervous system.
To begin tracing the GFP’s evolutionary pathway, Scripps researcher Dimitri Deheyn and a colleague at the Institute for Research on Cancer and Aging in France analyzed the genome of the two-inch (5-centimeter) long lancelet, alongside its closest living ancestor Asymmetron. Amphioxus, which spend their lives in shallow coastal regions burrowed in sand except for their heads, and Asymmetron diverged from each other on the tree of life approximately 120 million years ago.
Studying their genomes helped the researchers determine which of two different pathways the genes that produce the proteins were acquired by the organisms. The first, called vertical evolution, happens through the transfer of genes from one species to another through offspring. The second, termed horizontal transfer, occurs when it enters the genome over time from a food source or parasite.
They found that Asymmetron has two distinctly different GFPs while amphioxus has 16 different genes that produced the green glowing proteins. The two genes found in Asymmetron were also found in amphioxus.
“We discovered that these species, which separated more than 120 million years ago, have two of the same fluorescent proteins,” said Deheyn, a Scripps researcher who studies marine biomimicry, including in relation to light producing proteins.
Since both organisms carried two of the same GFP gene, the researchers theorize that the two common proteins likely diversified into 16 in amphioxus through vertical evolution over the last 120 million years.
“What we now know is that amphioxus did not acquire all 16 proteins through its common ancestor,” said Deheyn, the senior author of the paper recently published in the journal Scientific Reports. “Instead it evolved and diversified over time.”
In their natural environment, amphioxus are thought to use fluorescence for photo-protection (thus acting as sunscreen), as an antioxidant, and possibly for photo-sensing (using GFPs as receptors to the surrounding light). GFPs are used in a large variety of research fields from biomedicine, bioengineering to materials science.
This study provides the first connection for GFP in nature through common ancestors and helps put researchers closer to understanding the function of the green glowing protein and why it was retained by the animal over millions of years of evolution.
Deheyn and colleagues hope to fill in the evolutionary gap between jellyfish to lancelets to understand why the offspring of animals have so successfully passed on GFP through their genes.
“Our next question is what else does a fluorescent protein do in nature that it is preserved through evolution,” said Deheyn.
-- Annie Reisewitz