Fight For Light

UNC-TV Science: May 5, 2014
Ferns, Flowers and the Fight for Light

A species’ survival is a story of struggle and adaptation. It is very similar to the Rocky movies where the tough-as-nails slugger from Philly you can’t help but root for has to reinvent his fighting style to win the big boxing match against all odds.

These stories are the cornerstone of evolution as we know it. A seemingly implacable force disrupts a species’ way of life. Those that can’t adapt die off, but those born with the right genes to make the best of the new environment flourish.

A great example of this kind of adaptation comes from the very paragon of pugilistic plants; the fronds of fury; the pteridophyte terror; the... fern. About 70 to 80 million years ago, ferns faced a possible extinction, but a single adaptation, a gene, has allowed ferns to endure and thrive in the shady spots of forest floors.

Now biologists from Duke University, in collaboration with others around the country, have discovered how the ferns were able to adapt and where that gene came from.

About 100 million years ago, ferns were one of the dominant types of plants on Earth. They grew quickly and had broad leaves to capture lots of sunlight. Then the ferns had an encounter with flowering plants, called angiosperms.

If the ferns are Rocky Balboa, then the angiosperms are Apollo Creed. Angiosperms grew taller and faster, had even broader leaves than the ferns, and recruited birds and bees to help them pollinate and reproduce. The ferns didn’t really stand a chance. Angiosperms soaked up the sun, while relegating ferns to the shade.

This should have wiped out the ferns, but it didn’t. The ferns, armed with a single gene called neochrome, persisted. Plants absorb sunlight to make their energy, and because plants are green, they can absorb blue and red light. Most plants, angiosperms included, focus on blue light because it has more energy. What neochrome does is it absorbs both blue and red light, allowing the ferns to live off of the light the angiosperms don’t use.

Ferns Balboa couldn’t outcompete with Flowers Creed toe-to-toe. Flowers are just better suited to fast growth and absorbing sunlight. But neochrome gave ferns their Rocky II sequel opportunity. Instead of punching sides of beef, Rocky chased chickens and learned to fight with his opposite hand. Ferns grew low and fast and learned to exploit the opposite side of the visible light spectrum.

But there’s a gap in this story: a gap that Fay-Wei Li, Duke Ph.D student and lead author of the new study sought to close. Where did ferns get the neochrome in the first place? Who’s Mickey in this story, giving Ferns Balboa his chickens to chase?

HornwortLi scoured the genomes of other plants for neochrome. The idea is that if you can find other plants with neochrome, you can look at how they are related to each other and better guess how both plants got the gene. Li found that the other plant group that carries neochrome is actually a crotchety moss-like group called hornwort. Hornworts tend to live in damp or humid environments and can either grow in the soil or on the bark of some trees.

There are three plausible explanations for how plants can share a gene like neochrome. The first is they could have a common ancestor that had neochrome and passed it down to both of them. The second is that both naturally evolved the gene on their own. The third is that one of them gave the gene to the other in a process called horizontal transfer. Or to continue the Rocky, Mickey and chickens metaphor: either Rocky and Mickey had the same great-great-grandfather who left chickens to both of them in his will, both Mickey and Rocky decided independently to go out and buy a chicken, or Mickey saw that Rocky was too slow and gave him a chicken to chase.

In order to start narrowing the possibilities down, Li investigated the ancestry of ferns and hornworts and found that they do have a common ancestor from around 400 million years ago. Many other plant families share that common ancestor, and you would expect them to have neochrome as well. But if you watch Rocky II, you won’t see every family in South Philly taking their pet chicken for a walk, and the same goes for plants. Hornworts and ferns are the sole carriers of neochrome. All of the other plants that shared that ancestor would have to have lost neochrome sometime in the last 400 million years, which is very unlikely.

More unlikely still is the possibility that both families of plants put together a gene as complex as neochrome. It’s even more unlikely than, say, two random people in South Philadelphia going out and buying a chicken on the same day.

That leaves one option: horizontal transfer. In bacteria and other microbes, horizontal gene transfer happens all the time. As long as the individual cells touch each other, they can swap genetic material back and forth. Fungi do it and viruses make a their living by shuttling their genetic material from one cell to another.

In plants, though, this process is more mysterious simply because plants are larger and are made up of billions of cells. And giving a single leaf cell neochrome will not help that plant pass neochrome down to it’s children or even really help it survive.

But the way ferns reproduce may lend itself to horizontal gene transfer. In a fern’s life cycle, it produces spores. The spores grow into what are called gametophytes. The gametophytes produce sperm and eggs that combine to make new ferns. In ferns, the sperm and eggs are not protected by the gametophyte, so they can brush up against other ferns and other plants, or be attacked by a virus or bacterium. Any of those processes could have allowed neochrome to enter a sperm or egg cell, which would have guaranteed that neochrome got passed down.

This study does not provide the answer as to how neochrome moved, but Li’s genetic analysis shows that it most likely moved from the hornworts to the ferns, and just in time to survive the rise of flowering plants.

This study appeared in the journal, Proceedings of the National Academy of Sciences.

- Daniel Lane

Daniel Lane covers science, medicine and the environment as a reporter/writer. He is currently pursuing a master's degree in medical and science journalism at UNC - Chapel Hill.