Ancient plants learned to survive on land by stealing other species' DNA

The first plants on Earth were algae that lived in water, and moving onto land required completely new survival skills.

Now a new study involving Canadian researchers shows plants got those skills by stealing genes from a totally different species — soil bacteria that had already been thriving on the land for billions of years.

That's a surprise because gene transfer to plants and animals from other species is quite rare in nature, the researchers say.

That event eventually led plant-eating animals to follow onto land and the greening of the Earth into a world covered in forests and grasslands that we know today.

Gane Ka-Shu Wong, a University of Alberta biology professor and co-author of the study, calls the colonization of land by plants "one of the most important events in the evolution of life on this planet — without which we as a species would not exist."

Five hundred million years ago, plants were mostly algae that we would recognize as "pond scum." Bacteria had already been there for a "long, long, long time" — billions of years, Wong says. 

"They evolved the ability to tolerate dry conditions… in some sense, they perfected a lot of these processes."

Rather than reinventing the wheel, early plants stole those genes and incorporated them into their own genome, suggests the new study published Thursday in the journal Cell.

Biologists call the movement of genes from one species to another "horizontal gene transfer." Wong likens it to genetic engineering used in food crops today, where genes from ones species are incorporated into another to give them new abilities like, well, drought tolerance. 

In order to figure out what happened, Wong and his colleagues did a genetic analysis of some algae that are most closely related to land plants and share a common ancestor that was likely the first land plant.

Sort-of-new species

Fortunately, Michael and Barbara Melkonian, a couple of algae specialists at the University of Cologne in Germany who also co-authored the paper, had just the algae for such an analysis. They had found the "gelatinous blob," as Michael Melkonian called it, during an algae-hunting hike with some students in 2006 on a rock near the banks of a brown, bog-fed river near Cologne.

Freshwater algae live in water, but lakes, ponds and creeks do dry up from time to the time, forcing them to survive in drier environments like mud where soil bacteria also live. The algae the Melkonians found was a single-celled organism that took things a step further, living on rocks that are more often dry than wet. 

"It's already terrestrial," Michael Melkonian said. 

It turned out to be a species first discovered in France in 1845, but never properly scientifically described until now — the researchers have named it as the new species Spirogloea muscicola. And a DNA analysis showed it was extremely closely related to land plants.

"With these microscopic organisms like these microalgae, you can actually go out your doorstep — you don't have to go to exotic places, and you can find new things," said Melkonian, who has found new species all over the world, including in Canada.

By looking at the genes found in this algae, a related species, and land plants but not other algae, the researchers reasoned they could learn more about the transition to land.

Sure enough, they found a family of genes known to be important for plants to survive dryness and life on land. Then they did a search through gene databases to see where the genes might have originated.

"And the only other place they could be found… was in soil bacteria," said Wong, who leads a larger project that aims to catalogue the genomes of all plants.

Natural GMOs?

But how did the genes get from the soil bacteria into the algae? 

Wong says like animal-like single-celled organisms such as ameobas, Spirogloea eats soil bacteria.In fact, he said, one individual the researchers tried to analyze had eaten so many bacteria that its own DNA couldn't be extracted because there was too much bacterial DNA in the sample.

That said, the researchers know the genes were now part of the algae's genome and weren't bacterial contamination because they're interrupted by chunks of DNA called "introns," which are common in the genes of complex organisms, but don't occur in bacteria.

"It's surprising," Michael Melkonian said, "because horizontal gene transfer was believed to occur among bacteria, and not really between bacteria and eukaryotes [organisms with complex cells]."

He added that when that kind of gene transfer is seen, it typically happens in places where complex organisms need to adapt to new environments, such as hot springs.

Wong said it also seems to happen mainly in more primitive complex organisms, as larger organisms have more cell walls and membranes that make it hard for bacteria genes to get into.

Still, he said, the fact it happens at all in nature may have something to say about genetic engineering and genetically modified organisms (GMOs).

"It's not as unnatural as people believe."

Andrew Roger, an evolutionary biologist at Halifax's Dalhousie University who was not involved in the study, said the new paper builds a reasonable case that two of the gene families in plants involved with coping with environmental and biological stress originally came from soil bacteria — and that horizontal gene transfer was involved in the "major evolutionary transition" from which land plants originated.

The role of that kind of gene transfer in the evolution of eukaryotes like plants and animals has been debated, he said in an email.

"But this finding coupled with a number of other recent findings within other major kingdoms of eukaryotes shows that it truly is an important mechanism by which complex life evolves."