Horizontal Gene Transfer Helped Ferns Survive In Low Light

By Ben Wolford on April 14, 2014 3:41 PM EDT

Horizontal gene transfer likely helped ferns, like this Haplopteris Fern, to survive in low-light areas. (Photo: Fay-Wei Li)

Horizontal gene transfer likely helped ferns, like this Haplopteris Fern, to survive in low-light areas. (Photo: Fay-Wei Li)

When bacteria grow resistant to antibiotics, they usually aren't evolving via the traditional route — that is, by accidental, beneficial mutations passed down to offspring. It happens much quicker than that by swapping genes with other organisms, a process called horizontal gene transfer. Microbes do it all the time.

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But new research on ferns suggests horizontal gene transfer is more widespread than scientists thought. In a study published today in Proceedings of the National Academy of Sciences, a team led by a Duke University doctoral student discovered that ancient ferns borrowed a gene from a moss-like plant that helped them survive in low-light areas. It was a crucial evolution at the very moment when forest canopies were blocking out sunlight and threatening ferns' energy supply. How the plants arranged the gene transfer, though, remains a mystery.

"We're actually seeing more and more incidence of horizontal gene transfer in plants, but there's no definite answer as to what mediates it," says lead author Fay-Wei Li, in a statement announcing the findings. To absorb new genes and incorporate them into DNA, two organisms must come into close contact. In most microbes, it's easy because cells are simple. But in eukaryotes, like plants and fungi, cell structures are more complex, and the genetic information is hidden inside a nucleus. To reach it would be more difficult.

The fern story begins with the 2005 naming of a new type of photoreceptor called neochrome. Most plants get their energy from sunlight at the blue end of the visible spectrum. But some mosses and algae have red light photoreceptors, which are advantageous in darker places where the blue light gets filtered out. Before Li's study, nobody knew how ferns acquired neochrome.

They began by looking through a database of genomes of other plants with neochrome that might be related to ferns. If there was a common ancestor that had the neochrome gene, then they simply inherited it. If there was no common ancestor, then that could mean that two different plants evolved the same genetic trait independently. Li found a neochrome-containing candidate in the hornwort, a cousin of moss. Tracing their ancestry, they found that ferns and hornwort had a common ancestor 400 million years ago.

But the genetic data didn't support the idea of that ancestor having the neochrome trait. In fact, the evidence more strongly suggested that the fern version of neochrome had evolved from the hornwort version about 179 million years ago. This was surprising. It would be like acquiring blue eyes when not a single person in your bloodline has ever had blue eyes — the genetic information just showed up.

Their leading theory is that hornworts gained access to ferns' relatively exposed reproductive cells. "They have no protective layer on top, no cuticle," said Duke fern expert Kathleen Pryer in the statement. "A gametophyte [which produces the reproductive cells] is also a really compact structure, and the sex organs are right there, with lots of contact with other plants that are all competing for light and space." Li is still trying to get to the bottom of the mystery; it's for his dissertation.

Whatever the case, the ferns got lucky. It was a do-or-die point in the Earth's history when canopy shade could've killed them off. Neochrome saved a lot of ferns.

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