In the debate over genetically modified crops, one oft-said word is “unnatural.” People typically use it when describing how scientists move genes from one species into another.
But nature turns out to be its own genetic engineer. Genes have moved from one species of plant to another for millions of years.
Scientists describe a spectacular case this week in The Proceedings of the National Academy of Sciences in which ferns acquired a gene for sensing light from a moss-like plant called hornwort. Gaining the gene appears to have enabled the ferns to thrive in shady forests.
The new research builds on a 2004 study on ferns carried out by Kathleen M. Pryer of Duke University and her colleagues. They traced the evolutionary history of ferns by comparing samples of DNA from 45 fern species.
The scientists found that roughly 100 million years ago, ferns exploded into a number of new lineages. Eighty percent of today’s fern species can be traced to that evolutionary burst.
Intriguingly, these successful ferns also evolved a new kind of light-sensing protein. Known as a neochrome, it makes ferns sensitive to dim levels of light. These neochromes may have enabled ferns to thrive on shady forest floors.
In 2011, one of Dr. Pryer’s graduate students, Fay-Wei Li, set out to discover the origin of neochromes. It was possible, he speculated, that an older light sensor that was sensitive to brighter light became adapted to dim forest shade.
To find that ancient gene, Mr. Li examined the neochrome gene, and then he looked at genes for other light sensors in ferns.
But as hard as Mr. Li looked, he couldn’t find a light-sensor gene in ferns that was closely related to the neochrome gene.
“I remember walking to my adviser’s office and telling her my Ph.D. was doomed because I couldn’t figure it out,” Mr. Li said in an interview.
But just as Mr. Li was despairing, a team of scientists at the University of Alberta unveiled a new database of DNA from hundreds of plant species. Mr. Li renewed his search, searching this new cache of genes for a neochrome-like gene.
He found one. To his surprise, however, the gene was not in a fern. Instead, it belonged to a hornwort. These primitive plants, which lack roots or stems, grow in mats on damp banks or on trees. It was a strange connection to find because hornworts are only distantly related to ferns.
“The first thing that came to my mind was that this must be a contamination,” Mr. Li said. A neochrome gene must have somehow been mixed into a sample of hornwort DNA.
The only way to know for sure was to look at more hornwort DNA. Mr. Li obtained hornwort tissue from other scientists and gathered some of his own from a roadside ditch near Duke. In all five species, he found variants of the neochrome gene.
Comparing all the data, Mr. Li and his colleagues came up with an unexpected hypothesis for how ferns got their neochromes. Neochromes did not gradually evolve in ancient ferns. Instead, a single lineage of ferns picked up the neochrome gene from hornworts about 180 million years ago.
Mr. Li speculates that the transfer took place between a hornwort and a fern growing in intimate contact. Once a fern picked up the neochrome gene, his research indicates, it moved into other fern species as well. It’s possible that acquiring this gene enabled ferns to thrive in dark forests.
“The results look to be strong and convincing,” said Jeffrey D. Palmer, an evolutionary biologist at Indiana University.
Dr. Palmer has found evidence for DNA moving between plant species, as have other researchers in recent years. Those previous studies suggested that plants sometimes replaced one of their genes with a version from another species. “It’s like swapping out one part of a machine for another part made in another country,” said Dr. Palmer.
The fern study, on the other hand, shows that plants have also gained functions by acquiring new genes from other plants. And Dr. Palmer expects that Mr. Li’s study will prompt other researchers to hunt for other cases. “We’re bound to find more,” Dr. Palmer said.
Source: New York Times
Image credit: National Geographic