Ancient algal mixup sorted
Sequencing one of Carl Linnaeus's seaweeds shows modern samples aren't what we thought.
As the year of Carl Linnaeus's 300th birthday draws to a close, researchers in Northern Ireland have for the first time sequenced the DNA of a specimen gathered by the 'father of taxonomy' himself — and used it to solve an algal enigma.
In 1753, Linnaeus described a type of seaweed commonly called sea lettuce, named it Ulva lactuca, and put a sample, or 'type specimen', into his herbarium. Today, much of the similar-looking translucent green seaweed found on European shores is given that name. But Christine Maggs, a seaweed expert (phycologist) and taxonomist at the Queen's University Belfast, reckoned that this sea lettuce looked subtly different from the specimen Linnaeus had gathered in 1753. With the help of graduate student Frédéric Mineur, she turned to DNA forensics to determine the possible imposter’s true identity.
The Linnean Society of London gave them a precious fragment of the original type specimen — LINN 1275-24 — from which they extracted the DNA and sequenced the rbcL gene, which encodes a protein that is important in photosynthesis and is often used to work out evolutionary, or phylogenetic, relationships between plants.
As Maggs suspected, what most scientists know as Ulva lactuca is in fact another species.
"The specimens called Ulva lactuca over the past 200 years, and all the specimens that were sequenced and had DNA sequences deposited in GenBank, are not the same species as Linnaeus's Ulva lactuca," says Maggs, who is preparing the work for publication. She thinks that what scientists call Ulva lactuca might instead be what was called Ulva stipitata by another botanist. But further work needs to be done to clarify matters, she adds.
"It's the first time that herbarium material from Linnaeus’s own herbarium at the Linnean Society of London has been sequenced," says Charlie Jarvis, a researcher at London's Natural History Museum and the society's curator of plants.
There were fears, says Jarvis, that that the dried type specimen might never reveal its DNA secrets. Not only was it old, but it had been soaked in water in the 1960s for scientists to get a better look at it, and then redried. "We thought it might not yield usable DNA, so everyone was delighted when this proved not to be the case," he says.
It's not the first time that Maggs has notched up a success in using DNA to sort out the nomenclatural mess that exists in many seaweed groups. Scientists, it seems, sometimes don't know their Ulva from their elbow. In 2003, Maggs and co-workers showed1 that the genus Enteromorpha, comprising tubular green algae that look very different to the flat-fronded Ulva, and which was given its name in 1820, is in fact part of the genus Ulva, as Linnaeus had said all along.
Phycologists are increasingly turning to genetics to sort out algal taxonomy, says Robert Waaland, a seaweed scientist at Washington University. "Chris Maggs has certainly done a splendid job of integrating molecular phylogenetic analyses with classical and life-history studies," he says.
"It is a technique that can be used to resolve certain nomenclatural questions, provided the taxonomic background is well understood, as was the case here," adds Jarvis.
In a related event, Linnaeus’s herbarium itself is moving into the twenty-first century by going online. In November, the society opened a website offering digital images of its 14,300 specimens — the largest and most important of Linnaeus’s plant collections, which includes 4,000 species from Asia, Europe and the Americas.
- Hayden, H. et al., Linnaeus was right all along: Ulva and Enteromorpha are not distinct genera. Eur. J. Phycol. (August 2003), 38: 277 – 294.