Symbiotic Association

Lichen species provide clues to evolution
August 1, 2011
 

More than meets the eye: Lichen DNA reveals divergent evolutionary paths.

More than meets the eye: Lichen DNA reveals divergent evolutionary paths.

Lichen, those drab, fuzzy growths found on rocks and trees, aren’t as cuddly and charismatic as kangaroos or intriguing as opossums, but they could be a fungal equivalent, at least evolutionarily.

A Duke research team has found that two types of lichen that seem identical in all outward appearances and produce the same internal chemicals are in fact two different species, one living in North America and one in Australia. They’re an example of “convergent evolution,” in which two species evolve separately but end up looking very similar, like the Tasmanian wolf and the American wolf.

The lichens developed the same adaptations to survive and thrive in vastly different regions of the world. Because they show the same evolutionary patterns as marsupials and mammals, but are easier to study, they could become model organisms to further probe how mammals and other groups of organisms evolve, says Brendan Hodkinson Ph.D. ’11.

“Lichen can often seem dull and uncharismatic, but these two species turned out to be quite intriguing,” says Hodkinson, who worked in the lab of Duke lichenologist Francois Lutzoni Ph.D. ’95. “They’re like sugar gliders and flying squirrels or wombats and groundhogs. They’re fungal examples of convergent evolution.”

In 2009, Hodkinson and James Lendemer, of the New York Botanical Garden, gathered lichen samples from North American and Australian specimens preserved in herbaria at Duke and the New York Botanical Garden. They studied the organisms’ body structure and chemical composition and found no difference. But then the lichenologists looked at the organisms’ DNA, which nobody had done before.

Hodkinson and Lendemer used this analysis and computer modeling of the lichens’ evolution to digitally reconstruct a family tree, which showed that the Australian lichen evolved on a branch completely separate from the North American lichen, suggesting that the organisms are separate species.

The lichenologists describe their work and rename the North American lichen species Xanthoparmelia hypofusca, following past lichen literature, in a paper that appeared in a spring issue of the journal Bibliotheca Lichenologica.

Hodkinson adds that lichen are important for another reason. Like canaries in a noxious coal mine, lichen die when the air is unhealthy. Scientists have already seen some species disappear in Europe since the beginning of the Industrial Revolution. A few were hardy and came back when air quality improved, he says, but “we may not always be that lucky, and we could see some lichen go the way of the Tasmanian wolf—extinct.”