Lichen-forming fungi and their evolutionary relationships

A lichen has two partners: a fungus and an alga, living in a relationship that benefits both. That's all we need from “symbiosis” for now. The problem is where the fungal partner fits in its family tree. Researchers already know a lot about other fungi, but these fungi have been hard to place.

Now a DNA study gives us an answer. Lichen-forming fungi are related to familiar fungi such as yeast and mushrooms. That also explains why some lichens look like mushrooms. So they aren't strange outsiders after all. My next question is how researchers figured this out, and why the connection was so difficult to establish before.

The second paragraph explains the obstacle. Fungi often live on or with another organism, so researchers can have trouble knowing whose DNA they're studying. With lichens, the two partners are especially hard to separate. The alga can be sandwiched between fungal layers, or the fungus can grow through the alga's cell walls. Picture trying to study one partner when its tissue is tangled up with the other's. The details illustrate that problem; we don't need to memorize each arrangement.

That uncertainty explains the old classification. Lichen-forming fungi were put in a separate group “by default.” That matters. Scientists hadn't established that these fungi had a completely separate evolutionary history. They couldn't yet determine where the fungi belonged. A separate category for things we can't place isn't proof that those things form their own branch.

New tools let researchers isolate the fungal DNA and compare a gene across different fungi. The study includes 75 species, 10 of them lichen-forming. The key is what that comparison accomplishes: researchers can now place varieties of lichen-forming fungi on five branches of the family tree. Those are five identified branches, not five competing guesses about where all lichens belong. More research may place other varieties on still more branches. We're discovering a spread of relationships, rather than one isolated lichen group.

The final paragraph asks what else follows from this new family tree. There was a longstanding assumption that parasites gradually become less harmful and eventually develop mutually beneficial relationships with their hosts. That direction seems understandable: destroying your host would threaten the relationship. But the writer says the new research challenges the assumption. How? Merely placing fungi on a family tree wouldn't explain that yet.

The next sentences supply the connection. Harmful and harmless fungi appear both early and late in the reconstructed history. The new layout suggests that fungi can move toward mutual benefit and later turn back toward parasitism. The history doesn't simply run from harmful to harmless. That's the broader implication of identifying where these fungi belong.

So our central idea is that new DNA research has clarified the place of lichen-forming fungi in the fungus family tree. It also changes what we can infer about how fungal relationships evolve. Keep the discovery at the center; the difficulty of separating the organisms explains why the discovery took new tools, and the final paragraph explains why the findings matter.