Learning about lichens

October 9, 2010

If you aren’t impressed by lichens, let me try to change that. This is another in the “things I learned over a cup of coffee while ignoring the kids and trying to prepare for teaching next week” series. As usual, teaching is the excuse for the addiction (obsessive learning, not the coffee).  I have covered lichens before in different classes, but never delved into their evolution.

What are lichens?

images from Natural History Museum
Lichens are multicellular fungi that are living with a single celled algae in a symbiotic relationship. The algae is able to photosynthesize, thus bringing energy into the partnership and the fungi, well, I’m not exactly clear on that. I have been told that the fungi provides “protection”, but I am doubtful that anyone has actually tested that idea. I can imagine that the fungus brings in minerals, and that IS something you can measure, but anyhow…
The algae live within the “body” of the fungi (the “hyphae”)…. they are not inside the cells of the fungus – sort of like if you had photosynthetic organisms in your skin, creeping around the outside of your skin cells bringing you energy from the sun.
One detail that had never thought of before: the fungal partner reproduces by spores that can move to a new location, but the spores don’t carry any algae with them – how does the new fungi recruit new algae?  Is there a stage in its life cycle where it exists without its little buddy?  Questions questions questions.

The running obsession in this blog is relationships between species (thus “interspecies”), especially those that blur the distinction of what constitutes an “individual” and lichen do this magnificently. Fungi are as un-related to algae as you and I are. In fact humans share a more recent common ancestor with fungi than with algae. Think of fungi as your uncle’s children and algae as your great great grandmother’s sister’s great great grandchildren. The point is that fungi and algae are only very distantly related.

image made by me on mindmeister.com

And here is the cool part. You might think that a lichen is a lichen right? You would expect that all lichens would be more related to each other than they would be to other fungi right? Nope. It turns out that this partnership has has evolved many times. We know this because the DNA and genes of different lichen species are more related to non-lichen fungi than they are to other lichen.

Here’s the data to back that up:

image from: Gargas, A., et al (1995) Multiple origins of lichen symbiosis in fungi suggested by SSU rDNA phylogeny. Science 268: 1492-1495.

The species in green are lichen fungi and those in black are not. Notice how there are several groups of lichen and the different groups are more related to non-lichen fungi than they are to each other. In red are fungi that are pathogenic to humans. Basidiomycetes are mushroom-forming fungi and Ascomycetes contains the mold that makes penicillin (the green one that grows on old citrus fruit).

Think about that for a second: at least 5 times in history, a fungi has winked at an algae and they decided to shack up for all time.  How does that even happen?   Is this like the bacteria that grow all over and in our bodies, only more organized and exclusive?  I won’t be offended if you un-friend me on Facebook

Something that occurred to me an hour later at Costco

Actually this doesn’t mean that this evolved “at least 5 times”.  In fact the simplest explanation is that it happened once and many of the descendants of this organism dropped the partnership, although that would mean that (according to the phylogeny I show) last common ancestors of basidiomycetes (mushroom formers) and ascomycetes was a lichen, which seems more unlikely to me.  Sequencing and comparing the DNA of the algal symbionts of present day lichens would solve this: if the partnership evovlved once then all of the algal partners should show a similar pattern of ancestry as their fungal buddies.  If the algae inside present day lichen don’t show that pattern of relatedness, then it had to happen more than once.  I wonder if anyone has done that.  I wonder if we could collect and grow local lichens, invent a mechanism to tease out their algal passengers and uh, sequence some DNA.  Can the algae even survive outside their hyphael Winnebago?

If someone does this later and wins the Nobel Prize, remember – you read it here first.

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