It is Spring! The sun is warm and strong now and days of it are strung together with lustrous rainy ones. Plants are waggling their petaled parts, exuding fragrant messages and beckoning to whatever vector makes the magic happen for them. People are shedding Winter-worn layers for brighter, lighter and smaller articles, revealing ever more of themselves to the hungry eyes and fluttering hearts of their compatriots. The air is ripe with pollen grains enough to excite any wayfaring sinus or epiglottis into a crescendo of…mucus? Hmmm. Luckily for yours truly, northeastern Spring doesn’t have such an expectorant effect and indeed, I am all too happy to be whisked away into blissful reverie by the pungent, floral-scented Atlantic breezes. Swoon!
Now I want to tell you about lichen sex. I cannot refrain from anthropomorphizing this subject, so I appreciate your forgiveness in advance. Thank you. As a burgeoning scientist, I’m not supposed to attribute human characteristics to other organisms but I think it’s helpful to have some kind of context within which to understand this fairly complicated and unusual subject. Plus, it’s far more fun to explain it in ways that appeal to our desire for relatable drama.
The most basic way to put what I have to say is this: some lichens have sex and some do not. At least, we haven’t seen them do it.
Ah-break it dowwwnn:
Sexy lichens have sex. The most outstanding thing about this evolutionary mechanism is the dynamic between the symbiotic partners–the fungi wear the bossy pants and actually suppress the reproductive abilities of their algae or cyanobacteria. Although the fungi and algae don’t have sex with each other, they are domestic partners in that they share living quarters and the algae also feeds the fungi food that it’s photosynthesized for itself. What’s more, the fungi gets to have all the reproductive fun they can muster but refuses to allow the photobiont to reproduce. No sexy time! It’s not the most fair relationship by my standards but it’s apparently working well for them, so obviously I am not an appropriate judge. It is thought that this measure ensures that the algae doesn’t evolve into less suitable genetic configurations that wouldn’t work as well for the fungus to partner up with. To complicate things a bit, let me blow your mind with this interesting, though somewhat irrelevant fact: fungi have so many sexual types that there is no male or female–if you’ve read my previous posts, you have to know how much I love this divergence from boring old binary gender systems.
As I was saying, it is only the fungus which thrusts its way through the surface of the lichen, creating structures we refer to as a “fruiting body”, which are analogous to genitals in that they release reproductive spores. It could look like these parts:
So, their spores are disseminated into the world, not just to make new genetic combinations, but specifically to search for that special someone to move in with: an algal symbiotic partner. They’re really into interspecific relationships. You see, the fungi that wants to become a lichen is dependent upon algae–it can’t live without it! Isn’t that terribly romantic? Less romantic is the fact that fungi can switch algal partners. If it doesn’t like the one it’s with, it can pick a more suitable partner either by stealing it from another lichen or acquiring it from a free-living algae community.
Non-sexy lichens do not have sex. They can also be referred to as sterile, but that makes them sound like they’re lolling about at the end of their genetic lineage…which is just not true! It might be that some of them are just very discreet and only have sex occasionally which we just haven’t seen enough of to say that they do. The ones who don’t have sex are not prudes devoted to practicing abstinence, nor are they barren curmudgeons–they simply don’t need sex to reproduce. These crafty little darlings have evolved into this lifestyle because it’s clearly successful for them.
They produce asexual propagules, which are little bundles of cells that can grow into genetic replicas–a clone, if you will. They come in a couple of forms, being isidia, which are finger-like or soredia, which are more like balls or granules. These propagules are released by the lichen into the environment, typically via water, animals or wind, and if they land on their preferred surface (we call that a “substrate”) and are surrounded in the right conditions, they can become a new lichen with the same genetic makeup. It’s kind of crazy. That’s like if you grew fingers which fell off and then grew into a whole new you! This method is not unique to lichens–some plant species also produce “pups” that can grow into new plants…investigate stonecrops, agave, yucca, cacti and spider plants for more info on how they do such magnificent things. The picture below is a succulent plant with three smaller pup propagules around it.
Unfortunately, sterile lichens don’t get much lichenology play because so many scientists aren’t willing to take the time to understand them. They’re challenging. Dunh-duh-duh-nuhnnn, James Lendemer to the rescue! James, whom you may recall from my previous post, is a lichenologist at the New York Botanical Garden who has championed the sterile crustose lichens for years now–researching them, redefining their taxonomies and even the ways in which we look at and describe them. He’s written keys for them, which are necessary to identification and thusly make it possible for us to get to know who we’re working with.
Sterile lichens are every bit as important as reproductive lichens because they provide similar services to humans and they have their own role in their environment–they deserve to be protected from habitat destruction for the very same reasons. They are extra important even, because they tell us a different story about evolution and the perpetuation of species. I, for one, am listening.