September 5, 2012 | 3
It’s a bit embarrassing to admit you were recently on your hands and knees excitedly filming a cow pie. But I was.
And the reason was this:
There were five or six of these polka-dotted mounds in close proximity. Gorgeous orange cup fungi on a cow pie! I’d never heard of such a thing. Most dung fungi are *not* known for their, er, comeliness. What could it be? With a little searching, I believe this fungus is somewhere in the neighborhood of the dung-loving fungus (only in mycology is that not a slur) Cheilymenia fimicola.
But even better than the orange cup fungus on the cow pie was a special trick that the orange cup fungus on the cow pie could do.
What’s going on here? It’s not a dusty cow patty. It’s a simultaneous spore discharge, AKA a “puff”.
Cup fungi belong to a group of fungi called “ascomycetes” that form their sexual spores in a (usually) long sac called an ascus (“ask-us,” plural asci “ass-eye”). That is, inside the ascus, a diploid fungal nucleus undergoes meiosis — or sexual, reductive cell division — to produce ascospores. In you, meiosis generates eggs or sperm — either four sperm, or one egg and three degenerate polar bodies. In an ascomycete, a subsequent cell division or divisions often results in 8 (usually), 16, 32 or more ascopsores.
A classic ascus is sausage-shaped, like this one, which belongs to a tasty morel mushroom, a type of large ascomycete with a compound cup head. You can count the eight ascospores in this ascus:
Incidentally, due to the filamentous fungal body plan, the spores that are the products of meiosis are ordered in most of these fungi. That is, unlike the indistinguishable groups of cells that are the product of plant and animal meiosis, the meiotic products of Neurospora and many ascomycetes are all kept in a neat row. That means it’s possible to know which spores were the product of which cell division of meiosis. That matters little for the fungus, but it mattered a great deal to some famous biologists who saw in the ascomycete Neurospora crassa a golden opportunity.
These scientists used Neurospora crassa to work out many of the fundamentals of sexual recombination or “crossing over” of homologous parental chromosomes in the mid-20th century. Recombination is one of the chief engines by which sex produces genetic variation in offspring and helps organisms adapt to environmental change. Biologists studied Neurospora for other reasons too. Edward Tatum and George Wells Beadle studied Neurospora‘s response to radiation to generate the “one gene, one enzyme” hypothesis that individual genes generally produce one individual protein. They won the Nobel Prize in Physiology or Medicine for this work in 1958.
I know about Neurospora because I studied it in college genetics class. And although we diagrammed “ordered tetrads” (the spore product sof Neurospora‘s meiosis in its skinny, order-preserving ascus) ad infinitum, no one told me what the other members of the ascomycetes looked like or were capable of until I took mycology, and finally understood what an ascus *really* was.
That is, it’s a weenie-shaped spore canon.
Here’s Bryce Kendrick describing an ascus in his classic but increasingly inaccurately named mycology text “The Fifth Kingdom”:
[The ascus] seems originally to have evolved as a tubular spore gun: an elongated cell within which, once the spores have matured, turgor pressure builds up until the tip of the cell bursts and the ascospores fly out.
The job of this gun, Kendrick says, is to get the spores into the layer of turbulent air flow above the fungus’s fruiting body. If you’re an orange dung fungus, say, you need to get your spores *off* of the non-delicious (to a cow) dung you’re living on and onto a tasty plant that a cow might eat. The solution is a gun with a pre-set pressure release valve. Some asci have elastic rings at their tips that only allow spores out once a set pressured is reached. Others have little lids called operculi that literally pop open when the pressure is sufficient (See pictures of both types here). Either way, those spores are getting launched like it’s going out of style.
In ascomycetes that build their asci inside flask-like structures called perithecia or pseudothecia, the asci take turns snaking to the little opening, poking themselves out, and firing. But some discomycetes — fungi that produce their asci in a cup-like structure in which the fertile surface is completely exposed to the environment — have learned to synchronize their firing.
Here’s the structure of a typical cup fungus, like the orange cups on the cow pie. The only difference between them and this diagram is that they lack a stalk:
Those tangly lines are the body of the fungus, which, like nearly all fungi except yeast, is made up of a series of tubular connected cells. At the spore surface (which mycologists call the “hymenium”), meiosis takes place and asci sprout up. In the diagram above, you can see the asci at various stages in meiosis. The ripe ones sporting eight spores are ready to blow.
It just so happens that in many cup fungi, a sudden change in humidity triggers a massive multiple discharge — a “puff”, like you saw above. According to Kendrick, the working theory on this is that a simultaneous discharge helps create an air current that is more likely to carry spores away. I happened to notice that this fungus was capable of puffing when I was setting up my specimens for a foray display. Someone waved a sheet of paper very near the cups I’d collected, and I suddenly saw a little cloud of spores. Perhaps some factor other than just humidity is involved? Wind currents, perhaps? The way I got the spores to puff in my video — and the way you can get obliging ones to do party tricks at home — was to blow on them suddenly. Humidity and air pressure may both be involved.
Some ascomycetes, like morels (source of the ascus pictured in this blog post), are capable of continuously firing their ascospores when conditions are right. I will leave you with this video of a fine haul of delicous morel mushrooms busily firing their asci, blissfully unaware that their spores are going to end up in a dust pan or vacuum bag, and that they are about to become dinner. Perhaps it is because the sunlit kitchen is a warm, humid place.
This video does a brilliant job, thanks to some fortuitous backlighting, of showing spores being ejected into the turbulent air layer above the mushrooms,where they are carried away on a gentle current. In the light from the sun, the discharge looks like the steam rising from a boiling cauldron filled with some sort of magic brew. The fusillade is so heavy that the guy shooting the video is, at the end of the clip, able to write with his finger in the pile of spores that has collected on the table.