Fungal Dispersal Mechanisms

Nearly everyone has seen mushrooms poking their frilled caps up through the litter in a forest, and probably many have assumed they were plants. They are not; they are the fruiting bodies of a fungus, and their sole purpose is to produce and distribute spores.

Fungi, members of the kingdom that includes mushrooms, yeasts, molds, and rusts, are not plants (or animals), but many are fixed in place the way plants are. Since some can’t move around to spread their spores, and others require very particular environments to thrive, fungi have evolved unique dispersal mechanisms for their genetic material.

Philobus crystallinus is a fungus that lives in animal dung. It propels its sporangiospores (spores formed inside a spore case) on a water jet (whose initial pressure has built up to 100 pounds per square inch) a distance up to six feet, quite rapidly. This water jet is actually aimed toward the lightest area in sight, so that the spores land on grass or leaves where they can be eaten by a grazing animal such as a horse and germinate in dung.

Stinkhorn mushrooms produce a sticky mass on the top of their cap which smells like garbage, rotting flesh, or dung. This passive technique attracts flies which pick up the spores on their legs and disperse them. The fast-growing common stinkhorn, Phallus impudicus, is a remarkable sight in the woods and fields of America
and Europe.

The Cannonball Fungus, Sphaerobolus stellatus, actively shoots a peridiole (spore container) out of a cup, sending it as far as 6 meters, to a spot where it will stick and its spores will grow. The young Cannonball fruiting body is a sphere, with an inner and outer layer. As it matures, the top splits open into 4 to 8 petals or teeth, so that it looks like a sort of an idealized tulip, although only 2 millimeters in diameter. Eventually, when fully open, it looks like a cup with toothed edges, with a reddish ball sitting in some liquid in the center. The red ball is the peridiole, soon to be launched. To launch it, the cup suddenly turns its inner layer inside out, with great force. The inversion of the inner membrane of the cup takes at most a thousandth of a second. The initial speed of the peridiole is 3-5 meters a second.

The Birds Nest fungus also distributes its spores in peridioles
from a cup, but its method of spore dispersal is passive. The open cups that contain ripe spores in the Birds Nest are so shaped that raindrops striking them force the peridioles out the way streams of water in a kitchen sink are sprayed sharply out of a teaspoon. The peridioles ride this stream and end up, depending on the species, as much as 2 meters from the parent.

Basidiomycete yeast Itersonilia perplexans is a fungus that generates
ballistospores. The spores form at the tips of sterigmas, launchers. When the spore is ready to be launched, a drop of fluid, called Buller’s drop, forms at the base of the spore, and a separate drop forms on the side of the spore.

Buller’s drop grows for a few seconds and then both the drop and the spore disappear. They have been ejected from the fungus at an acceleration of more than 10,000 gees. That is, the spore accelerates so fast that the force on it is like 10,000 times the gravity of earth. It is not clear how this works. Mycologists currently believe that the launcher is a sort of catapult which uses the release of the energy of the surface tension in the Buller’s drop to propel the spore into flight. About 30,000 species of fungi produce some kind of ballistospore.

Puffballs and Earthstars use passive methods to distribute their spores. Each mushroom is a flexible ball that is filled with loose spores and has a small opening on top. The touch of an animal or even the wind can compress part of the ball and force a spray of spores out into the air, where they form a dusty cloud that is spread by the wind. Some mushrooms of this type have a relatively fragile shell that degrades readily, leaving a soft pile of spores to float away.

Chytridiomycota, called chytrids, may be the first type of fungi to evolve that still exists. There are about one thousand known species and almost all are aquatic, as all the first fungi are believed to have been. Their reproductive stage is the flagellated gamete, which uses its flagella (whip-tail) to swim off to become a new chytrid.

Spores may be released actively, when a fungus flings them away, or passively, when some other agent such as an animal, wind, or water spreads them. Active spore dispersal ensures that the spores are at the correct stage for release, but passive dispersal saves the fungus energy, which it may use to produce more or higher quality descendants. In either case, fungal dispersal mechanisms are fascinating examples of the ingenuity of life.