Spore Guns

©Else C. Vellinga
Lab
Original publication: Mycena News, February 2007

Elfin saddles have, despite their romantic name, a violent way of sending their spores into the world. They belong to the very diverse group of fungi known as ascomycetes, which includes cup fungi, morels, earth tongues, cramp balls, and many others. The spores in this group are arranged, often in a single row, within an elastic, liquid-filled tube called an ascus (plural, asci). These balloon-like tubes stretch only lengthwise and are arranged side by side, like the bristles on a brush, in the cup of the fruitbody or on the outside of the head of the morel, elfin saddle, or earth tongue. Pressure mounts on the walls and the top of the asci during the development of the spores; when the time is ripe, the spores shoot off, along with the liquid, in one big squirt through an opening at the top. Some asci have a lid—others just a weaker spot in the ascus tip—that breaks open. You can easily see the release of the spores. Just put a ripe morel, elfin saddle, or other cup fungus in a box; let it sit for a few minutes, and when you open the box, a cloud of spores is released. More surprisingly, you can even hear the sound of the spores being fired by putting your ear close to the fruitbody! (Do not forget to wash your ear afterwards!) Expect to hear a fizzing noise, like the bursting of bubbles in a glass of soda.

Peziza

A cup fungus, Peziza violacea

Some ascomycetes, however, do not follow this general pattern. For example, there is no reason for truffles to maintain this elaborate apparatus when all they want is to have their spores eaten and dispersed by animals. Accordingly, there is no squirting or shooting, and the ascus walls just degrade.

The distances travelled by the ejected spores are small—just enough to get through the layer of still air around the surface of the asci and into nearby air currents. You can indeed pick a morel without being hurt by a bombardment of spores, but it seems wise to be a bit more careful around dung, as we’ll soon discuss.

First, one more fascinating tidbit of information: The asci direct themselves towards the sun. It has been shown that when the asci are on the steep side of a cup, they may not be able to turn far enough, so that the whole tip faces the sun; the lid of the ascus through which the spores escape is not mounted centrally on the tip, but on the edge of the tip nearest the light. The purpose of this orientation is, presumably, to have a clear line of fire.

A special case is presented by ascomycetes growing on dung. Here, the whole purpose of spore dispersal is to get spores beyond the substrate (the cowpat) into the grass, where they will be eaten by a cow or other herbivore. In due course, they emerge in a fresh dropping, where they can germinate and form new individuals.

Ascobolus

One species, Ascobolus immersus, has been a subject of study for almost a hundred years, starting with Buller in 1909. This species forms fruitbodies that are only a few millimeters across, but its asci and spores are enormous. Every day, a few asci ripen and stick halfway out of the top of the fruitbody. The spores are dark and have a sticky layer around them. This jelly serves several purposes: It keeps the spores together when they are ejected and increases the mass of the projectile; it makes the spore mass stick to the grass when it lands and keeps it there while the spores wait to be eaten; and it serves as some protection when the spores get inside the cow. The upper half of the asci of this fungus points toward the sun, guided by light coming through the top of the ascus. The spores are shot off around noon, when the sun is highest. (This works fine in temperate regions but is not a good strategy in the tropics, where the spores might be slowed by heavy air and land back on the firing ascus.) At the optimum angle, the spores may land up to 40 centimeters from the launching pad. Later, in the cow interior, the spore package dissolves and the spores disperse, to prevent inbreeding in the cowpats. (The literature mentions a record distance of 70 centimeters, but this is almost unbelievable. I have not been able to discover what the species was—or whether it had been taking steroids.)

The spores are pushed out by the huge osmotic pressure inside the ascus, caused by glycerol and, to a lesser extent, proline in the Ascobolus. The ascus sap in another species, Gibberella zeae, was also analyzed; mannitol was found to be the main component. (Mannitol is also the substance in Buller’s drop, which forms on the spores of basidiomycetes just before they launch, but that is another mechanism and another story entirely.) As the asci of Ascobolus are huge, the pressure within them just before the asci shoot their spores off can be measured. It turns out to be three atmospheres, which is comparable to the pressure in car tires. This is why you need protective clothing to approach them (just kidding)! The asci collapse as soon as their contents shoot off, and shrink to half their former size.

Another ascomycete on dung, Sordaria fimicola, has a different but equally fascinating way of getting its spores out into the world. This fungus has flask-shaped fruitbodies with very narrow necks. One by one, the asci grow into the neck and, when they have reached the top, the spores shoot off. In this species the neck of the fruitbody, like the barrel of a cannon, is aimed at the sun. After the launch, the ascus shrivels at the bottom of the flask, and the next one follows suit. Just as in the Ascobolus species, the spores are dark, covered in jelly, and are shot 10 to 15 centimeters beyond the dung into the grass.

These are just a few examples of the ingenious ways in which ascomycetes disperse their spores. For further reading, I especially recommend Ingold’s The Ballistics of Sordaria (the MSSF library has a copy) and those by Buller. Buller has particularly beautiful illustrations, and my husband tells me that Ralph Emerson’s account 25 years ago of Buller’s work is still the most memorable MSSF talk he ever heard.