Spore Dispersal in Fungi (continued)

Mushrooms & Puffballs

In the previous section, we have gone over experiments that demonstrate the ease with which many spores are able to stay afloat, which is an important feature if the spore is to be dispersed over long distances. There are several different mechanisms by which fungi release their spores into the air, which then allows them to be dispersed by wind. One that was mentioned, in the previous section, was the dispersal of basidiospores of mushrooms. This release is accomplished by the forcible ejection of the basidia from the basidiospores. The force that ejects the basidiospores comes about from the internal pressure that is built up in the basidia. When the basidiospores are mature, the pressure in the basidia literally shoots the basidiospores between the gills of the mushroom. Although the actual distance that the basidiospores are ejected is very short, it is enough to  allow them to drop between the gills, without getting trapped (see Figure 5a-b, on previous page). Once free of the gills, they can be carried great distances by wind, away from the parent mycelium. While this is a simple mechanism, it should not be underestimated. In one species of mushroom, Schizophyllum commune, research was carried out, by Raper, et al (1958) that genetically demonstrated that this dispersal mechanism has led to a world-wide distribution of this species. We will, however, not cover the details of this experiment since it is well beyond the scope of this course. There are other mechanisms that serve the same functions of initially ejecting the spores into the air so that they may be picked up by air currents.

A similar means of dispersal occurs in the Ascomycota. In most species in this division, fruiting bodies are produced that bear ascospores, in asci (Figures 6a & b). The ascospores are forcibly ejected through the top of the asci and are then carried away by wind (Figure 6c): 

Fig 6a: Fruiting body of Pseudoplectania, a member of the Ascomycota Fig. 6b: Longitudinal section through fruiting body showing numerous asci & ascospores
Fig. 7: Animated Ascus dispersing ascospores

An equally ingenious means by which spores are initially ejected into the air is the mechanism used by certain puffballs. Although, puffballs are also members of the Basidiomycota the basidium does not forcibly eject the basidiospores into the air to be carried away with the wind. Instead, several different mechanisms have evolved in "puffballs" to disperse the basidiospores.

The most familiar example is in those species in which the basidiospores mature within a pliable globular sac called the peridium, which is entirely closed except for the terminal ostiole where the spores will be ejected (Figures. 8a and b). The energy to eject the spores is entirely external and is usually provided by either the impact of raindrops and/or the inadvertent bumping of the peridium by small animals. The depression of the pliable peridium, usually by one of the two external force, causes the spores within to be ejected in a "puff" of smoke-like spores. Thus, the common name "puffballs".

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Fig. 8a: Animated Lycoperdon showing spore puffing from ostiole from a pencil eraser applying external pressure on peridium Fig. 8b: Geastrum, the earthstar genus is another example of a puffball with pliable peridium in which spores are puffed out the ostiole.

 

Distances That Spores are Dispersed by Wind

The distances that fungal spores are dispersed, outdoors, are equally phenomenal. Puccinia graminis (Wheat Rust) has been studied extensively because of its economic importance. The disease has probably been known since the beginning of agriculture and even today the occurrence of wheat rust results in billions of dollars in losses, annually. During the Spring, the urediospores from infected wheat plants are carried northward, from northern Mexico, into the United States, from southern Texas, over the Great Plains and into Canada. During the Fall, the urediospores are carried southward, back down into the wheat growing region where the young winter wheat is beginning to grow. Studies carried out over almost a thirty year period, have traced the path of wheat rust epidemics along this route.

Related to how far spores can travel is how high can spores be found. Not only are spores known to travel great distances, but have also are known to go up to high altitudes. In the early days of aerobiology, during the 1930s, planes flying at 10,000 feet commonly recovered fungal spores from that altitude. It is probable that they could have recovered spores at much higher altitudes, but because of the cold and the requirement of oxygen mask at higher altitudes, the scientists doing such studies were not quite as curious about fungal spores beyond 10,000 feet. Even at the altitudes in which studies were carried out, it was the graduate students that actually went up in the planes to sample for fungus spores. After their return from their great scientific effort, the graduate students involved were often welcomed back as great heroes with the graduate students usually exaggerating the significance and daring of their mission. In the 1930's, even Charles Lindbergh, in collaboration with the United States Department of Agriculture, participated in surveying spores while he was flying over the Arctic Circle. Although he was flying lower, only 3,000 feet, compared to the 10,000 feet above, Lindbergh was able to catch what was described as a "considerable number of spores". This was of interest since Lindbergh was above the open ocean far from land, giving us an indication as to how far these spores must have traveled.

More sophisticated experiments utilized balloons to find spores in still higher elevations. In 1935, the balloon Explorer II, containing a spore trapping device was released at an altitude of 71,395 feet and was set to close once the balloon reached 36,000 feet. Although only five living spores were recovered, think of the conditions in which the spores faced at elevations between 36,000-71,000 feet. The air must have been very thin at that altitude and the temperatures must have been below freezing. Wind was also measured by the Explorer II. At the elevations in which the spores were trapped, winds were measured at 40-60 miles/hours. If winds remained constant at those elevations, it was calculated that fungal spores up in this jet stream could be carried 8,400 miles in a week.

The above experiments not only provided results that demonstrated that fungal spores are capable of traveling long distances, but must and can survive adverse environmental conditions in doing so.

Although wind dispersal of fungal spores is the means by which they travel all over the world, other means of spore dispersal are also found in other fungi. Although these other mechanisms are utilized by far fewer number of species, they are nevertheless interesting mechanisms that deserve a cursory coverage.

Water Dispersed Spores

Where wind dispersed spores are hydrophobic, water dispersed readily absorb water and are said to be hydrophilic.  Water dispersed spores often produce their spores in "slime". Due to the weight of the slime and the fact that the slime masses the spores together, wind dispersal is impossible or at least impractical. What occurs in these spores is that when large amounts of water is present, during a rain or in area where there is water flowing freely, such as in a stream, the spores are carried away, passively. The spores are characteristically shaped, usually with long appendages or are coiled (Figure 9). The spores stay afloat due to the surface tension of the spore or air pockets in the spores. The major source of food for these fungi are from leaf litter and other plant material that may fall into streams. 

Fig. 9: Characteristic shapes of water dispersed spores.

There are a large number of fungi that produce flagella and are motile. However, there is probably too much emphasis placed on their motility. Considering that these spores are microscopic and have a very low food reserve, it is unlikely that even though they have motility that their flagella can take them any significant distance away from the parent mycelium. Water, however, does play a role in the dispersal of flagellated fungi. As in the case of the non-flagellated spores, water currents and flowing rainwater will disperse spores. However, their flagella do play a role in finding food. Flagellated spores are usually chemotaxic and will swim towards a chemical source.

It is difficult to know where to put some mechanisms of dispersal. There are actually more than one mechanism involved in the group of fungi known as the bird's nest fungi (Fig. 7). The common name is due to the strong resemblance that the fruiting body has with a birds nest. Prior to 1790, they were thought to be flowering plants and the eggs, which contains the spores of the fungus, were thought to be the seeds of the plant. The actual dispersal mechanism of this fungus was not discovered until the 1940's by Dr. Harold Brodie, a mycologist that would devote his career on studying this group of fungi.

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Fig. 10: Cyathus, commonly referred to as the birds nest fungus because of their resemblance to a birds nest. The "eggs" contain the spores of this fungus.

How Dr. Brodie determined the mechanism is an interesting story. However, before telling his story, let's look at the actual fruiting body of the bird's nest fungus. The 'eggs' that contain the spores are called peridioles. There are usually several peridioles per nest  attached to the inside by means of a slender connection that is folded up called a funiculus. If we moisten a peridiole and pull the funiculus out, it may stretch up to 6 to 8 inches and at its base is a stick area, the hapteron that will adhere to any surface that it touches.

Before Dr. Brodie determined the mechanism, mycologists believed that the peridioles must have been shot into the air by some explosive force generated by the fungus itself since such of mechanisms are known to occur in some groups of fungi. However, long observations failed to detect any such explosive mechanism. Brodie determined that the nest was so constructed that when a raindrop splashes into the nest, the force will eject the peridiole out of the nest up to a distance of 3-4 feet. The force of ejection causes the funiculus to unwind and if the now wet and sticky hapteron comes in contact with any object as it flies through the air, it will stick to that object. Once attached the cord stretches and winds around the object. This all takes place very quickly. The peridiole is now in contact with a substrate where it can grow or it is in a position where it may be eaten by an animal. Once in the animal, the peridiole can pass through the digestive system unharmed and grow on the dung pad.

Insect Dispersals of Spores

The most interesting dispersal mechanism can be found in the group of fungi that are commonly referred to as stinkhorns because of their unpleasant odor. These fungi produce their spores in a usually liver-brown slime, which is on top of a colorful part of the fruitbody.  When the spores are mature and exposed to the external environment, the odor of the spores will attract flies that will eat up the slime and spores thereby dispersing the fungus (Figures. 11a and b).

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Fig. 11a: Phallus rubicundus, a stinkhorn, has a strong, foetid odor that attracts flies when the spores are mature. Slimy apical portion contains spores Fig. 11b: Aseroe rubra,  The brownish slime on top of the red contain the spores. This species is the most common stinkhorn that occurs in Hawai‘i.

Another mechanism that is also very interesting that involves insect was discovered during the 1990's by Dr. Barbara Roy, now at the University of Oregon. She discovered that a plant pathogen, Puccinia monoica induces the host, usually a species of the genus Arabis, to produce a "pseudoflower". The pseudoflower is actually a modification of the leaves that mimics the appearance of the flower and attracts insect pollinators that have come to pollinate the pseudoflower. Instead what occurs is that the insect completes the sexual cycle of the fungus. An easy to read article and a nice picture may be found here. The article is in the Adobe Acrobat format (.pdf) and may be downloaded so that you can read it at your leisure.

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