Spore Dispersal in Fungi

Introduction

When I was an undergraduate and long before that time, Fungi were classified as plants. While we no longer classify them as such, Fungi and plants have one characteristic in common that should be addressed. The vegetative growth in both are not motile and they are attached to the substrate on which they are growing. Thus, individual plants and Fungi must spend their entire lives rooted in one place. This may not sound interesting and even boring, but this is actually the most interesting quality of plants and Fungi. They have evolved mechanisms that have allowed them to get around this handicap. Plants for example are able to photosynthesize and are able to make their own food, thereby eliminating the need to "hunt" for their food. Fungi being heterotrophs have resolved the problem of their sedentary existence by living on/within their food source. Their inability to move has also led to interesting mechanisms by which they can sexually reproduce, which is the subject of this topic.

There are approximately 100,000 species of Fungi. This number is small relative to other organisms. However, Fungi are found just about everywhere in the world because their spores, both sexual and asexual, are produced in large numbers and have evolved interesting mechanisms by which they disperse their spores. One reason that they seem to occur everywhere is that they produce large number of spores that often can be dispersed long distances by different mechanisms that have evolved. Moreover, fungal spores also have other attributes that ensure their survival. Spores are often less susceptible to adverse environmental conditions than the mycelium or yeast cells  and germination of spores oftentimes will not occur until environmental conditions are optimal for their survival.

How Many Spores do Fungi Produce?

When Carl Sagan, the noted astronomer, spoke of the numbers of stars in galaxies, he is always stereotypically thought of as saying that there are "billions and billions" of stars (He actually never used this catch phrase but was associated with it by its use from various comedians who satirized his use of the word "billion" which he did use quite frequently, but never "billions and billions"). Sagan's use of billions usually referred to the number of galaxies and stars, but it is number less frequently used with respect to number of organisms. For example, the world's human population at approximately 6.5 billion, but probably no other mammal exceeds a billion in number. However, observations by researchers have observed that billion can readily apply to the number of spores of some species of fungi. Some examples will be given here to demonstrate the enormous numbers of spores in some species of fungi. Ustilago maydis, the Corn Smut (Figs. 1-2) may infect any part of the corn plant.  When infected, the corn plant will have black galls of various sizes. A gall that is about 1in3 may contain approximately 25 billion spores (Ingold, 1965)! Multiply that by all of the galls that may be present in a single corn plant and we will literally "have billions and billions" of spores. By far the species that produce the largest number of spores are those in which spores are borne in fruiting bodies. Ganoderma applanatum, the Artist Fungus (Fig. 3), a bracket fungus, produces a perennial fruiting body, which may disperse approximately 30 billion spores a day and maintain this rate for a five month period. The basidiospores of this species are borne, inside the pores, on the lower surface of the fruiting body (Fig. 4). The giant puff ball, Calvatia gigantea, has been estimated to contain up to 7 trillion spores.

Figure 1: Two corn smut galls of Ustilago maydis on corn cob. Figure 2: Numerous corn smut galls on corn cob. Figure 3: Ganoderma applanatum fruiting body growing on a live tree. Figure 4: Pores typical of Ganoderma applanatum fruiting body. Figure 5: The giant puffball, Calvatia gigantea, is 60 cm or more in its longest dimension.

 

Figure 5: Sporangia of Rhizopus stolonifer. Figure 6: Penicillium notatum colony approximately 2.5 cm in diameter. Figure 7: Penicillium sp. conidia and conidiophores.

And while the number of spores produced by microscopic fungi are not of this magnitude, they are nevertheless still significant in the numbers that are produced.  Rhizopus stolonifer, the common bread mold, can be recognized by the grayish, aerial mycelium covered with numerous black "dots". These dots are the sporangia containing spores of this fungus. Each sporangium contains upwards of 50,000 spores (Christensen, 1965)! A single spore grown from this species, in three to four days, will produce hundreds of millions of spores. Many species of microscopic fungi are capable of producing comparable number of spores. A culture of a species of Penicillium, with a colony approximately 2.5 cm in diameter (Fig. 6) may produce up to 400,000,000 conidia.

If even a small fraction of these spores were to survive and produce more fungi, the world would be entirely covered with fungi. Since we are not, it is safe to assume that most of those billions of spores produced will not live long enough to reproduce themselves or perhaps even reach a food source that will even allow for germination of the spores. However, there are more than enough spores that survive to make the world the moldy place that it is.

Non-Dispersed Spores

The majority of fungal spores are dispersed, away from the parent mycelium, where they are more likely to have a fresh food supply thereby increasing their chances for survival. However, some fungi may produce resting spores that are not dispersed, but instead, when conditions become unfavorable for growth of the parent fungi, these resting structures weather these conditions and remain viable due to the thick walls that are present in such spores. Some common examples of thick walled resting spores include the zygospores in the Zygomycota (Fig. 7), teliospores in the Uredinomycetes (Fig. 8) and resistant sporangia in Allomyces (Fig. 9)

Figure 7: Thick-walled zygosporangium, of Zygomycota, containing zygospore Figure 8: Thick-walled teliospore of Puccinia coronata overwinters. Figure 9: Resistant sporangium of Allomyces arbuscula remains viable when rest of thallus dies off. 

Dispersal Mechanisms

The production of large number of spores is only partly responsible for the abundance of fungi around us, in order for them to become widespread, there must also be mechanisms by which they can disperse their spores, over long distances. There is also a practical reason why it is important for fungi to disperse their spores. If spores germinate and grow among the parent mycelium, food will become limiting since the new mycelium must now share the food and the probability of survival will not be as great. Some of the different mechanisms by which fungi disperse their spores are quite ingenious.

Air Borne Spores

The most common means, by far, that most fungi have of dispersing their spores is by riding air currents. The wind dispersed fungi often produce what are referred to as dry spores. These spores do not readily soak up water and when clusters of these spores are splattered by water, as may often occur in those fungi that produce their spores directly on their mycelium, rather than absorbing the water, the impact dislodges the spores and scatters them into the wind. Because these spores do not readily absorb water, they are said to be hydrophobic. Although this may not be very intuitive, the initial resistance of these spores to water makes a great deal of sense. The absorption of water by spores would give them extra weight, making it more difficult for them to stay afloat. The majority of the known species of fungi disperse their spores by wind.

The importance of airborne reproductive propagules, to which I am including all spore producers, e.g., algae, fungi and plants, as well as pollen and other air borne organisms, is such that there is a discipline, aerobiology, that is dedicated to their study. One aspect that has been much studied is the cause of allergies by organisms in the air. Fungal spores are the cause of a significant number of allergies each year. Unlike pollen, however, it was not until 1924 that fungal spores were thought to cause respiratory allergies. Specifically, Puccinia graminis, the wheat rust was identified to be the cause of allergies of three Canadian who were threshing wheat, but further studies indicated that the allergies were caused by mold spores of Cladosporium, Alternaria and Penicillium spores (Feinberg, 1946). Wherever spores have been monitored, an abundance of these genera may be observed. In 1937, in Minnesota and the Dakotas, it was estimated that thousands of tons of spores of Cladosporium and Penicillium were present in the air that blew eastward into the ocean and may possibly have blown across the Atlantic (Feinberg, 1946). Considering the size of spores and the fact that there was an estimated thousands of tons of spores, the number of spores present must have been astronomical.  It is not any wonder then that the above two genera of fungi are significant factors in the cause of allergies.

Although simple, the efficiency dispersal of air borne spores should not be underestimated since most fungi utilize this method to disseminate their spores. An air sample may contain as many as 200,000 spores/meter3, but just how effective is dispersal of airborne fungal spores? Since air-borne spores cannot be seen, it is difficult to appreciate the number of spores that are in the air. However, in order to give you an idea as to the number of spores that are in the air, let us make an indirect comparison with small air-borne objects that are visible to the naked eye. The ability of any small objects to stay afloat can be readily observed.  When we look at the morning or afternoon sun shining through a window, in a room, where the air is still, numerous small particulate pieces of "lint" or dusts, in the light beam can be observed to be kept afloat by the convection of heat generated by the light beam. So it should not be difficult to imagine that spores, which are far smaller and lighter, would and probably are also present in such a light beam. The extent that spores can travel indoors where the air is still was nicely demonstrated with an experiment carried out by Dr. Clyde Christensen (1975), at the University of Minnesota St. Paul Campus, in the plant pathology building.

The Preliminary Experiment

The experiment used Cladosporium resinae as a "marker fungus" whose spores are not usually found in the air. In nature this fungus is found only in resin permeated soil, and in wood that has been impregnated with coal tar creosote in order to protect them from decay, such as telephone poles and railroad ties.  Because of its requirement for creosote, a "selective medium" containing this compound is not only required for growth of C. resinae, but at the same time, will prevent the growth of other common air-borne or soil fungi.

Christensen demonstrated the selectiveness of this medium by inoculating decaying plant material with known fungi, and soil samples, on campus, infested with common and uncommon fungi into creosote agar plate medium (Fig. 10). Neither the fungi known to be infecting the plant material and fungi present in the soil samples were able to grow on this selective medium, nor was C. resinae recovered, indicating that this species was absent in these substrates.

Figure 10: Experiment demonstrating selectivity of Creosote Agar medium and absence of Cladosporium resinae in soil and on plants.

Tests were also carried out by exposure of blocks of creosote impregnated wood and agar plates to the air around and inside the plant pathology building. Again, C. resinae was not recovered from the substrate and agar, indicating that spores of this species were not air-borne in this area (Fig. 11).

Figure 11: Cladosporium resinae was not recovered from creosote agar plates and impregnated wood, from air and experiment sight, indicating its absence in experiment sight and area around sight.

The Cladosporium resinae Experiment

The four storied, plant pathology building, in which the experiment was carried out, has stairways at each end, with a hallway in the middle of each floor and does not have a central ventilating system. In testing the extent to which the C. resinae spores could remain afloat in the still air of the plant pathology building, agar plates with coal tar creosote were exposed throughout the building. A culture of C. resinae was then placed on the first floor hallway and the spores of the fungus were then brushed off the agar surface into the air of the building. Remember that Christensen had earlier exposed plates of the creosote agar prior to dispersing the spores and had not recover C. resinae. Therefore, any plates that were now discovered to have this fungus growing on it would be due to the brush dispersal of C. resinae, by Christensen, to that part of the building.  Two of the several tests that were carried out are summarized in Tables 1 and 2. Table 1 summarizes the number of colonies recovered on creosote plates exposed at successive five minute intervals. In Table 2, seven sets of plates were exposed at each location for intervals of 0-5, 5-10, 10-20, 20-30, 30-60, 60-120 and 120-240 minutes. All plates were incubated and later examined for the number of colonies of the fungus formed on each plate. Colonies were recorded because it is assumed here that each colony was produced from a single spore. Plates with colonies of C. resinae were isolated throughout the buildings where the creosote plates were placed. As might be expected, there were generally more colonies on plates closest to the source, i.e. on the first floor, where the spores were dispersed and fewer occurred on those plates that were placed on the upper floors, more colonies were recovered in the hallway than in the rooms on the same floor and more colonies were recovered in those rooms with open doors than those with close doors.
 

Table 1. Number of colonies of Cladosporium resinae recovered on creosote plates in successive five-minute periods on the second, third and fourth floors after spores were liberated in the first-floor hallway.
 

Number of colonies/plate

Floor

First 5 Minutes Second 5 Minutes Third 5 Minutes Fourth 5 Minutes
2 85 70 27 20
3 20 33 9 16
4 4 17 10 7

 
 

Table 2. Number of colonies of Cladosporium resinae recovered on creosote agar plates exposed for different periods of time after liberating spores in room on first floor

Number of colonies/dish exposed for given period of times (in minutes)

Location of exposed plates 0-5 5-10 10-20 20-30 30-60 60-120 120-240 Total
First floor hall 228 49 43 31 26 15 5 397
Second floor room, door closed 0 0 1 1 3 10 2 17
Third floor greenhouse connected to the main building by a 30 foot passage 0 0 0 0 1 0 1 2
Third floor laboratory, door open 8 0 5 19 55 28 8 123
Third floor hall 0 8 70 35 44 12 0 169
Fourth floor hall 68 78 50 34 51 7 3 291
Fourth floor room, door open 0 0 2 3 7 14 5 31

Total

304 135 171 123 187 86 24 1,030

Christensen's demonstration not only showed us the remarkable ability of fungal spores to disperse themselves in the still air of a building, but also serves to remind us that if you hear someone sneezing in a building, on the floor below you, they are literally sneezing their germs right in your face.

Another experiment that you can carry out to demonstrate fungal spores ability to stay afloat can be done with a mature mushroom and an elongated cardboard box approximately 10" high and a yard long (Fig. 12). The basidiospores from the mushrooms are initially "shot off" from the basidia to the area between the gills (recall from the last lecture that these structures are characteristic of the Basidiomycota). Under these conditions it would be expected that all of the spores would drop directly below the cap of the mushroom. Although most spores will fall directly beneath the cap of the mushroom, some will manage to stay afloat and travel the length of the box, a yard away (Figure 13).
 

Experiment1.jpg (4206 bytes) Experiment1b_Top.GIF (7339 bytes)
Figure 12: Mushroom spore dispersal in a covered cardboard box without air circulation. The spores will land on the cardboard bottom where we can record the number of spores. Figure 13: Top view of spores on the bottom of the box. The greatest number of spores landed directly below the mushroom, as you might expect, but some spores stayed afloat until reaching the other end of the box.

 

coprinus1a.GIF (76272 bytes) Spore Dispersal.gif (542797 bytes)
Figure 14: A section through a mushroom gill showing the path of two spores that have been ejected between the gills and away from the mushroom. Figure 15: A magnified animation showing the ejection of a single spore from the basidium to an area between the gills. 

The movement of spores indoors is of significance to the aerobiologist. Most of us may not suffer from any respiratory symptoms while outdoors, but once inside we may suffer from difficulty in breathing, have sniffles and perhaps even feel ill. This may occur at work in air-conditioned buildings or at home. Symptoms are more likely to occur at the latter when you are sweeping and vacuuming causing the dust in your house to shift around. It is commonly believed that it is the dust that is the sole cause of your misery, but instead it is more probable that the cause is due to fungal spores (more about this topic later).

There was one extreme case of fungi over running a house that occurred in Mānoa. The owner of the house called me to ask my advise as to what he should do because of an apparent fungal allergy that had become progressively worst over time. The owner's brick planter was cracked and he was in the process of repairing it. However, , each time he approached the planter his eyes became watery and he had difficulty breathing. Finally, he hired someone to demolish the old planter and rebuild it. Upon demolishing the old planter, he discovered that there was a large fungal infestation that was probably causing his illness. Having discovered this, he had the parts of the planter taken away and believed his troubles to be over. Unfortunately, they had just began. Shortly after the planter was demolished, his symptoms grew worst and soon he was unable to enter his house without having these symptoms. That was when he called me to inquire as to what happened. I explained to him that in demolishing the planter, the fungal spores became scattered throughout his house and that with the high humidity in Mānoa, the fungus growth was probably now growing throughout his house instead of being restricted to his planter.

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 (Figs. 14-15). 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 phylum, fruiting bodies are produced that bear ascospores, in asci (Figs. 16-17). The ascospores are forcibly ejected through the top of the asci and are then carried away by wind (Fig.18): 

Figure 16: Fruiting body of Pseudoplectania, a member of the Ascomycota Figure 17: Longitudinal section through fruiting body showing numerous asci & ascospores
Figure 18: 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 (Figs. 18-19). 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".

Lycoperdon Dispersal.gif (416956 bytes) Geastrum.GIF (62734 bytes)
Figure 18: Animated Lycoperdon showing spore puffing from ostiole from a pencil eraser applying external pressure on peridium Figure 19: 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 (Fig. 20). 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. 

Figure 20: 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 (Figure 21). 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.

 

cyathus.GIF (59558 bytes)
Figure 21: 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 (Figs. 22-23).

Fly Dispersal.gif (479537 bytes)
Figure 22: Phallus rubicundus, a stinkhorn, has a strong, foetid odor that attracts flies when the spores are mature. Slimy apical portion contains spores Figure 23: 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 here so that you can read it at your leisure.

Active Mechanisms of Spore Release

Active mechanism of spore release refer to those fungi that are able to eject their spores using energy from within their mycelium or fruitbody. These can be divided into subcategories:

Explosive Mechanisms: A common means of forcible spore discharge is that in which a cell contains a large vacuole through which greater and greater pressure is applied as water comes into the cell. Eventually, the vacuole will explode releasing the spores in a jet stream of water and the fruiting structure collapses. You saw an example of such a fungus in the video, The Moldy World About Us.

The genus Pilobolus (Figure 12) forcibly ejects an entire Sporangium (Zygomycota) approximately 4 feet away and the swollen portion of the sporangium also serves as a light receptor that curves the stalk of the sporangium so that it shoots the spores in the direction of the light. The spores of the sporangium are in a slime layer so that the entire mass of spores are dispersed as a unit. As you recall the distance here is important because Pilobolus is a dung inhabiting species and the spores must be dispersed beyond the dung heap that it is growing on because the cow will not graze within a certain distance of their dung heaps. And it is important that the cow eat the spores, in order that they will go through their digestive system, and come out with the dung to start the next generation. An animated "gif" demonstrating dispersal in this genus can be found here that was created by Bob Fogel, at the University of Michigan:

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Figure 24: Pilobolus, the howitzer, ejects its entire sporangium.

Animal Dispersal

By examining the fruitbodies of some puffballs, there do not always appear an obvious means of dispersal (Figs. 25-26). In some genera, it is believed that such puffballs are adapted for dispersal by animals. Foraging animals often consume these fruiting bodies. The spores of these fungi are able to pass through their digestive systems unharmed and will be dispersed in their fecal material.  

Rhizopogon.GIF (65414 bytes)
Figure 25: Rhizopogon, a puffball that does not have an obvious dispersal mechanism is thought to be dispersed by small mammals. Figure 26: Tuber melanosporum (truffle), the most expensive fungus in the world is animal dispersed

Speaking of fecal material, there are a large number of species that grow only on animal dung. and utilize the dung as their food. 

I hope that you can now appreciate the means by which fungi are able to get around. There are more interesting mechanisms and we do not have time to cover these others. The mechanisms mechanisms covered represent the most common I have included a few of the more interesting ones.

Literature Cited and Further Readings

Christensen, C.M. 1975. Molds, Mushrooms, and Mycotoxins. University of Minnesota Press, St. Paul.

Feinberg, S.M.  1946. Allergy in Practice. 2nd. Ed. Chicago. The Yearbook Publisher, Chicago.

Ingold, C. T. 1965. Spore liberation. Clarendon Press, Oxford. 210 pp.

Raper, J. R., Krongelb, G. S. & Baxter, M. G. 1958. The number and distribution of incompatibility factors in Schizophyllum commune. American Nat. 92, 221-232.

Important Terms and Concepts 

Aerobiology (âr´o-bě-ňl´e-ję) noun
The study of the sources, dispersion, and effects of airborne biological materials, such as pollen, spores, and microorganisms.

- aer´obiolog´ical (-e-lňj´î-kel) adjective
- aer´obiolog´ically adverb

The American Heritage® Dictionary of the English Language, Third Edition copyright © 1992
by Houghton Mifflin Company. Electronic version licensed from InfoSoft International, Inc. All rights reserved.

Dry spores: Referring to air-borne spores that are hydrophilic. Their ability to not take up water enhances their ability to remain air-borne

Funiculus: Elastic cord, of peridiole of bird's nest fungus, functions in wrapping peridiole around substrate anchored by sticky hapteron at base of cord.

Hapteron: Stick portion of end of funiculus that serves in anchoring peridiole to substrate.

Hydrophilic (hě´dre-fîl´îk) adjective

Having an affinity for water; readily absorbing or dissolving in water.

- hy´drophile´ (-fěl´) noun

- hy´drophilic´ity (-fe-lîs´î-tę) noun
The American Heritage® Dictionary of the English Language, Third Edition copyright © 1992
by Houghton Mifflin Company. Electronic version licensed from InfoSoft International, Inc. All rights reserved.
Hydrophobic (hě´dre-fo´bîk, -fňb´îk) adjective

1.  Repelling, tending not to combine with, or incapable of dissolving in water.

2.  Of or exhibiting hydrophobia.

- hy´drophobic´ity (-bîs´î-tę) noun
The American Heritage® Dictionary of the English Language, Third Edition copyright © 1992
by Houghton Mifflin Company. Electronic version licensed from InfoSoft International, Inc. All rights reserved.
 

Marker Fungus: a fungus such as Cladosporium resinae that has specific nutritional requirements so that it can be selected for in a given medium and at the same time exclude other species of fungi utilizing the same medium. 

Ostiole: A small opening of a fruiting body. Used here to refer to the opening in the peridium of puffballs.

Peridiole: Egg-shaped structure containing basidiospores, in bird's nest fungi.

Peridium: Outer layer of puffball fruitbody that encloses the basidiospores.

Pseudoflower: Vegetative leaves, stimulated by the rust fungus, Puccinia monoica, to morphologically mimics a flower, thereby attracting insect pollinators that complete the sexual life cycle of the fungus

Questions to Think About

  1. Relative to other organisms, the number of species is not large, but fungi appear to be everywhere. What characteristics do fungi have that enable them to be just about everywhere?

  2. Although fungi have many mechanisms by which they disperse their spores, wind dispersal is the most common. Describe some of the mechanisms that are used by fungi to propel their spores into the air so that they can be carried away by wind.

  3. The term hydrophobic refers to spores that repel water. This is a characteristic of wind dispersed spores. If that is the case, how does it absorb water so that they can grow.

  4.  Some fungi do not utilize wind to disperse their spores, at all. Describe some mechanisms of spore dispersal that some fungi have that do not utilize wind.

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