Pilobolus: the Shotgun Fungus

Pilobolus. the

CHARLES R. COBLE

CHARLES E. BLAND Downloaded from http://online.ucpress.edu/abt/article-pdf/36/4/221/369026/4444751.pdf by guest on 24 September 2021

T HE STUDY OF FUNGI in high-school biology is a rather unexciting phase of the course for most ~~~~~WI students (and teachers). Classroom and laboratory work with fungi, when it is undertaken, usually is limited to a study of the comparative morphology of the Ascomycetes and the Basidiomycetes-the higher fungi. This is understandable (and even com- mendable), because the larger varieties, such as Morchella, bracket fungi, puffballs, and mushrooms, and the more "glamorous" Penicillium, yeasts, and rusts belong to these classes. Laboratory or field experiments using fungi are X_.~ even less frequent in high-school biology. Granted, a few standard laboratory activities make use of fungi; for example, mating the plus and minus strains of the common bread mold, Rhizopus, to ob- serve zygote formation. However, from the stand- Fig. 1. Living young fruit-body of Pilobolus. Note the nu- point of exciting student interest, teachers often find merous droplets present on the subsporangial swelling and that the standard "canned" laboratory activities in- stipe. S, sporangium; SS, subsporangial swelling; ST, stipe; WD, "water droplets." Photo, by C. E. Bland. reproduced by volving fungi often fall short of their mark. permission of Mycologia. There are, however, some relatively little-known fungi that display rather interesting features and that lend themselves quite well to some open-ended experimentation by high-school students. One such fungus is Pilobolus, which is equipped with a "shotgun." ' Charles R. Coble (left) is assistant professor of science Pilobolus is a member of the same class as Rhizo- education, and Charles E. pus: the Zygomycetes. Like many other members of Bland (right) is assistant its class, Pilobolus is a saprobe and a common in- professor of biology, at East habitant of horse and cow dung. Of particular in- Carolina University, Green- ville, N.C. 27834. Coble has terest is the mechanism of spore dispersal developed _4 his A.B., M.A.T., and Ed.D. by Pilobolus, in which the entire sporangium (spore degrees from the University case) is violently shot off the sporangiophore and of North Carolina, where he taught before joining the ECU adheres to the first solid object it strikes. The spor- faculty, in 1972; earlier he taught in Charlotte, N.C., schools, angiophores are specializing in planetarium and television instruction. Bland positively phototropic; that is, they has his A.B. and Ph.D. degrees from the University of North eject their sporangia in the direction of light. Carolina, where he taught botany and plant anatomy, 1967- 69, before accepting his position at ECU. He has published Structure and Asexual nearly 20 papers in mycology and has recently prepared Reproduction (with J. N. Couch) a section on the family Actinoplanaceae for the next edition of Bergey's Manual of Determinative The fruit body (asexual reproductive structure) Bacteriology. of Pilobolus (fig. 1) consists of two main parts: the

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Fig. 2. Schematic drawing of Pilobolus, illustrating development of asexual reproductive structures. For explanation see text. sporangium and the sporangiophore. The terminally stick to any object it strikes. In nature this frequent- located, black sporangium contains numerous ly is a nearby blade of grass. Being so deposited, the spores, which under suitable conditions may germi- sporangia are in a good position to be eaten, along nate (sprout) to give rise to new plants. The highly with the grass, by a horse, cow, or other grazing specialized sporangiophore, which supports the spor- animal. The sporangia and spores pass unharmed angium, consists of the enlarged subsporangial swell- through the digestive system of the animal and are ing and the stalk, or stipe (fig. 1 and A in fig. 2). excreted. In the fresh, moist dung the spores within Usually, droplets of "water" are visible over both the subsporangial swelling and the stipe. During development, the subsporangial swelling, acting as a primitive lens, and the stipe function together in the orientation of the sporangium toward light. This is brought about by a growth response, which is triggered whenever light entering the subsporangial swelling is brought to focus at any point other than the orange-pigmented area at the top of the stipe. Therefore, through light-triggered directional growth of the stipe, the sporangium is very accu- 1k' rately aimed at the light source. On reaching maturi- ty, the sporangium is violently discharged ("shot") from the sporangiophore (B in fig. 2). This is ac- complished by means of a "water-squirt" mechanism, which is capable of projecting the sporangium to a distance of 1.8 m. On discharge, the sporangiophore is thrust against the substrate where, being no longer functional, it disintegrates (C in fig. 2). However, the sporangi- Fig. 3. Collecting dish containing freshly collected horse um, by means of a ring of mucilage at its base, will dung.

222 THE AMERICANBIOLOGY TEACHER, APRIL 1974 A

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B Fig. 4. Collecting dishes arranged to maintain moist conditions necessary for growth of Pilobolus. -r . the sporangia germinate (D in fig. 2) to give rise to rapidly growing, vegetative filaments, called hyphae. The hyphae grow throughout the dung (E in fig. 2). After two or three days' growth, asexual reproduction is initiated when enlarged, orange-colored -'6- -% structures, called trophocysts, are formed along the hyphae (F in fig. 2). It is from the trophocysts that the sporangiophore and sporangium develop. This C~~~~~~~~~~~~~~~~~~~~~~~~~

Fig. . Reslts o ligh expeiment wit ilobous. Fr ex plnto e et

developmental process, which is illustrated by G-K in fig. 2, requires about 12 hours for completion and results in synchronized maturation of the sporangia at approximately noon of the day of discharge. Fol- Fig. 5. Light experiments with Pilobolus. For explanation see lowing the first "crop" of sporangia, new but smaller text. numbers develop daily for two or three days.

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F'ig. 7. Growth response of fruit bodies of Pilobolus.

Experimenting with Pilobolus gle, centrally located opening, 4 mm in diameter, is made in the cover. Clear tape is placed over the Pilobolus can be easily obtained for observation opening, in order to catch any sporangia that make a and experimentation in the classroom or laboratory. "bull's-eye." After a sufficient number of days (2-3) For this, simply collect fresh horse or cow dung (fig. have elapsed to allow for sporangial discharge, the 3) and keep it under moist conditions (fig. 4). After covering is removed, to reveal the pattern shown in three or four days the clear mycelia, sporangio- fig. 6, A. Under these conditions the sporangia are phores, and black sporangia should have developed discharged very directly at the light source, and the and be plainly visible. A good hand magnifying glass high accuracy of the phototropic-response mechan- or dissecting microscope will reveal most of the ism can be plainly seen. If concentric lines are external anatomy of Pilobolus. drawn at regular intervals around the hole, students After examining the gross features of Pilobolus, can make some mathematical estimates of the ac- the students can undertake a number of interesting curacy of sporangial discharge. experiments with this remarkable organism. The fol- Effect of light intensity. For this experiment a lowing are suggested subjects for investigation; they collecting dish is wrapped in aluminum foil and then should serve only as guides for the development of covered with semiopaque paper in which two holes other types of experiments. of different sizes have been made (B in fig. 5). Clear Effect of a diffuse light source. In a room with tape is placed over the holes-again, to catch any diffuse lighting, the sporangia will be discharged in sporangia that make a bull's-eye. The pattern a random fashion and will adhere to the first object formed by the discharged sporangia is seen in fig. they touch. This can be observed by simply covering 6, B. Unquestionably, the brighter (larger) light a transparent collecting dish with another, similar source is shown to be more effective in bringing dish. about a phototropic response. The growth response Effect of a light source. To test the effect of a of the fruit bodies is seen in fig. 7. single-point light source on the phototropic response Effect of colors of light. To test the effect of colors of Pilobolus, a collecting dish is covered with paper and wrapped in aluminum foil (A in fig. 5). A sin- (Concluded on p. 242)

224 THE AMERICANBIOLOGY TEACHER, APRIL 1974 Today he went to see his family physician, for he troversial issues have been disregarded in education. has not been feeling too well lately. Suddenly, it Recently, educators such as Merrill Harmin, seems that everything has fallen out from under Howard Kirschenbaum, and Sid Simon (1973: him. He was told that he has incurable cancer and Clarifying Values through Subject Matter, Winston has only two years to live. Press, Inc., Minneapolis) have advocated the presen- If you were told by your doctor that you had an tation not only of facts and concepts but also of incurable disease, with only a few years to live, clarifying values. Teachers who provide this for their what would you do with the last days of your life? students aid them in examining their beliefs about Much has been written questioning whether mer- various issues and help them find a personal mean- cy killing is immoral or humane in the case of a ing in their education. patient who is suffering a great deal and is destined Other subjects that lend themselves well to eval- to die within a few days. uation techniques are ecology, organ transplantation, If one of your loved ones was a patient suffering genetic engineering, sex education, drug abuse, evo- and near death, would you feel mercy killing was lution, and biologic warfare. immoral or humane? If we want students to make responsible decisions If you were the patient who was suffering, would on the type of topics listed above, we biology teach- you want your life ended by mercy killing? ers must provide them with opportunities to think Put yourself in the place of an attorney defending about their beliefs. But it cannot be emphasized Downloaded from http://online.ucpress.edu/abt/article-pdf/36/4/221/369026/4444751.pdf by guest on 24 September 2021 a doctor accused of practicing mercy killing. Pre- enough that our place is not to force values on our pare a paragraph defending his actions. students; rather, we should supply them with a Now put yourself in the place of the prosecuting means to clarify their own. attorney. Prepare a paragraph in an attempt to con- vict the doctor of mercy killing. Charles R. Barman The above is an example of how values concern- Nathan Hale High School ing biology can affect our lives. Yet, how often are 11601 W. Lincoln Ave. value situations probed in biology instruction? Con- West Allis, Wis. 53227

Pilobolus. . . from p. 224 intensity but from opposite directions are focused on a single fruit body? What is the approximate weight of light on sporangial discharge, a collecting dish is of each sporangium, and how much force is required prepared as in the second experiment (above), ex- to achieve "lift-off"? Statistically, how accurate are cept that the two widely separated holes placed in the sporangia in reaching their target? the covering are of the same size and are covered with cellophane of two different colors-in this Acknowledgments.-Fig. 1 (photo by C. E. Bland) is repro- case, red and yellow (C in fig. 5). In our perfor- duced, with permission, from Mycologia, ref. Bland and mance of this experiment we hoped to obtain a Charles 1972. Fig. 2 (drawing by R. M. Page) is reproduced, rough measure of the effects of two wavelengths of with permission, from Science, ref. Page 1962; copyright 1962 light on the phototropic response; but the results by the American Association for the Advancement of Sci- ence. were less than dramatic (fig. 6, C). Only very slight differences can be observed in sporangia crowding REFERENCES around the two holes covered with different-colored cellophane. However, with the use of a greater Persons interested in pursuing these and other questions about Pilobolus are referred to the following variety of colors one might establish an "action publications: spectrum" for Pilobolus. BLAND, C. E., and T. M. CHARLES.1972. Fine structure of Pilobolus: surface and wall structure. Mycologia 64:774. BULLER, G. H. R. 1934. Researches on fungi. Longmans, Green, Conclusion & Co., London. Vol. 6, p. 1-189. PAGE, R. M. 1962. Light and the asexual reproduction of The simple experiments described in this paper Pilobolus. Science 138:1238. . 1964. Sporangium discharge in Pilobolus: a photo- should merely serve as a starting point for experi- graphic study. Science 146:925. mentation with Pilobolus. There are many other facets of its growth and development that lend themselves to classroom experimentation. Examples of the types of questions around which additional experiments might be designed are as follows: What is The Lesser Wonders the greatest vertical distance and horizontal distance that sporangia can be shot? What will the After marvelling at swifts and eagles, gazelles and sporangia do in the absence of light? Does gravity elephants, let us remember that the world is in- have any effect on the growth of Pilobolus? What habited chiefly by sparrows and rats. response will occur when two light sources of equal David Lack, Swifts in a Tower

242 THE AMERICANBIOLOGY TEACHER, APRIL 1974