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Teaching fungal systematics
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TEACHING FUNGAL SYSTEMATICS R SEVIOUR Bendigo College of Advanced Education, PO Box 199, Bendigo, Victoria 3550, Australia
It hardly needs repeating that fungi
have an immense impact on human society, both from the massive economic losses they cause on a global basis to food production, and from their increasing importance as producers of compounds like antibiotics, organic acids and enzymes. In the current climate of encouraging applied aspects of science, this alone is probably sufficient reason for studying them and aspects of their taxonomy at undergraduate level. Yet their appeal to students on first encounter is not always immediate. It tends to be a love affair which grows from slow beginnings, and many students are left disenchanted along the way. This reflects the methods used to present the material, with its endless and often confusing terminology, and also the initial struggles many students have examining large numbers of cultures of microfungi. My experience is that they are often irretrievably discouraged by their failure to see and recognise structures under the light microscope presented so lucidly as line diagrams in textbooks. Many microbiology students also see the approaches to fungal characterisation, relying as they do on morphological features, as somewhat old-fashioned and intellectually unfulfilling. One of the most significant points to emerge from the recent discussion on the teaching of fungal systematics conducted in The Mycologist is that the student's negative response is often a reflection of the negative attitude of the
teaching staff towards the subject matter and its presentation. Students need to be shown that observing fungi and describing their life-cycles can be made challenging and attractive. This is probably more important than submerging them with information at this stage. Most fungi are photogenic, and in practical session, students may be allowed to exploit the ability of the scanning electron microscope (SEM) to obtain high resolution three-dimensional images. Even the most cynical student is excited by the prospect of being able to use, under careful guidance, an expensive and sophisticated instrument like the SEM. Each student may be given two to three carefully selected fungi from different taxonomic groups, so with a usual class size of 15-20 students, a wide range of fungal types from all the major taxa can be included. The student then has to work out, with assistance, how to culture these fungi, so they produce the structures involved in either sexual or asexual reproduction, which can then be examined at different development stages. The students also have to prepare their own specimens for SEM examination. The methods described here are very simple, and almost always successful, so in our experience most students obtain impressive results. For agar grown cultures, the simplest method is that described by Samson et al (1979), except that for safety and economic reasons we avoid osmium tetroxide as a fixative. Briefly, the method consists of plac-
73
~
Fig. 1. SEM of Allomyees. (a) Gametophyte of A. javanieus showing the hypogynous male gametangium, x 450; (b) A. maerogynus showing epigynous, male gametangium and dissolved papillae, x 400; (e) male gametes with single posterior whiplash flagellum, x 2300; (d) planogametic conjugation with larger female gamete, x 2000; (e-f) A. arbuscula sporophyte showing thin-walled zoosporangium (e) with papillae (x 1150) and thick-walled resting sporangium with rupture lines visible (x 1500).
ing the specimen in 4-6% w/v gluteraldehyde at 4°C for 24h (although specimens can be left much longer), rinsing in two changes of distilled water for 10 min each, and then dehydrating in a single step, using methoxyethanol for at least 10 min. Specimens are then washed in two changes of acetone before being critically point dried in CO2 , coated with gold and then examined in the SEM. With unicells or liquid grown
cultures, samples can be fixed in the same way but need to be packaged in containers for dehydration. The simplest system is that described by Gadd & Griffiths (1980) where membrane filters (Unipore polycarbonate filters, 13 mm diam with 0.4um pore size) are stuck with silicon adhesive to the base of 1 em sections of hollow glass-rods. Dehydration carried out using 25-100% v/v isopropanol series works well, and the membrane filters, after critical
74
Fig . 2. Thraustoth ecu clavata. (a-b) Rupture of sporangial wall and primary cysts with in the sporangium. x 400 . x 1000 ; (c-d ) gametangial contact showing branched antherid ia and oogoni a , x 450 . x 1000 .
point drying, can be easily removed before attaching them to alu minium stubs . Examples of the results obtained by our students for four of the fungi we use , Allomyces spp , Thraustoth eca d avata, Emericella variecolor and Schizosaccharom yces octosporus, are shown in Figs. 1-4 . Each student then presents the results, consisting of micrographs and a taxonomic discussion, either as a short talk or as a poster presentation to the rest of the class, who are required to take relevant notes . A total of 15 h practical time is allowed, and the students are third-year microbiologists who have already been given some general information on fungal biology as part of a second-year 'Microbiologycrm rse ' .
It is not designed to turn out professional fungal taxonomists who, in my opinion, are better developed at postgraduate level. Its .main aim is to overcome th e resistance barriers to fungal systematics by both staff (in particular) and students, so that some longer lasting relationship with the fungi can develop. Our experience over th e past six years gives us some cause for optimism. It or something similar might be worth trying. REFERENCES
M & GR IFFITHS. A J (1980). Effect of cop per on morphology of Aureobasi dium pullulans . Tran sactions of the British Mycologica l Society 74, 387-392. SA MSON, R A , STALPERS, J A & VERK EREKE, W (1979). A simplified techn iqu e to pr epare funga l speci me ns for scann ing elec tron m inmsnnnv . Cvtohios 24 .7-11. G ADO. G
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Fig . 3. Emeri cella variecolor. (a) Cleistotheca with rupture line and associated liberated ascospores , x 90; (b) Hulle-cells , x 1100; (c-d) ascospores with paired stellate flanges , x 4500 , x 10,000.
Fig . 4. Schi zosoccharomyces octo sporus. (a) Vegetative cells mating, x 7000 ; (b) conjugation canal between two mat ed cells, x 4000 ; (c) four-spored ascus, x 4000 ; (d) eight-spored ascu s , x 4500 .
TEACHING FUNGAL SYSTEMATICS R SEVIOUR Bendigo College of Advanced Education, PO Box 199, Bendigo, Victoria 3550, Australia
It hardly needs repeating that fungi
have an immense impact on human society, both from the massive economic losses they cause on a global basis to food production, and from their increasing importance as producers of compounds like antibiotics, organic acids and enzymes. In the current climate of encouraging applied aspects of science, this alone is probably sufficient reason for studying them and aspects of their taxonomy at undergraduate level. Yet their appeal to students on first encounter is not always immediate. It tends to be a love affair which grows from slow beginnings, and many students are left disenchanted along the way. This reflects the methods used to present the material, with its endless and often confusing terminology, and also the initial struggles many students have examining large numbers of cultures of microfungi. My experience is that they are often irretrievably discouraged by their failure to see and recognise structures under the light microscope presented so lucidly as line diagrams in textbooks. Many microbiology students also see the approaches to fungal characterisation, relying as they do on morphological features, as somewhat old-fashioned and intellectually unfulfilling. One of the most significant points to emerge from the recent discussion on the teaching of fungal systematics conducted in The Mycologist is that the student's negative response is often a reflection of the negative attitude of the
teaching staff towards the subject matter and its presentation. Students need to be shown that observing fungi and describing their life-cycles can be made challenging and attractive. This is probably more important than submerging them with information at this stage. Most fungi are photogenic, and in practical session, students may be allowed to exploit the ability of the scanning electron microscope (SEM) to obtain high resolution three-dimensional images. Even the most cynical student is excited by the prospect of being able to use, under careful guidance, an expensive and sophisticated instrument like the SEM. Each student may be given two to three carefully selected fungi from different taxonomic groups, so with a usual class size of 15-20 students, a wide range of fungal types from all the major taxa can be included. The student then has to work out, with assistance, how to culture these fungi, so they produce the structures involved in either sexual or asexual reproduction, which can then be examined at different development stages. The students also have to prepare their own specimens for SEM examination. The methods described here are very simple, and almost always successful, so in our experience most students obtain impressive results. For agar grown cultures, the simplest method is that described by Samson et al (1979), except that for safety and economic reasons we avoid osmium tetroxide as a fixative. Briefly, the method consists of plac-
73
~
Fig. 1. SEM of Allomyees. (a) Gametophyte of A. javanieus showing the hypogynous male gametangium, x 450; (b) A. maerogynus showing epigynous, male gametangium and dissolved papillae, x 400; (e) male gametes with single posterior whiplash flagellum, x 2300; (d) planogametic conjugation with larger female gamete, x 2000; (e-f) A. arbuscula sporophyte showing thin-walled zoosporangium (e) with papillae (x 1150) and thick-walled resting sporangium with rupture lines visible (x 1500).
ing the specimen in 4-6% w/v gluteraldehyde at 4°C for 24h (although specimens can be left much longer), rinsing in two changes of distilled water for 10 min each, and then dehydrating in a single step, using methoxyethanol for at least 10 min. Specimens are then washed in two changes of acetone before being critically point dried in CO2 , coated with gold and then examined in the SEM. With unicells or liquid grown
cultures, samples can be fixed in the same way but need to be packaged in containers for dehydration. The simplest system is that described by Gadd & Griffiths (1980) where membrane filters (Unipore polycarbonate filters, 13 mm diam with 0.4um pore size) are stuck with silicon adhesive to the base of 1 em sections of hollow glass-rods. Dehydration carried out using 25-100% v/v isopropanol series works well, and the membrane filters, after critical
74
Fig . 2. Thraustoth ecu clavata. (a-b) Rupture of sporangial wall and primary cysts with in the sporangium. x 400 . x 1000 ; (c-d ) gametangial contact showing branched antherid ia and oogoni a , x 450 . x 1000 .
point drying, can be easily removed before attaching them to alu minium stubs . Examples of the results obtained by our students for four of the fungi we use , Allomyces spp , Thraustoth eca d avata, Emericella variecolor and Schizosaccharom yces octosporus, are shown in Figs. 1-4 . Each student then presents the results, consisting of micrographs and a taxonomic discussion, either as a short talk or as a poster presentation to the rest of the class, who are required to take relevant notes . A total of 15 h practical time is allowed, and the students are third-year microbiologists who have already been given some general information on fungal biology as part of a second-year 'Microbiologycrm rse ' .
It is not designed to turn out professional fungal taxonomists who, in my opinion, are better developed at postgraduate level. Its .main aim is to overcome th e resistance barriers to fungal systematics by both staff (in particular) and students, so that some longer lasting relationship with the fungi can develop. Our experience over th e past six years gives us some cause for optimism. It or something similar might be worth trying. REFERENCES
M & GR IFFITHS. A J (1980). Effect of cop per on morphology of Aureobasi dium pullulans . Tran sactions of the British Mycologica l Society 74, 387-392. SA MSON, R A , STALPERS, J A & VERK EREKE, W (1979). A simplified techn iqu e to pr epare funga l speci me ns for scann ing elec tron m inmsnnnv . Cvtohios 24 .7-11. G ADO. G
75
Fig . 3. Emeri cella variecolor. (a) Cleistotheca with rupture line and associated liberated ascospores , x 90; (b) Hulle-cells , x 1100; (c-d) ascospores with paired stellate flanges , x 4500 , x 10,000.
Fig . 4. Schi zosoccharomyces octo sporus. (a) Vegetative cells mating, x 7000 ; (b) conjugation canal between two mat ed cells, x 4000 ; (c) four-spored ascus, x 4000 ; (d) eight-spored ascu s , x 4500 .