Topic #7: Fungi
REQUIREMENTS: Powerpoint presentations.
1. List characteristics of the Fungi. Identify the groups of organisms included in the Fungi. For the sake of completeness, note the group that BOT 3015 excludes as a means of simplification.
2. Draw the generalized sexual life cycle referred to as zygotic meiosis.
3. What are saprobes, parasites, endophytes, and mycorrhizae?
4. Discuss the organization of the hyphae. Define mycelium, rhizomorph.
5. What is the mode of nutrition of fungi? How does it relate to the fungal body plan? Discuss the body plan in relation to the efficacy of diffusion as a mechanism of acquiring external nutrients.
6. Describe the typical fungal habitat.
7. Describe hyphal growth.
8. Describe the cell wall of a fungus. Describe two attributes of the cell wall that must be considered with respect to the mode of nutrition.
9. Describe the limited amount of “division of labor” that occurs in the hypha.
10. Describe the biochemical and biophysical mechanisms by which fungi acquire nutrients. Compare with mechanisms of plants. How do fungi acquire nutrients from more dilute soil solutions than can plants? Relate this ability to mycorrhizal associations.
11. Compare the sizes of the plant, animal, and fungal genomes. Which genes are represented by many copies in all these eukaryotes? To what extent do the various groups have repetitive DNA?
12. Contrast different aspects of nuclear division in fungi with those other eukaryotes.
13. Indicate briefly the role of fungi in the ecosystem.
14. What is heterokaryosis? What is a dikaryon? What advantages accrue to this condition?
15. Compare the Fungi to plants in terms of carbohydrate-storage product, cell-wall composition, source of nutrition, mitosis, proximity of cells to environment, growth, ubiquity of high rates of metabolism, and transport of cellular substances (= cytoplasmic streaming and lack of complete septation).
16. Discuss the competitive environment around the fungal hyphae.
17. How does asexual spore formation in the Zygomycota differ from that of other fungal taxa?
18. Name the three groups of Fungi (as described in BOT 3015) and explain their distinguishing characteristics. Which two are the most closely related?
19. What are Fungi Imperfecti? How do they reproduce? To which of the major groups are they usually considered related?
20. What are lichens? Which organisms are typically involved? Briefly, what is the basis for their symbiosis?
21. Draw the life cycle of bread mold. Include both sexual and asexual reproduction.
22. Draw the life cycle of a typical ascomycete. Emphasize the ascus, hook, ascus mother cell, karyogamy, meiosis, ascospore formation.
23. Draw the life cycle of a mushroom (one type of basidiomycete). Contrast it with the life cycle of an ascomycete.
POWERPOINT SLIDES: A series of slides that detail attributes of fungi.
Attributes of the Fungi:
(A) Fungi are a large group of organisms that are quite distinct from plants and from animals. To gain perspective, consider the existence of about 100,000 named species and perhaps 200,000 yet unnamed.1,2
POWERPOINT SLIDES: Overview of fungal groups, perspective for BOT 3015.
(B) Fungi are saprobes3 (organisms that secure their food from nonliving organic matter) or parasites,4 or they form mutually beneficial relationships with other organisms.5 The primary mode of nutrition is absorption—because of the filamentous habit of the multicellular ones (and most are multicellular), no part of the fungal body (mycelium) is ever far from the environment and, therefore, a source of nutrition. Sometimes, however, the hyphae organize themselves into reproductive structures of one kind or another (such as mushrooms). In other cases, hyphal cords, rhizomorphs, are formed by many hyphae organized into a complex filament. Rhizomorphs “explore” new areas for potential nutrients and export these nutrients back to the origin.
POWERPOINT SLIDE: Osmotrophic mode of nutrition (self-made).
POWERPOINT SLIDE: Rhizomorphs (mulch pile, north Leon County).
(C) Fungi are primarily terrestrial, but some are marine and xerophilic—those that can live under challenging conditions of water insufficiency (such as in salted fish or jam, where bacteria cannot grow).
(D) Growth occurs primarily at the tips of the hyphae, but constituents such as protein and organelles are synthesized throughout the body, giving fungi a high rate of growth. (New components are delivered to the tip by cytoplasmic streaming.)
POWERPOINT SLIDE: Growth at tip of hypha.
(E) No fungal cell is without a wall. The cell wall, as indicated earlier, is composed primarily of chitin (a polymer of N acetylglucoseamine) and various linked glucans, which are linked to peptides, giving the wall a fairly high protein content.6 (As perspective, and painting with a broad brush, say that 30% of the dry wall mass of a fungus is protein, compared with, say, 10% for a plant cell wall7).
A brief reflection indicates that fungal cell wall must be special. First, it must be somewhat porous, to allow egress of the secreted enzymes, which break down the substrate. Second, it must be resistant to the fungus's own enzymes, as well as to the enzymes secreted by other organisms, like plants, as a response to fungal attack.8
(F) Unlike plants, fungi have no “quiescent” centers (i.e., areas that are relatively inactive metabolically). As you may have inferred from an earlier point, the absorptive mode of nutrition and the high surface-to-volume ratio mean that each cell’s activity (e.g., secretion of a hydrolytic enzyme) has an immediate effect on the surroundings, but the hypha does have a limited amount of “division of labor.” The very tip, full of vesicles for building activities, is involved mostly in growth. The next region is specialized for absorption. In this absorption zone, active excretion of H+s may lower the external environment to pH 3 and create a membrane potential of –200 to –300 mv.9 This huge H+ electrochemical potential gradient is used to drive uptake of K+, of PO42, of glucose, and of amino acids via a proton symport. (Because of the large driving force for proton influx, proton symport in fungi is effective in taking up very dilute external solutes. The magnitude of this driving force, along with the filamentous body plan of fungi, explains their value in mycorrhizal associations.) Further back is a storage zone, where polyphosphate and protein are stored in vacuoles and glycogen in the cytosol. The sizes of the zones depend, to an extent, on nutrient availability.
(G) Most fungi produce spores, which are dispersed by the wind or by insects or small animals.
(H) The mycelium is haploid10—meiosis occurs immediately after zygote formation (i.e., no alternation of generations occurs). The fungal genome is small—ca. 40 × 106 bp, or less than an order of magnitude larger than that of E. coli. (As usual, there are exceptions; some fungi have large genomes.) The genome is “compact,” however—only 10 15% of fungal DNA is repetitive, whereas a typical value for animals is 30%, and repetitive sequences in plants can form up to 90% of the total genome. In investigated fungal species, most of the repetitive DNA codes for rRNA sequences. All organisms, including prokaryotes, have repetitive rDNA, but plants and animals have other repetitive sequences of various functions, such as chromosome organization. The functions of some of this repetitive DNA in plants and animals are unknown.
(I) Many architectural aspects of the molecular and cell biology of fungi differ from those of other eukaryotes. (1) As a rule, the nuclear membrane does not disintegrate during mitosis and meiosis. The nucleus persists through these processes, and after the chromosomes are allocated, the nucleus is “pinched” into the daughter nuclei. (2) Fungi lack flagellated cells and, therefore, centrioles (the microtubule-organizing centers that effect chromosome migration), but they do have spindle pole bodies (SPB) attached to the outside of the nuclear envelope, which function like centrioles. SPB’s have not been characterized chemically, and they differ from one group to another. Depending on the group, the SPB remains outside the nuclear envelope during nuclear divisions or is inserted into holes in the envelope. (3) During mitosis, the chromosomes do not become aligned on a metaphase plate, as the chromosomes of animals and of plants do. In addition, the migration of chromosomes during anaphase is not synchronous, as it is in plants and animals. (4). Earlier, it was believed that fungi entirely lacked or had little histone protein. This erroneous conclusion resulted from the proteolytic loss of the histones during extraction. All fungi appear to have the major classes of histones that are involved in the formation of the nucleosome, but some lack H1, which complexes with the “linker” DNA between nucleosomes.
(J) Some fungi are heterokaryotic (i.e., have genetically different nuclei). Heterokaryosis can result from fusion of different hyphae (a common event in nature) or from mutation. (As you will see later, hyphae are often aseptate or multinuclear, so different nuclei may be operating against a common cytoplasmic background.) Heterokaryosis is somewhat similar to the diploid condition, because it confers a larger genetic potential to interact with the environment.
Distinct from being part of a true sexual cycle, rarely, haploid nuclei (genetically the same or different) fuse, giving rise to one diploid nucleus per 1000 haploid nuclei (this phenomenon is called “parasexuality”). The diploid nucleus does not undergo meiosis; instead the haploid condition is restored by loss of chromosomes. The importance of this phenomenon is not yet clear, but it would seem to offer genetic and evolutionary diversity.
(K) The role of fungi in ecosystems cannot be overstated. As an example, lignin (ca. 20% of total biomass produced per year; total= 50 x 1012 kg) is an important structural element. Whereas fungi do not use lignin (much?) themselves, they do degrade it so that it becomes available to other organisms. It appears that fungi need to remove the lignin to gain access to carbohydrate polymers, which they can utilize. Because fungi break down complex organic molecules outside the hyphae, the areas around the hyphae form niches for bacteria and other fungi. For this reason, perhaps, fungi developed an arsenal of antibiotics to combat these competitors.
(L) Fungi are adaptive, in an evolutionary sense (Neocallimastix frontalis, an obligate anaerobe in the rumen lacks mitochondria) and on a shorter time scale (such as the yeast11 that convert glucose to ethanol12 in the absence of O2 but switch back to oxidative metabolism when given oxygen).
Three groups of fungi will be considered below; the classification of the first group is historically based. A master table and charts at the end of this topic summarize characteristics of the three groups and simplified life cycles; for an expanded listing of fungi, see an introductory footnote.
POWERPOINT SLIDE: Overview of sexual reproduction in the three focal groups of fungi in BOT 3015 (custom).
POWERPOINT SLIDES: Series of slides on attributes of zygomycetes.
Attributes of the Zygomycota:
(A) They are a relatively small group of organisms, about 1000 species. They cause a few flower and fruit diseases, such as fig spoilage,14 but a major environmental role is in the formation of mycorrhizae, as discussed in a previous unit. (Recall, for completeness, that some classification schemes do not place the mycorrhizal ones in the zygomycetes.)
(B) A distinguishing characteristic of the Zygomycota is the sexual production of thick walled resting spores.15
POWERPOINT SLIDE: Mucor on summer squash (tentative identification by D. Chellemi by gross morphology and host) (north Leon County). Mucor is a relative of Rhizopus stolonifera (black bread mold), which is the name species for this group. The black dots are asexual sporangia with spores inside.
POWERPOINT SLIDE: Zygospore16 of Rhizopus nigricans (bread mold) (FSU lab; special thanks to R. Hebert and K. Riddle for preparation and microphotography.) Note that several stages of development are shown in this slide.
As we will explore in more detail in a few moments, in the Zygomycota, gametes of equal size fuse, after being released from appressed gametangia (left), to form a resting spore (right).
(C) Usually, cross walls are absent, except where reproductive structures form the hyphal tips.
(D) Always, asexual reproduction occurs on the tips of hyphae in specialized sporangia.
POWERPOINT SLIDE: Rhizopus stolonifer life cycle (custom).
(A) Asexual reproduction occurs at hyphal tips in sporangia. The fungal body can be divided into, essentially, three regions: the rhizoid, which grows into the substrate; the stolon or lateral filaments; and the sporangiophore.17
(B) When different but morphologically indistinguishable mating strains are near, hormones cause the hyphal tips to grow to one another and develop gametangia (singular, gametangium = gamete producing entity).
(C) The walls separating the gametangia dissolve, and the multinucleate protoplasts from the two gametangia come together. Nuclei (in pairs, usually, “+” plus “–”) fuse, resulting in a multinucleate diploid zygospore, which develops a thick wall. (Do not focus on the details—some zygomycetes produce several zygotes and in other species, all diploid nuclei except one disintegrate.)
(D) Under appropriate conditions, the zygote divides meiotically, about the time that the zygosporangium opens (“germinates”), and new haploid spores (like those produced asexually) are released (directly or indirectly). (Again, do not focus on the details—in Mucor, for example, only one of the haploid spores survives.)
THE KEY GENERAL POINT is that equal gametes fuse, that the zygote is the only diploid cell, that division of the zygote meiotically restores the haploid condition, which, in one way or another, generates spores that form a new mycelium.
POWERPOINT SLIDES: Series of slides on attributes of ascomycetes.
Attributes of Ascomycota:
(A) The Ascomycota are terrestrial or aquatic. They are among our worst enemies, causing, for example, Dutch elm disease, apple scab, brown rot of stone fruits, powdery mildews (along with other taxa), foot rot of cereals,18 and others.
POWERPOINT SLIDE: Podosphaera leucotricha (powdery mildew19—tentative identification by BBO made on the basis of gross morphological features and host) on Granny Smith apple (north Leon County).
POWERPOINT SLIDE: Botryosphaeria dothidea20 = B. berengeriana = Physalospora piricola; anamorph = Fusicoccum aesculi, apple ring rot (tentative identification made by BBO on the basis of gross morphological features and host) on Adina apple (north Leon County).
POWERPOINT SLIDE: Diaporthe perniciosa = D. eres; anamorph = Phomopsis mali, phomopsis fruit decay (tentative identification made by BBO on the basis of gross morphological features and host) on Anna apple (north Leon County).
POWERPOINT SLIDE: Cercosporella rubi, rosette, double blossom, witches broom (firm identification made in clinical pathology lab) on Shawnee blackberry (north Leon County).
Among the ascomycetes are also benign or beneficial organisms (such as the esculent morel). The following few slides are other examples of ascomycetes that are found in the Tallahassee area.
POWERPOINT SLIDE: Xylaria sp. (Birdsong, south Georgia; special thanks to W. Petty21 for identification.) In this genus, the asci line depressions in the club shaped fruiting body.
POWERPOINT SLIDE: Helvella sp. (Birdsong, south Georgia; special thanks to W. Petty for identification).
POWERPOINT SLIDE: Hypoxylon sp. (Birdsong, south Georgia; tentative identification by BBO).
(B) With the exception of the unicellular yeast (most of which are derived through reduction from this group), Ascomycetes are filamentous. In general, cross walls are present, but the cross walls are perforated, allowing organelles (including nuclei) and cytoplasm to pass through.22
POWERPOINT SLIDE: Perforated septum (Fig. 24.1 of Weier, Stocking, and Barbour).
POWERPOINT SLIDE: Nucleus through perforation (Fig. 12-6 of Raven, Evert, and Curtis).
(C) Usually, asexual reproduction is by the formation of specialized, multinucleate spores (conidia, which resemble fine dust), which are borne on conidiophores. You may wish to think of the conidium as a deciduous part of the hyphal tip. Note that asexual reproduction in the Zygomycetes took place by the mitotic production of spores within a structure, the sporangium. Conidia, on the other hand, are not enclosed as they mature. The following slide shows one example of a conidiophore, but there are other configurations also.
POWERPOINT SLIDE: Conidiophore (Fig. 12-15a of Raven, Evert, and Curtis).
(D) The distinguishing characteristic of the Ascomycota is that sexual reproduction always involves the formation of an ascus23 (plural, asci).
POWERPOINT SLIDE: Unidentified ascomycete (north Leon County). This fungus apparently is not among the common ones as it is absent from guidebooks and could not be identified by G. Bates, a colleague who is handy with mushroom identification. A main value of this slide is that this fungus “looks” like an ascomycete, and gives you a visual image of a prototype. At this stage, a microscopic examination revealed that the inside lining of this bowl shaped fruiting body was densely lined with asci, similar to those that are seen in the following slide.
POWERPOINT SLIDE: Asci of Peziza sp. (FSU lab; special thanks to R. Hebert and K. Riddle for preparation and microphotography).
POWERPOINT SLIDE: Ascomycete life cycle (custom).
(A) The mycelium is initiated with the germination of an ascospore (about 2 o’clock in the slide). Genetically different mating types develop from different spores. Asexual reproduction through conidia may occur several times in the course of a season and is the primary method of reproduction.
(B) Multinucleate gametangia differentiate and are “cut off” (about 5 o’clock in slide). The male structure is called an antheridium; the female structure, an ascogonium.
(C) Plasmogamy (fusion of the protoplasts) occurs through a structure that is an outgrowth of the ascogonium (about 6 o’clock in the slide).
(D) Cell division occurs, so each cell of the future ascus-bearing hypha has a pair of haploid nuclei (a dikaryon) now at “cradle” of fork at about 6 o’clock = “developing ascogenous hyphae”). The haploid hyphae (either “+” or “–”) are sexually sterile.
(E) Ascus formation develops at the tips of hyphae growing from the ascogonium. (This process occurs simultaneously at many hyphal tips, in a complex structure shown at top.) A dikaryotic cell grows over to form a hook or crozier.
(F) Mitosis occurs, with the spindles parallel—see nuclear orientation at 7 o’clock. One nucleus is shown as a darkened circle and the other as an open circle.
(G) Four daughter nuclei (haploid) have resulted. Two septa form, resulting in a dikaryon at the top and two genetically dissimilar haploid cells below.
(H) In the dikaryotic cell (8–9 o’clock), karyogamy (fusion of nuclei) occurs, forming a diploid zygote, which is the only diploid nucleus in the life cycle of the ascomycete (or for that matter, “any” fungus).
(I) The diploid nucleus undergoes meiosis (reverting to the haploid) and usually mitosis, which results generally in 8 nuclei.24 Each nucleus is cleaved off in a segment of cytoplasm to form a (haploid) ascospore. Ascospores germinate and develop into mycelia. As mentioned earlier, many hyphal tips can grow from the ascogonium.
Lichens—an alliance between kingdoms!
Lichens are composite organisms. Each is a symbiotic relationship between a fungus (usually an ascomycete but sometimes a basidiomycete) and a green alga or cyanobacterium.
POWERPOINT SLIDE: Cladonia sp. (south Georgia roadside; tentative identification by BBO).
POWERPOINT SLIDE: Collection of diverse species of lichens (gift of R. Hebert)
General attributes of lichens :
(A) Lichens are very widespread—usually the first colonizers of bare rock.
(B) They are usually of one of three types, based on morphology: crustose (crustlike), foliose (leaflike), or fruticose (bushlike).
(C) In brief general summary, the alga provides a source of photosynthate (carbohydrate, and in some cases, reduced N compounds), and the fungus provides a suitable physical environment.
Finally, the so-called “imperfect fungi,” which do not have a known sexual cycle, are usually mentioned along with the Ascomycota. Without a sexual stage, they cannot be classified as zygomycetes, ascomycetes, or basidiomycetes, but most of them are ascomycete-like, and they reproduce by conidia. A few, however, appear to be basidiomycetes that have lost the ability to reproduce sexually. Sometimes the Fungi Imperfecti are called Deuteromycetes.26
POWERPOINT SLIDES: Series of slides on attributes of basidiomycetes.
General attributes of the Basidiomycota:
The Basidiomycota include the common fungi with which most people are familiar (mushrooms, puff balls, shelf fungi). They also include some of the most important plant diseases, such as rusts28 and smuts, and certain kinds of root rot. Some are edible and others highly poisonous. In general, one sees only reproductive structures. It has been claimed (Moore et al., 1998, Botany, WCB/McGraw Hill, New York) that some mycelia live for hundreds of years and weigh several metric tons.29
The following are local basidiomycetes:
POWERPOINT SLIDE: Phallus ravenelii30 (Ravenel’s stinkhorn, tentative identification by BBO) (north Leon County).
POWERPOINT SLIDE: Leucopaxillus sp. (?) with the late Timber Outlaw (Leon County, specimen courtesy of R. Brymer).
POWERPOINT SLIDE: Linderia columnata (= Clathrus columnatus, columned stinkhorn, tentative identification by BBO) (north Leon County).
POWERPOINT SLIDE: Geastrum spp. (earthstar, tentative identification by BBO) (north Leon County).
POWERPOINT SLIDE: Pycnoporus cinnabarinus (cinnabar polypore growing on Quercus nigra (water oak) in north Leon County).
POWERPOINT SLIDE: Marasmius sp. (tentative identification by W. Petty from photograph) (north Leon County).
POWERPOINT SLIDE: Stevenum sp. (tentative identification by W. Petty from photograph) (north Leon County).
POWERPOINT SLIDE: Hericium erinaceus sp. (identification by W. Petty) (Wakulla Springs, Florida).
POWERPOINT SLIDE: Amanita cokerii (?) (white amanita,31 tentative identification by BBO) (north Leon County). This is an example of a gilled mushroom (similar in structure to the culinary mushrooms available in supermarkets). In this kind of basidiomycete, the basidia line the gills.
POWERPOINT SLIDE: Lentinus edodes (shiitake) (Herman Holley’s organic farm east of Tallahassee).
POWERPOINT SLIDES: Cantharellus cibarius (Chanterelle, identification by BBO) (north Leon County). This is an example of a mushroom that lacks gills and one in which the basidia do not match the prototype that is taught in this course. (In chanterelles, the basidiospores do not form on protuberances of the basidium; instead, the spores are formed within the basidium). The fertile surfaces are the blunt, shallow, branching ridges. The first slide is a close-up; the second, one of my little private patches; the third, my favorite way of seeing them.)
POWERPOINT SLIDE: Pleurotus ostreatus (wild oyster mushroom, identification by BBO) (north Leon County).
POWERPOINT SLIDE: Pleurotus ostreatus (oyster mushroom, cultivated by BBO) (north Leon County).
POWERPOINT SLIDES: Strobilomyces floccopus (Old man of the Woods, tentative identification by BBO; (north Leon County). . . . followed by three other example boletes from north Leon County.)
(A) The hyphae are septate, but the septa are perforated (like those of ascomycetes).
(B) The distinguishing characteristic of the Basidiomycota is production of basidiospores, which are borne on the outside of a club-shaped structure, the basidium.
POWERPOINT SLIDE: Basidia of Coprinus pileus (FSU lab; special thanks to R. Hebert and K. Riddle for preparation and microphotography).
(C) A distinctive feature of the basidiomycetes is the formation of so-called clamp connections over the septa of the dikaryotic mycelium near the tip. As shown in this slide, the connection is a temporary bridge formed during cell division. We will discuss how the nuclei divide and are allocated to daughter cells.
POWERPOINT SLIDE: Clamp connection (Fig. 12-27 of Raven, Evert, and Curtis).
(D) In general, asexual reproduction plays a less prominent role in the Basodomycota than it does in the Ascomycota; we will not cover asexual reproduction in basidiomycetes except to note its variable character (exemplified by budding in the few yeast that are basidiomycetes, formation of conidia on the primary mycelium, formation of conidia on the secondary mycelium).
POWERPOINT SLIDE: Basidiomycete life cycle32 (custom).
(A) (Starting at 5 o’clock) Most basidiomycetes pass through three phases: (1) Germination of a spore results in the primary mycelium, which is monokaryotic. (2) The secondary mycelium (6 o’clock) is dikaryotic and usually results from plasmogamy of different stains. The dikaryotic condition may, in some species, come from mitosis without cytokinesis, in which case each cell has two genetically similar haploid nuclei. (3) The tertiary mycelium (also dikaryotic) arises directly from the secondary mycelium. This tertiary mycelium is the “fruiting” body, e.g., a mushroom.
(B) Dikaryotic basidia (the club-shaped structure at 1 o’clock) differentiate along the lining of the basidiocarp.
(C) The terminal cell (1 o’clock) undergoes karyogamy (2 o’clock), which is immediately followed by meiosis (2:30 o’clock).
(D) Each resulting haploid nucleus migrates into one of the processes emanating from the cell tip; these nuclei are “cut off,” forming basidiospores.33
Chart: SOME CHARACTERISTICS OF FUNGI (not including chytrids, which are now considered by most to be fungi)a
Sexual reproduction (example)
Rhizopus sp. (black bread mold)
isogamy that yields resting spores (zygospores)
usually coenocytic (except for reproductive structures)
Hyphae of compatible strains are attracted and subsequently fuse. Pairs of haploid nuclei undergo syngamy, forming a multinucleate zygospore. This latter germinates and undergoes meiosis to produce haploid spores.
yeast, morel, truffle
sac-like reproductive structure, the ascus, which contains haploid spores (typically 8 = 1 meiosis + mitosis); hook
perforated septa (except unicellular yeast)
Plasmogamy produces dikaryotic ascogenous hypha, the tip of which is a hook cell. One round of mitosis plus cytokinesis isolates the heterokaryotic ascus mother cell. Karyogamye to form the diploid nucleus. Meiosis + mitosis to produce haploid ascospores.
mushrooms, puff balls, shelf fungi, rusts
production of basidiospores, which are borne on the outside of a club-shaped structure, the basidium; clamp connections
Binucleate condition usually from plasmogamy of different-strain hyphae (or mitosis without cytokinesis in a single hypha). Clamp connection on basidium ensures "correct" allocation of nuclei. Karyogamy. Meiosis to form basidiospores.
aFungi as we limit the word do not have flagellated cells. They are primarily terrestrial, and the primary mode of nutrition is absorption (either parasitic or saprophytic). The individual filaments are hyphae (singular, hypha). Collectively, hyphae compose the mycelium. Growth occurs at the hyphal tips; growth components synthesized back from the tip are transported to the growing tip by rapid cytoplasmic streaming. Walls of chitin. Zygotic meiosis. Persistent nuclear membrane during mitosis and meiosis. Little histone protein. Sometimes heterokaryotic and parasexual (the latter not being discussed in BOT 3015).
bFormerly, this group was called "Phycomycetes" (or alga-like fungi). They are not very closely related to the other two groups. This taxon is the usual partner forming endomycorrhizae, which are associated with a high percentage of vascular plants. As mentioned earlier, some taxonomic revisions affect this simpler view.
cThe basidiomycetes and ascomycetes are closely related phylogenetically. Again, as mentioned earlier, some revisions place these organisms together in one group.
dSexual reproduction is unknown in the so-called Fungi Imperfecti. Most of these organisms are probably ascomycetes. About 20,000 species of ascomycetes form symbiotic relationships with cyanobacteria or green algae; these "composite" organisms are lichens (e.g., reindeer "moss" (not, of course, a moss at all)).
eKaryogamy is fusion of nuclei. Plasmogamy is fusion of protoplasts. Syngamy (= karyogamy +plasmogamy), a similar process, is a specialized case of karyogamy in which the fusing nuclei are of gametes and the fusion product is a zygote.
fBasidiomycetes (and some ascomycetes) form ectomycorrhizal relationships with most trees and shrubs. Ectomycorrhizal fungi do not penetrate cells, but the hyphae do grow in the cell walls of cortical cells. Ectomycorrhizal relationships, unlike endomycorrhizal ones, are often specific.
After you have read your textbook, studied your own class notes, studied these class notes, written answers to objective questions as if to turn in, compared those answers to those of a classmate,
Do a self evaluation by taking this model exam.
The key and an explanation follow.
Then, if indicated, (a) consult with one of the TAs, or (b) consult with the instructor, or (c) bring unresolved questions to help session.
ntroductory Plant Biology Model Exam IV