Fungal Diversity (Continued)

Summary of Fungal Attributes

  1. From our discussion on how fungi are able to carry out their various activities, the following attributes can be used to characterize those organisms that we classify as fungi:
    1. They are heterotrophs. That is, they cannot manufacture their own food from simple compounds as plants are able to do. So they are dependent on other organisms to produce their foods, e.g., sugars, starches, proteins, fats, etc. Fungi can be further divided into saprobes, parasites, symbionts, facultative parasites and facultative saprobes.
    2. The food gathering part of a fungus is made up of either filamentous, hollow, branched tubes called mycelium or are single cells called yeasts.
    3. Structures called spores reproduce the fungus in the form of mycelium or yeast cells.
    4. They have cell walls. This is a characteristic generally attributed to plants, but unlike plants, most fungal cell walls are composed of chitin, a carbohydrate, and is the same material which makes up the exoskeletons of insects. Plant cell walls are composed of cellulose.
    5. Fungi are eukaryotes as are most other organisms with which you are familiar. However, bacteria differ from fungi in that they are prokaryotes.
    6. Absorption: The process by which fungi "eat". This differs from the way in which we eat in that a fungus will digest its food before eating it. Bacteria are the only other group of organisms that eat in this fashion.

So What do the Set of Characteristics mean?

We have gone over several working definitions for "fungi" that have been accepted in mycology over the last 50 years. Each definition, at the time that they were used was thought to include closely related organisms, i.e. they were all derived from a single common ancestor. However, as time passed, and we gathered more information about these organisms, it became apparent that this notion was not correct. So, we redefined "fungi" to reflect these changes. The above set of characteristics is the most recent definition as to what we believe constitute a fungus. However, in mycology, we continue to study those organisms that have been demonstrated to not being closely related to fungi. Thus, in mycology we have two definition for fungi. One in which only those organisms that have the above characteristics and are closely related to one another are recognized as belonging to the fungus kingdom. The second includes all the fungi that have been recognized as fungi since Alexopoulous' definition in 1952, i.e. includes slime molds and other organisms that have been excluded from the fungus kingdom. More will be said on this subject in our lecture on "How fungi get their name and how they are classified".

Post Scripts

Sexual vs. Asexual Reproduction

Sexual reproduction is a subject that can probably be best understood if we discuss it in human terms rather than using plants or fungi. You are all aware that this type of reproduction must involve two parents, and that the children from two parents will inherit characteristics from each parent. For example, all of you have features that can be recognized as being maternally or paternally inherited. This will also be true for any siblings that you may have. However, you and your siblings are genetically unique in appearance and personality because the process of sexual reproduction is such that no two individuals will be exactly alike unless they are identical twins.

Asexual reproduction requires only a single parents and the "children" produced would be genetically identical to the parent. Genetically, identical individuals are said to be clones. Asexual reproduction is currently more easily understood due to the extensive coverage of cloning in the news media of Dolly, the cloned sheep and the mice that have been cloned right here on the University of Hawai‘i, Manoa campus. Although some animals, naturally, have this type of reproduction, there are far more examples of asexual reproduction in plants. Asexual reproduction occurs when a part of an individual regenerates itself into another individual. Since this new individual was originally part of the parent, the two are genetically identical.  Many agricultural plants are reproduced asexually because if you have a plant with all the qualities that you want, growing clones of this individual will ensure that everything you are growing will also have these qualities. Examples of such plants are illustrated in Figures. 20-22. In potatoes, each "eye" can be used to produce a new potato plant, the leafy tops of the pineapple and carrot can be grown to produce other plants.

 

potato2.GIF (17688 bytes) pineapple2.GIF (17551 bytes) carrot2.GIF (11329 bytes)
Figures 8a, b and c. From Left to right, potatoes, pineapple, carrots and onion. These are example of plant structures that can be utilized in asexually reproducing the plant.

Both sexual and asexual reproduction has its advantages and disadvantages. For example, if a fungus is growing in an ideal environment that it is well suited for, it would be advantageous to reproduce asexually since the environment would also be ideal for clones of that individuals. However, if the environment should suddenly become unfavorable and lead to the death of that fungus, having an entire population of genetically, identical individuals will be a disadvantage since all of the individuals will be equally likely to die. In this situation, sexual reproduction will be advantageous since the individuals produced will all be genetically different. Being different, there may be one to several individuals that may be more suited for the new environment. 

A Few Misconceptions About Fungi

Temperature: Fungi grow best in warm temperatures. Some species of fungi do grow better at warm temperatures (70-90°F), but there are some that thrive in very high temperatures of 130-150°F and some that will thrive in very low temperatures below 32°F (below freezing). That is why when you refrigerate meat, it needs to be in a freezer at -20°F. Also, how many of you have found food in your refrigerator that has been contaminated with fungi?

Water: Fungi need lots of water to grow. For most fungi this is true. This is why fungi are more of a problem in the tropics than in temperate areas of the world. Personal property that is normally safe from fungi, such as clothing and shoes, can be damaged in the tropics. However, some fungi can grow in very dry conditions. Dried grains and fruits can become contaminated with fungi. Fungi contaminating grains have been a very serious health hazard. For example, when you go into a bar, it is not the beer that will kill you, but the free peanuts. At the other extreme, there are also fungi that can live under water.

Light: Fungi can only grow in the dark. For the most part, light does not play a role in how well fungi grow. There are some conditions where light is necessary for reproduction.

Mycelial Growth

Until the 1990's fairy rings were thought to be the largest examples of mycelial growth (actually rhizomorphs in this case) in fungi. In April 2, 1992 the war of the humongous fungus began. Smith, et al. (1992) published an article of what at that time was considered to be the world's largest organisms. This article reported that what genetically was determined to be an individual of Armillaria bulbosa, a species of mushroom, produced rhizomorphs that covered approximately 37 acres. It was estimated to be at least 1500 years old and weighing in at about 100 tons. This was published as a serious journal article, but captured the imagination of the news media that dubbed it the humongous fungus and a media blitz began that lasted for months. And just when things were going back to normal, on May 18, 1992, a still bigger mass of rhizomorph was discovered.  On Mt. Adams, in Washington State, rhizomorphs of Armillaria ostoyae covered 1500 acres. However, even this example does not represent the largest example of radial, mycelial growth. The most incredible growth of mycelium is one that was found in eastern Oregon, on August 1, 2000.  The mycelium was found to belong to a mushroom identified as Armillaria ostoyae. Researchers determined that the mycelium of this mushroom covered 2,200 acres and estimated it to be over 2400 years old. The full text of this story was written by Volks (2002). 

Literature Cited

Smith, M., J. Bruhn, and J. Anderson. 1992. The fungus Armillaria bulbosa is among the oldest and largest living organism. Nature 356:428-431

Volk, T. 2002. The Humongous Fungus -- Ten Years Later. Inoculum 53(2): 4-8. (Newsletter of the Mycological Society of America)

Mycological Terms

Absorption: The means by which fungi obtain their food. Process begins with the release of digestive enzymes, from the fungus, through their cell walls to digest the food that is around them. The digested food is then "absorbed" through their cell wall.

Budding: A form of asexual reproduction in which an outgrowth (="bud") developing on a parent yeast cell detaches to produce a new individual.

Cell Wall: The rigid outermost cell layer found in plants and certain algae, bacteria, and fungi but characteristically absent from animal cells.

Cellulose: A complex carbohydrate, (C6H10O5)n, that is composed of glucose units, forms the main constituent of the cell wall in most plants and some fungi.

Chitin: A complex, primarily nitrogen-containing carbohydrate, which forms the principal component of arthropod exoskeletons, ex. insects, and the cell walls of most fungi.

Clone: An organism descended asexually from a single ancestor, such as a plant produced by bulbs or fungi by asexual spores.

Conidium (plural: Conidia): Asexually produced spores produced on conidiophores.

Conidiophore: A specialized hypha on which conidia are borne.

Dimorphic: Fungi that have both a yeast and mycelial phase.

Eukaryote: Organisms whose cells contain a distinct membrane-bound nucleus.

Facultative parasite: An organism that primarily derives it nutrition as a saprobe, but may become a parasite if the opportunity presents itself.

Facultative saprobe: An organism that primarily derives its nutrition as a parasite, but may become a saprobe if the opportunity presents itself.

Fission: An asexual reproductive process in which a unicellular organism divides into two or more independently maturing daughter cells.

Fruiting body: A specialized spore producing structure found especially in fungi.

Heterotroph: An organism that cannot synthesize its own food and is dependent on complex organic substances, of other organisms, for nutrition.

Hyphae (sing: Hypha): Threadlike filamentous fragment of the mycelium of fungi.

Mitosis: The division of the nucleus, which results in the formation of two new nuclei. The two nuclei are identical and contain a complete copy of the parental chromosomes.

Mycelium: The vegetative part of the fungus that consisting of a mass of branching hyphae.

Parasite: An organism that derives its nutrition and is sheltered on or in a different organism (=host) while contributing nothing to the survival of that organism.

Rhizomorph: An aggregation of mycelial strands that forms a root-like structure formed in some fungi.

Saprobes: An organism that derives its nutrition from nonliving or decaying organic material.

Yeast: Unicellular fungi that reproduce asexually by budding (a small outgrowth, or "bud", on the cell's surface increases in size until a wall forms to separate the new individual) and fission.

Questions to Think About

  1. Why are facultative saprobes or facultative parasites more of a problem, as pathogens, than those that are obligate parasites?
  2. Even though fungi are all around us and are very common, people are generally unaware of their existence. What are some reasons that you can give for this lack of awareness? 
  3. Certain organisms are no longer considered to be fungi that were previously classified as such. Name these types of organisms and give the reasons that they have been excluded.
  4. How does sexual and asexual reproduction differ? What are the advantages and disadvantages of both?
  5. What are some common misconceptions about fungi?

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