The fungal world: Basic traits

Zygomycetes

Ascomycetes

Basidomycetes

Chytrids

Slime molds (P)

Oomycota

1. Deuteromycetes

2. Lichens


Fungi Introduction:

Before we undertake the job of interpreting the fungal world ( as, or more complex than that we encountered with the algae), we need to reflect on why it is that we should study such a divers group of organisms... after all they function in opposition to the autotrophic kingdom we've studied thus far...

Certainly with respect to the humans they can play positive roles:

  • they are a source of antibiotics ( penicillin, cephalosporin) and other medicinal compounds ( ergot + child birthing, headaches; cyclosporin)
  • they are the mainstay of our brewing ( beer & wine) as well as bread baking industry
  • they produce select if somewhat odoriferous cheeses that complement the wine, and in the orient, they aid in the production of tofu and miso ( recall the soup we cooked the algae in?)
  • the gallic acid they produce has been used in the the production of dyes, inks, photography materials as well in the manufacture of perfumes, chlorine, alcohols and acids
  • with respect to cleansing of the palate and hands, they are involved in the production of toothpaste, soaps, silvered mirrors and plastics...
  • for those who live on coke, the citric acid used for flavoring comes from Aspergillus
  • they even produce the steriods and hormones used for birth control pills...

In hand with the positive comes the negative implications of their existence:

  • Fungal diseases cause the loss of billions of dollars in crop and timber losses,
  • the consumption of bread produced with infected grains or grain in general can result in diseases associated with ergotism and other human and livestock diseases,
  • their very presence on many physical structures and materials had lead to the loss of its integrity i.e.. film, books, materials and even buildings built of wood...anything but plastics and some of the more toxic chemicals..
  • After crops are taken in they cause rot ...

In the more natural world, some theorize that plants could never succeeded on land initially without their association with mycorrhizae, and that nutrient flow in the forests today are controlled by these same organisms.

The degradation and recycling of dead organic material, especially of plant derivation depends on the combined efforts of the often competing fungal and bacterial species - without it organisms would be limited by lack of accessible minerals...

At the same time various lower fungi parasitize not only plants ( chestnut blight) but animals as well (for example the detested ich of fish, ringworm of pets and kids, and so on; over 100 fungi parasitize humans)... for a really fun page see

*Fungal diseases that must be overcome to have a traditional Thanksgiving dinner at ;

http://www.wisc.edu/botany/fungi/thanks.html

So enough! we need to know more......


Basic Characteristics of Fungi

Fungi by definition are eukaryotic,

  • based on whether their hyphae are septated or not, they can be uni- or multi-nucleate, the nuclei small with little repetitive DNA
  • they are chemoheterotrophs (requiring organic nutrition) and most are aerobic.
  • their body or thallus may be plasmodial, ameboid, pseudoplasmodial, unicellular or filamentous and may exist on land, in water or in the air. Generally they are not motile except for the spores...however those chytrids, ...
  • their membranes contain sterols (ergostrol but not cholesterol) but not peptidoglycans and their walls contain chitin and not cellulose ( except for the exceptions we'll discuss later)
  • they may produce wind or insect dispersed spores of sexual ( meiotic) or asexual ( mitotic)l derivation.. for some sex has become a thing of the past ( as far as we know), for others reproduction would fail without it.
  • Spores, millions of them are an necessity. Once fungi have digested available materials they must move to new ground...since the probability of a spore landing in an appropriate habitat is low and there are few food reserves to support them, massive production of spores is a necessity.As fungal spores show much variability by species they can used to identify genera and species status

    This site can be used to id the spores/sporangium of various common fungi:

    http://www.botany.utoronto.ca/ResearchLabs/MallochLab/Malloch/Moulds/ID_Plate_I.html

  • like plants they have an alternation of generations
  • unlike animals which ingest then digest, fungi digest then ingest, using exoenzymes to aid in the digestive process
  • unlike plants and like animals, they store their food in the form of glycogen
  • similar to some of the lower algae, mitosis does not require nuclear envelope breakdown

overall, the consensus is that as a group they are more closely related to animals than to eithe plants or protista with a common protistan ancestor.

Thus far there are over 100,000 species recognized but the actual number most likely far exceeds this estimate. As a result of complex life cycles, it can difficult to determine whether an organism in the field is an example of part of a life cycle, or a new species....
They are so successful out there for a number of the properites we discussed above:

A single hypha can produce thousands of spores... as single individual millions...and they move with the wind

These spores may remain dormant for years, so there is always a ready supply for new growth...

Genetically, they are diverse... sexual contact between different hyphae means a single individual may mate with many gneotypes...and incorporate many alleles...


Structure of fungi:

Most observers do not recognize the the white mat of hyphal strands in litter as fungi. Their identification of a fungus' existence is tied with the vision of a 'mushroom' - the basido or ascocarp they purchase in the store.

However most would readily recognize the mold growing on leftovers as of fungal derivation. In either case the complex structure or dripping mass are both composed of numerous, microscopic, branching hyphae known collectively as a mycelium.

In the higher fungi these hyphae have septa but in the less advanced groupings there are no crosswalls and the hyphae are considered coenocytic ( multinucleate as with the coenocytic algae). In the most advanced group the Basidomycetes, the 'hole' in each septation can be regulated with the dolipore, restricting flow of plasma.

 

 Cell Walls and growth:

Cell walls are present in the nonplasmodial form; they a typically contain chitin, beta 1,3 and beta 1,6 glucans. In the oomycetes cellulose along with beta 1,4 glucans are present and chitin is absent. In most fungi the structure is layered with an external sugar layer subtended by a proteinaceous and then chitinous support... compare the makeup of chitin to glucose....why can't they use cellulose in their walls? ( hint: how do they digest plant cells?)

Functions of wall:

The fungal wall protects cells against :

.

Growth occurs through the extension of the apical tip. The apical vesicles contain materials and enzymes that allow the formation of new hyphal wall. Chitosomes are micro vesicles which transport chitin synthethase to growing tip where upon fusion with the plasma lemma, the enzyme becomes activated. Micro fibrils are formed which then extend the wall. Due to the transient nature of hyphae, walls are Lysol as well, to recoup nutrients.

Older hyphae are less biochemically active and contain many storage vacuoles.

Diagram from:http://www.md.huji.ac.il/microbiology/book/ch073.htm


Reproduction:

From the reproductive mycelium, spores are formed.

a. Sexual reproduction

  • Karyogamy involves fusion of two haploid nuclei, followed by meiotic division of the resulting diploid nucleus
  • In some cases, sexual spores are produced only by fusion of two nuclei of different mating types, which necessitates prior conjugation of different thalli. This condition of sexual reproduction is known as heterothallism, and the nuclear fusion is referred to as heterokaryosis.
  • Plasmogamy = union of two hyphal protoplasts which brings the nuclei close together in the same cell is followed almost immediately by karyogamy but not always as we will see in the basidomycetes.
  • Homothallism, in which a nucleus within the same thallus can fuse with another nucleus of that thallus (i.e., homokaryosis).

b. Imperfect or nonsexual fungi

Some fungi are strictly asexual. These include the large group of imperfect yeasts (e.g., Candida species) and conidial fungi (e.g., Coccidioides immitis).

Most members of this group have permanently lost their ability to produce meiospores. A few undergo rare sexual reproduction, and perhaps for some species we have yet to discover their sexual or perfect stage.

The most common methods of asexual reproduction, includes:

  • budding in yeasts,
  • development of conidia from specialized hyphae (conidiogenous cells),
  • fragmentation of hyphae, and

conversion of hyphal elements into conidia or chlamydospores (thick-walled resting spores)

c. Parasexuality: from:http://www.md.huji.ac.il/microbiology/book/ch073.htm

Despite the absence of meiosis during the life cycle of these imperfect fungi, recombination of hereditary properties and genetic variation still occur by a mechanism called parasexuality.

The major events of this process include

Sexual and parasexual cycles are not mutually exclusive. Some fungi that reproduce sexually also exhibit parasexuality.

The parasexual cycle (genetic recombination without meiosis). Stages of the parasexual cycle are numbered as follows (1) Hyphal conjugation (plasmogamy). (2) Heterokaryosis. (3) Nuclear fusion (karyogamy). (4) Mitotic recombination and nondisjunction. (5) Haploidization and nuclear segregation leading to homokaryosis. l


Nutrition of fungi:

The true fungi obtain their carbon compounds from nonliving organic substrates (saprophytes) or living organic material (parasites & symbiosis) by absorption of nutrients through their cell wall.

Small molecules can diffuse directly through the membrane when in solution...

Macromolecules and insoluble polymers (e.g., proteins, glycogen, starch, and cellulose), on the other hand, must undergo preliminary digestion before they can be absorbed by the fungal cell. This process involves release of specific proteolytic, glycolytic, or lipolytic enzymes from the hypha or yeast, extracellular breakdown of the substrate(s), and diffusion of the products of digestion through the fungal cell envelope.

Roles in nature that tie in with their nutrition:

a. Decomposers or saprobes - as mentioned earlier, together with bacteria, fungi breakdown all forms of dead organic material... very often a succession of species occurs with groups specializing in the breakdown of families of compounds.. the first assortment absorbs the easily digested small molecules, and the last call involves those species that can breakdown the resistant cellulosic wall impregnated with lignin.. a period of 2-4 years may be required for complete decomposition.

Associated with this process is a competition between decomposers, especially between fungi and bacteria. Each group produce a number of secondary compounds that are used as antimicrobial agents by humans including the antibiotics penicillin from Penicillium chrysogenum, nystatin from Streptomyces noursei, and amphotericin B from S niveus.

b. Parasites: although we wince at the association between parasite and host, in the majority of cases, the association is not lethal. Generally the fungus with acts by making the hosts membrane 'leaky' or may insert its hyphae into the host ( haustorium) and tap into a nutrient pool.

They are nondiscrimainatory in their selection of hosts...most every group; algae, plants, animals have fungal parasites to contend with...

A specialized case ( termed Hyperparasites) relates to those fungal species that are parasitic on other fungi

c. Necrotrophs are less common - in this case the fungus will kill and then digest it's host. We see this in some fungi which kill plants by damaging their plasma membrane or by secreting excess 'hormones' during which the plant will respire itself to death, and then be digested by the fungus...

d. At the same time we can't forget the useful role of symbiotic fungi who have coevolved with their host ( most likely in an initially parasitic relationship) and which the host now feeds the fungus for 'services it provides'. That this relationship is controlled by the host is clear when we recognize that many plants will only support mycorrhizae when soil conditions are poor and not support them or limit it when conditions improve.

Besides mycorrhizae, the association between fungi and algae or bluegreens to form lichens is another good example of this association.


Over the next 1-2 days we will cover the true fungi and the protista with fungal like characteristics:

The Eumycota or true fungi

|======== Zygomycota (bread molds, Rhizopus, Mucor, etc.)

| === Basidiomycota (mushrooms, rusts, smuts, etc.)

=== Ascomycota (sac fungi, yeast, Penicillium, etc.)

=====Deutromycota

Athough the chytrids are fungi, they are quite primitive and we'll review them with 3 other protista:

========= Chytridiomycota (water molds, Allomyces, etc.)

========Plasmodial slime molds

========Cellular slime molds

========Oomycetes


Iglich 4/99