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Fungi I - Evolution and Diversity, Phyla Chytridiomycota and Zygomycota

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You should have a working knowledge of the following terms:

  • absorptive nutrition
  • chytrid
  • coenocytic hyphae
  • dikaryon
  • fungi
  • gametangium (pl. gametangia)
  • haustorium (pl. haustoria)
  • hypha (pl. hyphae)
  • karyogamy
  • mycelium (pl. mycelia)
  • mycorrhiza (pl. mycorrhizae)
  • plasmogamy
  • septum (pl. septa)
  • septate hyphae
  • zygomycetes
  • zygosporangium (pl. zygosporangia)
  • zygospore

Introduction and Goals

This tutorial will cover the vast and variable Kingdom Fungi, and its sizable impact on the environment. Fungi (fungus, sing.), like bacteria, are most commonly decomposers, assisting in the essential task of recycling nutrients in ecosystems. The majority of all plant species depend on symbiotic fungi for enhanced water and nutrient absorption. Other species have been exploited by humans for centuries for the production of bread, beer, wine, cheese, and edible fruitbodies (mushrooms). Antibiotic-producing fungi (e.g., Pennicillium chysogenum) have changed the face of modern medicine, saving millions of lives. Conversely, other species cause economically important and often devastating diseases to plants and animals (humans included). Fungal-induced plant disease epidemics have had dramatic historical influences, resulting in starvation, war, and human migration. Fungi are also responsible for the rots that damage timber, agricultural products, and human-made structures. Yet some fungi produce conspicuous fruiting bodies and are the source of endless fascination.

Figure 1. (Click image to enlarge)

By the end of this tutorial you should have a fundamental understanding of:

  • The roles and importance of fungi in the biosphere
  • Basic fungal nutrition
  • Basic morphological diversity within the kingdom
  • Basic reproductive features of fungi

Figure. 2(Click image to enlarge)

Basic Biology and Morphology

Although there are now more than 100,000 described species of fungi, mycologists (scientists who study fungi) estimate that there are probably more than 1.5 million species. The kingdom is comprised of four major phyla or divisions:

  • Chytridiomycota
  • Zygomycota
  • Ascomycota
  • Basidiomycota

The categorization of fungi is based on life cycle features. In some cases, a complete sexual life cycle has not been observed and some mycologists use a fifth phylum (Imperfecta or Deuteromycota) as a repository for these species. However, as scientists apply molecular techniques to understand fungal relationships, the artificial nature of this fifth phylum is becoming apparent and it is falling into disfavor. Hence, this tutorial will concentrate on the four phyla listed above.

Although originally grouped with plants, modern molecular systematists now think the lineage that gave rise to modern fungi arose before plants (possibly one billion years ago). Protists are related to fungi, with a possible link in the Chytridiomycota.

Figure. 3 (Click image to enlarge) |

All fungi are heterotrophic, and many are important and rather prolific decomposers, recycling organic carbon trapped mostly in plant cell walls as lignin and cellulose. A major distinguishing feature of the kingdom is *absorptive nutrition*. Food sources are digested externally, then absorbed into their cells. They produce copious amounts of organic acids to acidify their local environment, followed by powerful digestive enzymes to break down their food source. Fungi are almost unlimited in what they can digest. Everything from trees to human skin (e.g., athlete's foot) are subject to fungal decay. Fungi are also distinguished by their filamentous vegetative structures, *hyphae* (hypha, sing.; somewhat different structurally from the hyphae of the oomycota discussed in the tutorial coverying the oomyctes), and the presence of chitin as a primary cell wall polymer. A group or mass of hyphae (mycelium) may be visible to the naked eye as a white or gray fuzz on moldy bread or on a decaying tree. Extensive networks of mycelium give fungi a tremendous amount of surface area in a small space, making them ideally adapted to their absorptive mode of nutrition. Cellular specialization is somewhat limited in the kingdom. As depicted in this figure on the right, hyphae can be characterized as having cells clearly delimited by structures called *septa* (septate hyphae), or the absence of septa (coenocytic hyphae).

Figure. 4 (Click image to enlarge)

Many fungal plant parasites invade host tissue with specialized structures (haustoria) that are used to absorb the cellular contents of their hosts.

Environmental Impact

Members of the kingdom Fungi produce spores. Although they are mostly nonmotile (some members of the Chytridiomycota produce flagellated spores), spores (produced by the thousands or millions) can be dispersed by water and/or wind great distances to another available substrate or host. Spores are an important taxonomic character and, as expected, come in a myriad of shapes and sizes. Spores can be mitotically or meiotically produced, affording the organism flexibility to reproduce and proliferate over a variety of environmental conditions where sexual recombination may not be feasible.
Fungi play many roles in the biosphere. As you have already learned, they are important decomposers as well as plant and human pathogens. They also engage in extremely important symbiotic relationships with plants and algae. Mycorrhizae are an intimate association (symbiosis) between plant roots and fungal hyphae. The vast majority are mutualistic, with the plant or tree benefiting with increased water and nutrient uptake and the fungus obtaining carbon from the roots.
Symbioses with lichens are also common. Specific fungal species (usually Ascomycetes) grow in concert with these photosynthetic algae. The fungus gains carbon and provides moisture to the algae in a controlled parasitic relationship. Technically, it's not a mutualistic relationship since the alga can live on its own and is consumed by the fungus; however, it is a unique relationship since lichens can grow in extremely harsh climates (e.g., the arctic tundra and bare rocks).  Click here to see some spectacular images of fructicose and foliose lichens and here to see some crustose rock lichens.

Basic Biological and Reproductive Features

Fungi exhibit several different types of life cycles. This generalized diagram on the right depicts the typical sequence of events in the life cycles of fungi. Focus on the "mycelium" box for a starting point for both the sexual and asexual life cycles. Note, many species can produce asexual spores in specialized spore-producing structures. This allows the organism to clone itself while, often times, producing huge numbers of asexual spores. The hyphae of many species are haploid during the majority of their life cycles. Many fungi spend a good portion of their life in the asexual mode. The transition to the sexual mode can be triggered by certain conditions (e.g., light, temperature, moisture, availability of a sexually compatible partner, and limited nutrient availability).

Figure. 5. (Click image to enlarge)

The sexual stage is characterized by several features that are unique to the kingdom Fungi. Most fungal mating types are morphologically indistinguishable, and so, they are not referred to as male or female. Rather, they are simply referred to as "+" or "-" to let one know they are different (and hence, sexually compatible). In the case of sexual reproduction, compatible mating types fuse in a process that involves plasmogamy (fusion of cell membranes). This fusion produces a dikaryon (a mycelium with two genetically distinct nuclei), which can divide in the growing mycelium for a prolonged period. The prolonged dikaryotic stage is a feature unique to fungi. It is thought that the dikaryotic state provides the organism with genetic flexibility in a competitive environment while, at the same time, conferring it with many of the same advantages as a diploid because it possesses a complementary set of chromosomes. The dikaryotic state can last for months, or even years, while the fungus continues to grow and proliferate in its environment.
Another unique feature of the dikaryon is that paired nuclei (n+n) can, and often times do, migrate freely through the mycelium. Some cells may contain two or more pairs, and some (particularly actively growing hyphal tips), even more. When environmental conditions are suitable, the two haploid nuclei fuse (karyogamy) to form a highly transient diploid state. Meiosis follows, almost immediately, in a specialized spore-producing structure, and genetically distinct haploid spores are produced.

Features of Hyphal Growth

Hyphal growth occurs at the tip. As was mentioned, these actively growing areas are often characterized by the presence of multiple pairs of nuclei and a large number of vesicles moving toward the hyphal tip. Click hyphal growth to view a short film on the process of hyphal growth. Note the massive amounts of cytoplasmic streaming and the movement of vesicles along arrays of microtubules toward the actively growing tip. Near the end of the movie, also note the appearance of a nucleus.

Fungal Diversity and the Phylum Chytridiomycota

Modern fungal systematists have divided the fungal kingdom into four major phyla. We will start with what is thought to be the ancestral phylum of the kingdom, Chytridiomycota. Chytrids (members of the phylum) are mostly aquatic, which is why they are often called "water fungi." This phylum has only recently been determined to belong to Kingdom Fungi, therefore, not all textbooks include it in this kingdom. Biologically, they are mostly saprobes (introduced here- decomposers that absorb nutrients from dead organic matter), but some species are parasites of aquatic invertebrates, plants, and protists. They are also characterized as having absorptive nutrition and they can have septate or coenocytic mycelium, or be unicellular. Morphologically, they are important because some members possess flagellated spores. Until recently, systematists believed that the absence of flagellated cells was required for placement in the fungal kingdom. Molecular data have shown that chytrids are indeed members of the fungal kingdom. Ultrastructural and biochemical data (e.g., chitinous cell walls) support this designation as well. The molecular data also support the theory that they are the most primitive group in the kingdom; they retain the ancestral character of having flagellated cells similar to their protistan ancestors.

Figure.6 (Click image to enlarge)

Phylum Zygomycota

Of the roughly 600 described species in the phylum Zygomycota, most are terrestrial and exist as saprobes in soil or on decaying plant material. Several species are important because they form mycorrhizae with forest plants and trees.

Many common bread molds (e.g., Rhizopus stolonifer) are zygomycetes. Morphologically, they have coenocytic hyphae, with septa formed only in association with reproductive structures. The phylum gets it name from the production of the zygosporangium. This figure on the right illustrates the events in the life cycle of a zygomycete. Concentrate on the top portion of the diagram. As you have learned, two different haploid mating types are often required for sexual reproduction in the fungi. First, gametangia begin to form on dissimilar hyphae (step 1). The gametangia then fuse (step 2) to form the dikaryotic state (step 3). The dikaryotic zygosporangium then develops (step 4). The zygosporangium develops a rough and thickened cell wall, which renders it resistant to harsh "over-wintering" conditions. When conditions become favorable for zygospore germination, compatible ("+" and "- " mating types) nuclei fuse and a diploid is briefly formed (step 5). Meiosis immediately follows and millions of haploid zygospores are formed (step 6). The zygosporangium germinates and releases the aforementioned spores and the cycle begins again (step 7). The asexual stage alternates between mycelial growth and asexual spore production (as shown in bottom part of figure).

Figure. 7(Click image to enlarge)


This tutorial introduced the fungi. Fungi have an important saprophytic role in the biosphere. You might not find the subject of decomposition pleasant, but without the fungi (and other decomposers) a good proportion of nutrients would remain tied up in dead organisms and never get recycled back into the biosphere.

Fungi obtain their nutrition by secreting various hydrolytic enzymes into their environment. They then absorb the resulting monomeric units. This mode of nutrition is known as absorptive nutrition and is intimately related to the role that these organisms play in degrading organic matter.

The basic aspects of fungal life cycles were also introduced. Not all fungi have a sexual mode of reproduction, but when they do, the haploid state is the most prominent. An important feature that fungi demonstrate is the presence of a dikaryotic state, in which two haploid nuclei exist in the same cell. When these nuclei fuse, the resulting diploid nucleus undergoes meiosis very soon afterward to give rise to haploid spores. Thus, the diploid state (as defined by the presence of a true diploid nucleus) is very transient.

The hypha is the prominent cell type in the fungal kingdom. Hyphae can exhibit tremendous growth rates via tip growth, whereby new cellular material is continuously being added to the growing tip of the extending cell. Hyphae of fungi and oomycetes are analogous structures that look the same but have different wall properties. (Fungal walls are composed of chitin, whereas the walls of oomycetes are made of cellulose.)

We also examined two fungal phyla, the Chytridomycota (thought to represent the most ancestral state) and the Zygomycota (a common bread mold). Be sure you understand the life cycle of a zygomycete, and in the next tutorial you will learn how this life cycle distinguishes this phylum from others in the fungal kingdom.