Fungal Anatomy

There are four different phyla in this kingdom. They are based on characteristics of their spores. These include the following:

• Chytridiomycota or Chytrids—these are the oldest and earliest group, with one species adversely affecting the amphibian populations throughout the world.

They are motile with asexual and sexual spores. They have posterior flagella.

• Zygomycota—these are the Zygomycetes that have thick-walled zygospores formed during sexual reproduction. Black bread mold is an example of this type of mold.

• Ascomycota—these are called the Ascomycetes that have spores inside a sac known as an ascus. There are eight sexual spores in an ascus.

• Basidiomycota—these are fungi that have spores on the outside of a clubshaped structure known as a basidium.

There are a few fungi that do not fit well into any of the fungi phyla. The AM fungi (which stands for arbuscular mycorrhizal fungi) live only in the mycorrhizae of plants. They have large spores and only reproduce sexually. There are also the Fungi Imperfecti or “deuteromycetes” that do not have a sexual state within their life cycle and slime molds, which are similar to fungi but are not true fungal organisms.

Most fungi are beneficial to life; however, there are those that cause certain crop diseases and health problems to humans and other animals. Crop diseases such as smuts and rusts are from fungal organisms, while human diseases include those from aflatoxins (caused by moldy fruit and grains). Other human diseases, such as “sick building syndrome” and ringworm are from fungi. Yeast infections are from fungi as well. Certain psychoactive and hallucinogenic substances can be extracted from fungal fruiting bodies of some fungi.

FUNGAL ANATOMY

Fungal anatomy can be thought of in microscopic and macroscopic terms. Microscopically, these organisms form hyphae, which are cylindrical, thready-appearing structures that are only about two to ten micrometers in diameter but can be several

centimeters long. These structures grow at their tips with new hyphae forming as branches off of existing hyphae or forking off from an existing hypha. They can fuse together with other hyphae to form anastomoses. An interconnected group of hyphae come together as a mycelium.

Hyphae can be septate (developing septa), forming internal cell walls within the hypha, or coenocytic (having non-compartmentalized hyphae). These septa have large pores in them so that very large nuclei and other organelles can pass through. Coenocytic hyphae are like multinucleated cells or “supercells”. These are mainly saprophytic, living off of decomposing matter. They have haustoria, which are specialized food-uptake structures in parasitic strains of fungi. Some will have structures called arbuscules that can penetrate host cells in order to consume nutrients.

Fungi are considered opisthokonts, which is an evolutionary group that is characterized by having a single flagellum in the posterior section of the organism; however, except for the chytrids, the major species of fungi have lost his particular feature. The key feature is the cell wall, which contains glucans and chitin.

Septa will form in most filamentous fungi, particularly in the Ascomycota and Basidiomycota phyla. These are basically barriers across the filament. Some will have septa at regular intervals, while others are irregularly spaced. These allow the different “cells” or different compartments of the hypha to rapidly communicate. The main structure of the hypha is referred to as a thallus. Pores can close off in order to enable differentiation within the thallus. Secondary septa can form that prevent movement of cytoplasm out of damaged hyphae; these septa will not have pores. These secondary septa are irregularly spaced and so their appearance depends on stressors within the mycelium. As mentioned, these secondary septa can also form between the sexual structures and the main thallus. They will increase the rigidity of the hypha as it provides structural support for the hyphae that might otherwise rupture.

In yeast cells, the septum functions in reproduction. Their reproduction differs from that of hyphal organisms. In the ascomycetous yeasts, bud cells will separate from parent cells with a septum dividing the bud from the main yeast organism. There is a

wall deposited within the septum with subsequent dissolution of the septum so that the bud can separate off. This leaves a scar behind in the parent cell that will not be able to bud again. This will limit the number of buds that a single yeast cell can develop.

The pores will close off for a variety of reasons with blockage occurring sometimes rapidly. This leads to two separate functional units so that the thallus can grow differently on either side of the blockage. If a hypha is damaged, the pores will close in order to prevent the thallus from losing cytoplasm. The goal of pore closure is to keep the living thallus separate from that part which is damaged and that will die off.

Hyphal structure can be complex with things like fruiting bodies developing in some areas. There are three different types of hyphae. There are generative hyphae that have thin walls and septa. There are skeletal hyphae that are unbranched, having no septa and thick cell walls. Binding hyphae are highly branched and are also aseptate.

Macroscopically, fungal mycelia can be large enough to be seen by the naked eye. Spoiled food and damp surfaces can develop molds. These will have increased pigmentation and/or spores that make them visible. Figure 46 shows the macroscopic structure of a mushroom, which is a fruiting body of a fungus:

Some fungi are only noticeable when they produce spores as mushrooms or molds. The fruiting body is part of the sexual phase of the fungal life cycle. The sporocarp is a multicellular spore-forming structure seen only during the sexual phase of the life cycle of the fungus.

Dimorphic fungi have the capability to form two different shapes, depending on the circumstances. One dimorphic fungus, called Histoplasma capsulatum, has two different shapes depending on the temperature. In colder temperatures, the fungus forms a brown mycelium that looks like threads. In warmer temperatures, such as the human body temperature, they are single-celled organisms, causing the human disease called histoplasmosis.