Fungi

Classification of fungi

In 1866, Ernest Haeckel attempted to classify organisms and divided them into three categories: Animalia, Plantae, and Protista (microorganism). Haeckel placed fungi in the kingdom Plantae, and this classification was used until the 1950-1960s. In 1969, R.H Whittaker published a new classification that divided organisms into five kingdoms: Plantae, Animalia, Protist (eukaryotic microorganisms), Monera (prokaryotes such as archaea and bacteria), and Fungi.

Then, in 1990, Carl Woese published a study introducing a system classification system composed of six kingdoms: Eubacteria, Archaebacteria, Protista, Fungi, Plantae, and Animalia. These six kingdoms were clustered into three domains: Bacteria, Archaea, and Eukarya (Figure 1).

Fungi are classified based on phylogenetics, and it classifies the kingdom Fungi into seven phyla:

• Ascomycota

• Basidiomycota

• Glomeromycota

• Microsporidia

• Blastocladiomycota

• Neocallimastigomycota

• Chytridiomycota

Fungi characteristics

Now, let's talk about the characteristics of fungi. Fungi are eukaryotes, just like animals and plants. However, what mainly makes them different is their mode of nutrition. While animals engulf food and plants photosynthesize, fungi have the ability to absorb externally digested nutrients.

Fungi are achlorophyllous (they contain no chlorophyll) and heterotrophic, meaning that they use pre-existing organic sources of carbon in their environment. Most fungi are considered multicellular, but some fungi, such as yeasts, are unicellular.

Fungi can obtain their food in two ways: as parasites or as saprobes. Parasitic fungi obtain their food by infecting living organisms, whereas saprobes obtain their food by attacking dead organic matter. Some fungi can also form symbiotic relationships with plants.

What about reproduction? Multicellular fungi may have the ability to reproduce by sexual or asexual spores. Yeasts (unicellular fungi), however, reproduce by budding or binary fission.

Fungi structure

Now that we know the characteristics of fungi let's take a look at the basic body structure of a fungus (Figure 2).

The body of a fungus is called a thallus (or fruiting body), and it is used for reproduction. A fungus can have either a holocarpic or an eucarpic thallus.

• A holocarpic thallus means that the entire thallus is a reproductive structure.

• An eucarpic thallus means that only a part of the thallus is used for reproduction.

Most fungi are filamentous, and contain individual filaments called hyphae. A network of hyphae is called a mycelium, and it is used for nutrient absorption.

Since hyphae are very important when dealing with fungi, let's talk about their structure (Figure 3). Hyphae are filled with protoplasm (cytoplasm and organelles) and vary in thickness depending on function. Some hyphae might have septa (or septal pores) through which cytoplasm flows.

Fungal hyphae also have an inner cell wall composed of chitin (except for Zygomycota, which have chitosan) and β-glucans, and an outer wall containing glycoproteins and α-glucans. Hyphae also have a cell membrane.

Inside the hyphae, we also have structures such as mitochondria, nucleus, Golgi apparatus, endoplasmic reticulum, ribosomes, lysosomes, and peroxisomes. The fungal nucleus is small and can squeeze through the septal pores making hyphae either uni-, bi-, or even multi-nucleated! Fungi contain nuclear DNA, wrapped in histone proteins.

Figure 4 shows the cell structure of a fungus.

Types of fungi

Now that we learned how fungi are classified and their basic body structure, let's dive into some of the different types of Fungi that you might not even know existed!

Basidiomycota

The phylum Basidiomycota is a very large group of Fungi with around 32,000 known species. This group is very diverse, but they all have some features in common: they have a septate mycelium (remember that mycelium is a network of hyphae), and they produce exospores (basidiospores) on a structure called a basidium. Basidiomycetes also produce complex sporocarps (i.e., fruiting body).

Known representatives of this phylum include the famous button mushroom Agaricus bisporus, plant fungal pathogens such as smuts and rusts, hallucinogenic mushrooms such as Pcylocibe cubensis, and also deadly, poisonous mushrooms like Amanita virosa (Figure 5).

Basidiomycota life cycle

Basidiomycetes can reproduce either sexually (through the production of basidiospores) or asexually by the production of asexual spores. To better understand this, let's look at steps in the sexual reproduction of Basidiomycetes (Figure 6).

The cycle starts with basidiospores germinating into mycelia that can be of either + or - mating type. Then, the fusion between two mycelia with different mating types forms a dikaryotic mycelium.

After a dikaryotic mycelium forms, it undergoes mitosis to form a basidiocarp containing cells known as basidia. These basidia cells then go through the process of karyogamy (fusion of two nuclei) to form diploid (2n) nuclei zygote. The zygote will then undergo meiosis and cell division to produce basidiospores. Then, the cycle starts again with the germination of basidiospores!

Ascomycota

Like Basidiomycota, the phylum Ascomycota has a septate mycelium. However, they produce sexual endospores called ascospores. These spores are found inside a structure known as an ascus. The stages are fairly similar to what we saw in the sexual reproduction of Basidiomycetes. Ascomycetes can also reproduce asexually by the formation of asexual spores that undergo mitosis and then germination to produce more mycelia (Figure 7).

Some important fungi in this phylum include:

• Penicillium chrysogenum - a type of Penicillium fungus that can be used to develop the antibiotic penicillin.
• P. roqueforti - a fungus that is used in the production of blue-veined cheeses such as Roquefort cheese.
• Saccharomyces cerevisiae - a unicellular fungi (yeast) used in the fermentation of wine, beer, and bread.
• Gibberella fujikuroi - a type of fungal plant pathogen that causes disease in rice seedlings.
• Candida albicans - you have probably heard about this one before. C. albicans is a yeast that is part of our normal microbial flora. However, physiological changes may cause it to become pathogenic and cause diseases such as candidiasis in humans.
• Glutinoglossum glutinosum - a type of earth tongue fungus (tongue-shaped).
• True truffle (genus Tuber) - subterranean, potato-shaped fungi that is highly expensive.
• Microstoma floccosum - small, cup-shaped fungi that are mostly forest saprobes.

Figure 8 shows some examples of macroscopic fungi in the Ascomycota phylum.

Chytridiomycota

Fungi in the phylum chytridiomycota have a single posterior whiplash flagellum and mostly occur in aquatic environments. These fungi lack a true mycelium. Chytrids reproduce sexually or asexually through the formation of motile spores called zoospores. Important fungi in this phylum include parasitic fungi that kill frogs and other amphibians such as batrachochytrium dendrobatitis (Figure 9).

Glomeromycota

Fungi in this phylum were usually assigned to the phylum Zygomycota, but thanks to phylogenetic analysis of DNA sequences, researchers placed them in their own phylum known as the Glomeromycota phylum. Glomeromycetes lack septate hyphae and produce large multi-nucleated spores, and most likely reproduce asexually. They are mostly associated with mycorrhizal associations, meaning that they grow in mutualistic association with the roots of plant hosts (Figure 10).

Now, I hope that you feel more confident in your understanding of Fungi!