Classification of Fungi

The fungi are classified based on the characteristics of the sexual spores and fruiting bodies present during the sexual stages of their life cycles. Therefore, based on characteristics of their sexual stages and morphology of their asexual spores and thalli they are classified into different groups.

The imperfect higher fungi are placed at a special class known as Deuteromycetes. The sexual stage containing fungi is present mainly in the ascomycetes or basidiomycetes group.

In this article, the given classification of algae is proposed by the eminent American mycologist Alexopoulos in 1979. In this classification, the fungi and the slime molds are placed within the kingdom mycetae. The mycetae kingdom is divided into three divisions such as;

1. Gymnomycota
2. Mastigomycota and
3. Amastigomycota

Each division is further subdivided into different subdivisions, classes, sub-classes, and orders.

Division I: Gymnomycota

The distinguishable characteristics of these organisms are, they ingest particulate nutrients and also lack cell walls during the vegetative stage. This division is further divided into two subdivisions such as Acrasiogymnomycotina and Plasmodiogynomycotina.

Subdivision 1: Acrasiogymnomycotina 

The Acrasiogymnomycotina subdivision contains a single class known as Acrasiomycetes.

Class 1. Acrasiomycetes

Acrasiomycetes lack flagellated cells except for one species. In the vegetative stage free-living free living amoebae aggregate to form a stalked sorocarp bearing spores in a mucilaginous matrix. Some representative organisms of this class are Dictyostelium discoideum and Polysphondelium violaceum.

This class contains two subclasses such as Acrasiomycetidae and Dictyosteliomycetidae.

Subdivision 2: Plasmodiogymnomycotina

It contains two classes such as 

Class 1 Protosteliomycetes 

Class 2 Myxomycetes.

The true slime molds are included within the class Myxomycetes. Some important distinguishing characteristics of this class are; asexual reproduction by binary fission and sporangiospores formation, and the sexual phase is absent. Physarum polycephalum is the representative organism of this class.

This class contains three subclasses such as Ceratiomyxomycomycetidae, Mycogasteomycetidae, and Stemonitomycetidae.

1. Subclass I: Ceratiomyxomycomycetidae: This subclass contains one order known as Ceratiomyxales.
2. Sub Class II: Mycogasteomycetidae: This subclass contains four orders such as a. Liceales, b. Echinosteleales, c. Trichlales and d. Physarales
3. Sub Class III: Stemonitomycetidae: This subclass contains one order known as Stemonitales.

Division II Mastigomycota

This division contains flagellated lower fungi with absorptive nutrition, unicellular or filamentous, mycelium coemocytic. Mastigomycota division is consists of two subdivisions such as Haplomastigomycotina and Diplomastigomycotima.

Subdivision I: Haplomastigomycotina

This subdivision contains fungi with uni-or, bi-flagellate zoospores. Haplomastigomycotina contain three classes such as;

1. Class 1 Chytridiomycetes: Containing fungi can produce zoospores which are furnished with a single whiplash flagellum at the posterior end.
2. Class 2 Hyphochytridiomycetes: Includes motile cell containing single tinsel flagellum at the anterior end.
3. Class 3 Plasmodiophoromycetes: These are the parasitic fungi that can produce biflagellate motile cells containing both the flagella of whiplash-type at the anterior end.

Subdivision II: Diplomastigomycotima

The distinguishable characteristics of this subdivision are Sexual reproduction ooagamous and zoospores are biflagellate. It consists of one class;

Class 1 Oomycetes

The Oomycetes class contain four order such as a. Lagenidiales, b. Saprolegnailes, c. Leptomitales, d. Peronosporales.

Division III Amastigomycota

The major distinguishable characters of these fungi are absorptive nutrition, lacking flagella, mycelium aseptate, or septate. This division contains further four subdivisions such as; Zygomycotina, Ascomycotina, Basidiomycotina, and Deuteromycotina.

Subdivision I Zygomycotina

This subdivision contains two classes such as Zygomycetes and Trichomycetes.

1. Class I Zygomycetes: Consist of six orders.
2. Class II Trichomycetes: consist of five orders.

Subdivision II Ascomycotina

The major distinguishable characters of these fungi are, contain septate mycelium and can produce ascospores in a sac-like structure known as asci. This subdivision contains only one class such as

Class 1 Ascomycetes: This class contain five subclasses;

1. Subclass I Hemiascomycetidae Contain three orders.
2. Subclass II Plectomycetidae Contain Five orders
3. Subclass III Hymenoascomycetidae Contain Ten orders
4. Subclass IV Laboulbeniomycetidae Contain Two orders
5. Subclass V Lowloascomycetidae Contain five orders

Subdivision III Basidiomycotina

The major distinguishable characters of these fungi are Septate mycelium, which can produce basidiospores, and exogenously on various types of basidia. This subdivision contains only one class such as;

Class 1 Basidiomycetes:

The Basidiomycetes class is consist of three subclasses such as;

1. Subclass I Holobasidiomycetidae
2. Subclass II Phragmobasidiomycetidae
3. Subclass III Teliomycetidae
4. Subdivision IV Deuteromycotina

Classification of fungi based on spore formation

The kingdom of mycota is divided into six groups based on the organization of the vegetative thallus, spores production methods, the morphology of reproductive structures, and particular life cycle.

Phycomycetes

Phycomycetes is also known as Algae-Fungi, because it contains similar characters to algae. The thallus of Phycomycetes is unicellular or coenocytic or aseptate filaments. They are reproduced by asexual spore formation, mainly they form zoospores or non-motile spores.

It also contains isogamous or heterogamous sexual reproduction which is mainly taking place by gametangial contact. The zygote represents the diploid phase. There are two subclasses of Phycomycetes such as oomycetes and zygomycetes.

Oomycetes

Oomycetes can vary from a primitive unicellular thallus to a profusely branched filamentous mycelium. Some oomycetes are terrestrial and obligate parasites. They are reproduced by asexual with the formation of biflagellate zoospores.

Sexual reproduction is accomplished by oogamy which involves the mating of male and female gametes to develop oospore. After that, the oospore goes through the meioses process to produce haploid biflagellate zoospores. An example of Oomycetes is Phytophthora infestans.

Zygomycetes

During the life cycle, this type of fungi produces a diploid resting spore known as zygospore, that’s why this group is termed as Zygomycetes. Most of the members of this group are saprophytic, while some of them act as parasites on plants and animals.

They contain a well-developed mycelium which is profusely branched and coenocytic. Zygomycetes lack motile sexual or asexual cells. They mainly performed their asexual reproduction by sporangiospores, aplanospores or by conidia.

The sexual reproduction in Zygomycetes is accomplished by the conjugation of gametangia which leads to the formation of zygospore. Some example of Zygomycetes are Rhizopus, Mucor, etc

Ascomycetes

The members of this group produce sexual pores within the sac-like viscus, that’s why they are also termed sac fungi. Most of them are terrestrially occurring as saprophytes or parasites. All of them contain a well-developed, branched, septate mycelium except for yeast because yeast is a unicellular fungus.

Their asexual reproduction is accomplished by the formation of non-motile spores, conidia, oidia or chlamydospores. The sexual mode of reproduction in fungi is accomplished by the mating of gametangia of opposite mating types. The cells are non-motile. Some examples of Ascomycetes is Saccharomyces cerevisiae, Penicillium, Aspergillus, etc.

Basidiomycetes

These are can be saprophytic or parasitic. During sexual reproduction, the members of this group produce basidiospores at the club-shaped basidium, that’s why they are termed as Basidiomycetes.

They contain a highly developed, profusely branched, and septate mycelium. There are present different mating types of mycelia such as (+ve) and (-ve). The mycelium of this group’s members is divided into two classes such as primary mycelium and secondary mycelium.

In Basidiomycetes, asexual reproduction is mainly accomplished by fragmentation, budding, oidia, conidia, or chlamydospore. The during sexual reproduction a dikaryotic cell is formed. They contain non-motile cells throughout the life cycle.

The fungi of basidiomycetes are most advanced as their fructifications are often large and prominent. Some examples of Basidiomycetes are Mushrooms, Puccinia, Ustilago, etc.

Deuteromycetes (The Imperfect Fungi)

Deuteromycetes are known as the artificial class of fungi. This group is consists of more than 17000 species of diverse habits and habitats. The members of this group are saprophytes as well as parasites. The parasitic fungi of this group lead to serious diseases in plants, animals including human beings.

Some of the members are unicellular while others are multicellular. The asexual reproduction in Deuteromycetes is accomplished by the formation of conidia. Sexual reproduction is not present in Deuteromycete, that’s why they are known as ‘Fungi Imperfecti’.

Some examples of Deuteromycetes are Alternaria, Fusarium, Helminthosporium, etc. 

Classification of fungi based on Medical Importance

Based on the degree of tissue involvement and the host response to the pathogen Mycoses is classified into different groups such as superficial, cutaneous, subcutaneous, or systemic (deep) infections.

Superficial mycoses (or tineas)

These are mainly found in the tropics and are restricted to the outer surface of the hair and skin. Some example of Superficial mycoses are;

• Piedraia hortae: It is a filamentous member of the Ascomycota. This pathogen is responsible for black piedra. In this disease hair shaft characterized by brown/black nodules on the scalp hair.
• Trichosporon cutaneum: It is a yeast and member of the Basidiomycota. This pathogen is mainly found in soil, water samples, plants, mammals, and birds as well as act as normal flora. It responsible for piedra which is a superficial infection of the skin, and scalp, and pubic hair.

Subcutaneous mycoses

This type of fungus mainly found in soil in tropical and subtropical areas of Africa, India, and South America. They mainly invaded the host cell through the wounded skin. Some example of Subcutaneous mycoses are;

• Madurella mycetomatis and M. grisea: These are filamentous, Ascomycota, and responsible for human mycetoma or madura foot. Some common symptoms of this infection are chronic inflammation, resulting in swelling, distortion, and ulceration of the infected body part. They enter into the host cell through the wounds of skin and grow in cutaneous and subcutaneous tissues. They are resistant to chemotherapy, leaving surgery, even amputation, as the only resolution.
• Sporothrix schenckii: These are thermally dimorphic, Ascomycota, and responsible for sporotrichosis, which also known as ‘rose handler’s disease’. This fungus mainly occurs in soil. This pathogen infects the host cell by entering through the minor skin injury and then spread through the lymphatic system. It can produce septate vegetative hyphae, conidiophores, and conidia at 25°C, whereas at 37°C an oval to cigar-shaped budding yeast cells are formed. This pathogen also causes infections of the lungs and bones and joints, endophthalmitis.

Systemic mycoses

This infection can affects the whole body. Systemic mycoses are divide into mycoses due to primary (usually dimorphic) virulent pathogens, and those due to opportunistic pathogens. 

Cutaneous mycoses

There are present 3 genera of fungi such as Microsporum, Trichophyton, and Epidermophyton, these are responsible for disease in the non-living tissues of skin, hair, or nails/claws. They mainly grow at the zone just above where the protein keratin is deposited. These pathogens are filamentous Ascomycota.

These are often called dermatomycosis. This type of pathogenic fungi can degrade keratin and easily can grow as non-invasive saprotrophs on the skin and its appendages.

Classification of fungi based on Route of Acquisition

Based on route of Acquisition or infection fungi can be two types, either exogenous or endogenous.

1. Exogenous fungi: These types of fungi are transmitted through the airborne, cutaneous, or percutaneous routes.
2. Endogenous fungi: These types of fungi cause infection by the colonization or reactivation of a fungus from latent infection.

Classification of fungi based on Virulence

Based on Virulence fungi are classified into two groups such as Primary pathogens and Opportunistic pathogens.

Primary pathogens: These are cause infections in normal hosts. Primary pathogenic fungi enter the host cell through the respiratory tract. Some examples of Primary pathogenic fungi is Coccidioides immitis, Histoplasma capsulatum, Blastomyces dermatitidis, and Paracoccidioides brasiliensis.

Opportunistic pathogens:  These are mainly caused by an infection in individuals with compromised host defense mechanisms. The host’s defense system can be compromised by different types of diseases such as cancer, organ transplantation, surgery, and AIDS. 

They infect the host by invading through the respiratory tract, alimentary tract, or intravascular devices. Some examples of opportunistic fungal pathogens are Cryptococcus neoformans, Candida, Aspergillusspp., Penicillium marneffei, the Zygomycetes, Trichosporon beigelii, and Fusarium spp.