Classification of Fungi – Gymnomycota, Mastigomycota, Amastigomycota.

Fungi are a group of eukaryotic organisms placed under the kingdom Fungi. The cells contain a true nucleus and membrane bound organelles. Earlier these organisms were grouped with plants but later studies showed that fungi are more closely related with animals. The members of this kingdom include unicellular forms like yeast and multicellular forms such as molds and mushrooms.

A characteristic feature of fungi is the presence of a rigid cell wall which is mainly composed of chitin. In multicellular fungi the body is made up of thread like structures called hyphae. These hyphae collectively form a network which is known as mycelium. Fungi do not contain chlorophyll therefore photosynthesis does not occur in them. Because of this fungi are heterotrophic organisms and obtain their nutrition by absorption of nutrients.

Fungi are widely distributed in nature and occur in terrestrial as well as aquatic habitats. They act as important decomposers and help in the breakdown of dead organic matter. This process helps in recycling nutrients like carbon and nitrogen in the environment. Some fungi form symbiotic association with plants such as mycorrhiza while others may live as parasites causing diseases in plants and animals. Reproduction in fungi occurs by the formation of spores which may be produced by both sexual and asexual methods.

Division of Kingdom Mycetae or Classification of Fungi

The kingdom Mycetae is divided into three major divisions based on structural characters and mode of reproduction. Some of the main divisions are–

1. Gymnomycota – These organisms are phagotrophic in nutrition and the cell wall is absent. The body is generally amoeboid in nature and slime molds are included in this group. It is divided into two subdivisions namely Acrasiogymnomycotina and Plasmodiogymnomycotina.
2. Mastigomycota – The members possess absorptive mode of nutrition and the body may be unicellular or filamentous. The mycelium is generally coenocytic (non–septate). Motile cells such as zoospores are produced during the life cycle. This division includes Haplomastigomycotina and Diplomastigomycotina.
3. Amastigomycota – These fungi possess absorptive nutrition and the mycelium may be septate or aseptate. Motile cells are absent in this group throughout the life cycle. It is the largest division and includes Zygomycotina, Ascomycotina, Basidiomycotina and Deuteromycotina.

Division I: Gymnomycota

Division Gymnomycota includes phagotrophic organisms which are devoid of cell wall. These organisms obtain nutrition by engulfing food particles. The division mainly includes slime molds. It is divided into two subdivisions–

1. Subdivision – Acrasiogymnomycotina – This subdivision includes a single class Acrasiomycetes. Class – Acrasiomycetes – The members generally lack flagellated cells except in one species. The class is divided into two subclasses–
   • Acrasiomycetidae
   • Dictyosteliomycetidae
2. Subdivision – Plasmodiogymnomycotina – This subdivision includes organisms commonly known as slime molds. It is divided into two classes– Class – ProtosteliomycetesClass – Mycomycetes – It includes the true slime molds. The class comprises three subclasses–
   • Subclass – Ceratiomyxomycomycetidae
     • Order – Ceratiomyxales
   • Subclass – Mycogasteomycetidae
     • Order – Liceales
     • Order – Echinosteleales
     • Order – Trichlales
     • Order – Physarales
   • Subclass – Stemonitomycetidae
     • Order – Stemonitales

Division II: Mastigomycota

Division Mastigomycota includes fungi which show absorptive mode of nutrition. The vegetative body may be unicellular or filamentous and the mycelium is generally coenocytic (non–septate). The members produce motile cells known as zoospores. This division is divided into two subdivisions–

1. Subdivision – Haplomastigomycotina – In this group the zoospores possess one or two flagella. It includes three classes–
   • Class – Chytridiomycetes – The zoospores possess a single posterior whiplash flagellum.
   • Class – Hyphochytridiomycetes – The motile cells possess a single anterior tinsel flagellum.
   • Class – Plasmodiophoromycetes – These are parasitic fungi producing biflagellated motile cells. The flagella are inserted at the anterior end.
2. Subdivision – Diplomastigomycotina – The members show oogamous type of sexual reproduction and produce biflagellated zoospores. It includes a single class–
   • Class – Oomycetes – These fungi are commonly known as water molds. The class includes four orders–
     • Order – Lagenidiales
     • Order – Saprolegniales
     • Order – Leptomitales
     • Order – Peronosporales

Division III: Amastigomycota

Division Amastigomycota includes fungi which show absorptive nutrition. Motile cells are absent in all stages of life cycle. The vegetative body consists of mycelium which may be aseptate or septate. This division includes four subdivisions–

1. Subdivision – Zygomycotina – These fungi reproduce sexually by the formation of zygospores. It includes two classes–
   • Class – Zygomycetes – It includes six orders.
   • Class – Trichomycetes – It comprises five orders and many members are associated with invertebrate hosts.
2. Subdivision – Ascomycotina – The fungi usually possess septate mycelium. Haploid sexual spores called ascospores are produced in sac like structures known as asci.
   • Class – Ascomycetes – It is divided into five subclasses–
     • Subclass – Hemiascomycetidae – Comprising three orders.
     • Subclass – Plectomycetidae – Comprising five orders.
     • Subclass – Hymenoascomycetidae – Comprising ten orders.
     • Subclass – Laboulbeniomycetidae – Comprising two orders.
     • Subclass – Lowloascomycetidae – Comprising five orders.
3. Subdivision – Basidiomycotina – The members possess septate mycelium and produce basidiospores exogenously on structures called basidia.
   • Class – Basidiomycetes – It is divided into three subclasses–
     • Subclass – Holobasidiomycetidae
     • Subclass – Phragmobasidiomycetidae
     • Subclass – Teliomycetidae
4. Subdivision – Deuteromycotina – These fungi are known as imperfect fungi because sexual stage is absent or unknown.
   • Form class – Deuteromycetes – It includes three form subclasses–
     • Blastomycetidae
     • Coelomycetidae
     • Hyphomycetidae

Classification of Fungi Based on Spore Formation

Fungi are classified based on the type of spores formed during reproduction. Some of the main groups are–

1. Phycomycetes– These fungi possess a simple thallus which may be unicellular or filamentous. The mycelium is generally coenocytic (aseptate). Asexual reproduction occurs by the formation of zoospores or non-motile spores. Sexual reproduction results in the formation of zygote. It includes two subclasses–
   • Oomycetes– Asexual reproduction occurs by biflagellate zoospores. Sexual reproduction is oogamous and produces oospores.
   • Zygomycetes– Motile cells are absent. Asexual spores such as sporangiospores or conidia are formed and sexual reproduction results in zygospore formation.
2. Ascomycetes– These fungi possess a well developed septate mycelium. Motile cells are absent in this group. Sexual spores known as ascospores are produced inside sac like structures called asci. Asexual reproduction occurs mainly by conidia.
3. Basidiomycetes– The members possess septate mycelium and produce basidiospores. These spores are formed exogenously on a club shaped structure known as basidium. Asexual reproduction may occur by fragmentation, budding or conidia formation.
4. Deuteromycetes– These fungi are known as imperfect fungi because sexual stage is absent or not known. Reproduction occurs only by asexual spores such as conidia.

Classification of Fungi Based on Site of Infection

Medically important fungi are classified based on site of infection, virulence of the organism and route of acquisition. Some of the important groups are–

1. Superficial mycoses – These infections are restricted to the outer surface of skin and hair. The infection usually affects the hair shaft and outer keratin layer. Examples are Piedraia hortae and Trichosporon cutaneum.
2. Cutaneous mycoses – These infections involve skin, hair and nails. The fungi responsible are called dermatophytes. They utilise keratin as a nutrient source. The important genera are Microsporum, Trichophyton and Epidermophyton.
3. Subcutaneous mycoses – These infections occur in the tissues below the skin. The fungi usually enter through cuts or wounds in the skin. Examples include Madurella mycetomatis and Sporothrix schenckii.
4. Systemic mycoses – These infections involve internal organs and may spread throughout the body.

Classification of Fungi Based on Route of Acquisition

Fungal infections may also be classified based on the route by which the organism enters the body. It is divided into two main groups–

1. Exogenous infection – In this type the fungus is acquired from external environment. The infection may occur by different routes–
   • Airborne infection – The fungal spores are inhaled into the respiratory tract from air containing fungal particles.
   • Cutaneous infection – The fungus infects the skin by direct contact with contaminated objects or infected individuals.
   • Percutaneous infection – The fungus enters the body through wounds or breaks present in the skin.
2. Endogenous infection – In this type the fungus is already present in the body as normal flora and infection develops under favourable conditions.
   • Colonization – The fungus normally remains harmless but under certain conditions it begins to grow and cause infection.
   • Reactivation – A dormant fungal infection becomes active again usually due to weakened immune system.

Classification of Fungi Based on Virulence

Fungal infections may also be classified based on the virulence of the pathogen. It is divided into two main groups–

1. Primary pathogens – These fungi are capable of causing infection in healthy individuals with normal immune system. They do not require any predisposing condition for infection. The organisms usually enter the body through respiratory tract and produce systemic infection. Examples include Coccidioides immitis, Histoplasma capsulatum and Blastomyces dermatitidis.
2. Opportunistic pathogens – These fungi generally do not cause disease in healthy individuals. Infection occurs mainly in persons with weakened immune system. Conditions such as cancer, organ transplantation or AIDS increase the susceptibility to these infections. Examples include Candida species, Aspergillus species and Cryptococcus neoformans.

Significance/Importance of Fungi

Fungi are important organisms present in nature and they play several useful roles in environment and human life. Some of the main importance are–

• Decomposers in nature – Fungi break down dead plants and animals into simple substances. This process helps in recycling of nutrients in soil.
• Mycorrhizal association – Many fungi form association with roots of plants known as mycorrhiza. The fungal hyphae help in absorption of water and mineral nutrients from soil.
• Lichen formation – Some fungi form symbiotic association with algae to form lichens. These organisms grow on rocks and help in soil formation.
• Food source – Some fungi are edible and used as food. Mushrooms are commonly consumed and contain high nutritional value.
• Fermentation processes – Yeast is used in fermentation for the preparation of bread, beer and wine.
• Production of antibiotics – Certain fungi produce antibiotics used in medicine. An example is penicillin obtained from Penicillium.
• Industrial importance – Fungi are used in industries for production of enzymes, organic acids and vitamins.
• Use in scientific research – Some fungi such as yeast are used as model organisms in genetics and biological studies.
• Plant diseases – Many fungi cause diseases in crop plants such as rusts and smuts which lead to economic losses.
• Human and animal diseases – Some fungi cause infections in humans and animals which are known as mycoses.