Classification of Fungi – Gymnomycota, Mastigomycota, Amastigomycota.

Fungi are a group of eukaryotic organisms that can adopt different forms, such as yeasts, molds, or a combination of both. These organisms are ubiquitous and can be found in various environments, performing vital ecological roles. Some fungi are pathogenic and can cause a range of diseases, including superficial, cutaneous, subcutaneous, and systemic infections, as well as allergic responses.

Yeasts are a type of fungus composed of single-celled organisms. These cells reproduce primarily through budding, a process where a new cell forms from the parent cell. Molds, on the other hand, are multicellular fungi that grow in the form of long, thread-like structures called hyphae. These hyphae elongate by apical extension, allowing the mold to spread and colonize new surfaces.

Fungi are heterotrophic, meaning they rely on external sources for nutrition. They digest their food externally by releasing hydrolytic enzymes into their environment. These enzymes break down organic matter into simpler compounds, which the fungi then absorb through their cell walls. This method of digestion is known as absorptive nutrition.

All fungi share certain key features. They possess a chitinous cell wall, which provides structural support, and their plasma membranes contain ergosterol, a sterol unique to fungi. The cell’s ribosomes are 80S, and their microtubules are composed of tubulin, similar to those in other eukaryotic cells. Another defining characteristic of fungi is their ability to synthesize lysine through the L-α-adipic acid biosynthetic pathway, a metabolic feature that distinguishes them from other organisms.

The classification of fungi is based on various factors such as morphology, reproduction, and genetic characteristics. Understanding these characteristics allows for a deeper insight into the diverse roles fungi play in ecosystems and their potential impact on human health.

Classification of Fungi

Fungi are classified based on their practical applications and phylogenetic relationships. The system used for naming and grouping fungi follows a binomial nomenclature, with each species assigned a generic name and a specific name. For instance, Aspergillus niger refers to a species of fungus, where Aspergillus is the genus and niger is the species.

The hierarchical classification structure for fungi is as follows:

• Species are grouped into genera.
• Genera are grouped into families (identified by the suffix -aceae).
• Families are grouped into orders (with the suffix -ales).
• Orders are grouped into classes (ending in -mycetes).

Fungi are categorized into various divisions, each representing different groups with distinct characteristics. The division of fungi includes:

• True slime molds (Myxomycetes)
• Lower fungi (Phycomycetes)
• Higher fungi (Eumycetes)

In 1979, Alexopolous and Mims proposed a detailed classification for fungi, which placed fungi, including slime molds, under the kingdom Mycetae within the super kingdom Eukaryota. The kingdom Mycetae was further divided into three major divisions:

• Gymnomycota
• Mastigomycota
• Amastigomycota

These divisions group fungi based on their structural and reproductive features. Each group provides insight into the evolutionary and functional diversity within the fungal kingdom.

Division I: Gymnomycota

Gymnomycota is a division of organisms that includes phagotrophic species that lack cell walls. This division is divided into two major subdivisions: Acrasiogymnomycotina and Plasmodiogymnomycotina. Each subdivision contains distinct classes and subclasses with specialized characteristics.

Subdivision 1: Acrasiogymnomycotina

This subdivision includes a single class, Acrasiomycetes, characterized by a lack of flagellated cells, with the exception of one species. Within this class, there are two main subclasses:

• Acrasiomycetidae
• Dictyosteliomycetidae

These subclasses differ in their structural features and developmental patterns but share the common characteristic of not having flagellated cells in most of their members.

Subdivision 2: Plasmodiogymnomycotina

The second subdivision of Gymnomycota is Plasmodiogymnomycotina, which includes two primary classes:

1. Protosteliomycetes
2. Mycomycetes

Class 1: Protosteliomycetes

This class contains a variety of organisms that contribute to the broader Plasmodiogymnomycotina division, though specific details about its members are less defined in this context.

Class 2: Mycomycetes

This class includes the true slime molds and is divided into three subclasses:

• Ceratiomyxomycomycetidae
  • Order: Ceratiomyxales
• Mycogasteomycetidae
  • This subclass is further divided into four distinct orders:
    • Liceales
    • Echinosteleales
    • Trichlales
    • Physarales
• Stemonitomycetidae
  • Order: Stemonitales

Each of these subclasses and orders reflects the diversity within the Mycomycetes class, which plays a crucial role in the study of slime molds and their complex life cycles.

Division II: Mastigomycota

Mastigomycota consists of fungi that primarily obtain nutrients via absorption. These organisms can be either unicellular or filamentous, with a coenocytic mycelium structure. This division is split into two main subdivisions: Haplomastigomycotina and Diplomastigomycotina, each with distinct characteristics and classes.

Subdivision 1: Haplomastigomycotina

Fungi within this subdivision are known for producing zoospores, which can be uniflagellate or biflagellate. The subdivision is further divided into three classes:

• Class 1: Chytridiomycetes
  • Fungi in this class produce zoospores with a single whiplash flagellum located at the posterior end.
• Class 2: Hyphochytridiomycetes
  • These fungi feature motile cells with a single tinsel flagellum inserted at the anterior end.
• Class 3: Plasmodiophoromycetes
  • Parasitic in nature, these fungi produce biflagellate motile cells with both flagella of the whiplash type, positioned at the anterior end.

Subdivision 2: Diplomastigomycotina

The fungi in this subdivision undergo oogamous sexual reproduction and produce biflagellate zoospores. It includes a single class:

• Class 1: Oomycetes
  • This class is organized into four orders:
    • Order 1: Lagenidiales
    • Order 2: Saprolegnailes
    • Order 3: Leptomitales
    • Order 4: Peronosporales

Each of these orders represents a specific group of fungi with varying life cycles and ecological roles.

Division III: Amastigomycota

Amastigomycota is a division of fungi characterized by absorptive nutrition and the absence of motile cells. These fungi can have either aseptate or septate mycelium, depending on the species. The division includes four subdivisions, each distinguished by their unique reproductive methods, mycelial structures, and forms.

Subdivision 1: Zygomycotina

This subdivision includes two distinct classes:

• Class 1: Zygomycetes
  • Zygomycetes produce zygospores through sexual reproduction.
  • This class is further divided into six orders, each with specific characteristics related to spore production and ecological roles.
• Class 2: Trichomycetes
  • Trichomycetes are mostly associated with invertebrate hosts and are classified into five orders.
  • These fungi have specialized forms of reproduction and unique adaptations that distinguish them from other classes.

Subdivision 2: Ascomycotina

Ascomycotina fungi typically have septate mycelium and produce haploid ascospores inside specialized cells called asci. The class is divided into five subclasses, each with distinct reproductive methods and ecological niches:

• Class 1: Ascomycetes
  • Ascomycetes are classified into five subclasses based on their reproductive strategies and morphological characteristics:
    • Subclass 1: Hemiascomycetidae
      • Includes fungi with simpler ascus structures and typically three orders.
    • Subclass 2: Plectomycetidae
      • Characterized by five orders, often producing closed fruiting bodies.
    • Subclass 3: Hymenoascomycetidae
      • This subclass contains a wide variety of fungi, divided into ten orders.
    • Subclass 4: Laboulbeniomycetidae
      • Contains two orders, typically parasitic on arthropods.
    • Subclass 5: Lowloascomycetidae
      • Comprising five orders, this subclass includes fungi with specialized ecological roles.

Subdivision 3: Basidiomycotina

Fungi in Basidiomycotina produce basidiospores exogenously, which are borne on basidia. These fungi have septate mycelium and are typically associated with fruiting bodies like mushrooms. The class is further divided into three subclasses:

• Class 1: Basidiomycetes
  • This class is divided into three subclasses:
    • Subclass 1: Holobasidiomycetidae
      • These fungi produce basidiospores directly from the basidium.
    • Subclass 2: Phragmobasidiomycetidae
      • The basidia in this subclass are divided into segments.
    • Subclass 3: Teliomycetidae
      • This subclass includes fungi that produce teliospores as part of their life cycle.

Subdivision 4: Deuteromycotina

Deuteromycotina includes imperfect fungi, whose sexual reproduction is either absent or unknown. This subdivision consists of a single form class, Deuteromycetes, which is subdivided into three distinct forms:

• Form Class: Deuteromycetes
  • Subclass 1: Blastomycetidae
    • Contains fungi with yeast-like growth forms.
  • Subclass 2: Coelomycetidae
    • These fungi form conidia within specialized structures.
  • Subclass 3: Hyphomycetidae
    • These fungi produce conidia on aerial hyphae, typically associated with environmental conditions like soil or decaying organic matter.

Classification of Fungi Based on Spore Formation

Fungi are classified based on several factors, such as the structure of their vegetative body, reproductive organs, spore formation, and life cycle. The kingdom Mycota is divided into multiple divisions, each of which has distinct characteristics regarding spore production. Below is an overview of the classification of fungi, focusing on their spore formation mechanisms and reproductive structures.

Phycomycetes

• Characteristics: This division includes some of the simplest fungi, often referred to as Algae-Fungi due to their algae-like characteristics.
• Thallus Structure: The vegetative body is either unicellular or consists of coenocytic (aseptate) filaments.
• Asexual Reproduction: Asexually, they reproduce via the formation of zoospores or non-motile spores.
• Sexual Reproduction: Sexual reproduction is either isogamous or heterogamous, taking place through gametangial contact. The diploid stage is represented by the formation of a zygote.
• Subclasses:
  • Oomycetes: These fungi can be unicellular or have branched filamentous mycelium. Asexual reproduction occurs through biflagellate zoospores, and sexual reproduction is oogamous, leading to the formation of oospores, which undergo meiosis to produce zoospores. Example: Phytophthora infestans (causes potato blight).
  • Zygomycetes: Known for producing a diploid resting spore called a zygospore. They are primarily saprophytic, with a well-developed, coenocytic mycelium. Asexual reproduction occurs via sporangiospores, aplanospores, or conidia. Sexual reproduction happens through the conjugation of gametangia, forming a zygospore. Examples: Rhizopus and Mucor.

Ascomycetes

• Characteristics: These fungi are commonly called sac fungi due to the production of sexual spores within a sac-like structure called an ascus.
• Thallus Structure: They generally have a well-developed, branched, and septate mycelium. Yeast forms are unicellular.
• Asexual Reproduction: Asexual reproduction occurs through non-motile spores such as conidia, oidia, or chlamydospores.
• Sexual Reproduction: Sexual reproduction involves the fusion of gametangia from opposite mating types, leading to the formation of ascospores within the ascus. There are no motile cells at any stage of the life cycle.
• Examples: Saccharomyces cerevisiae, Penicillium, and Aspergillus.

Basidiomycetes

• Characteristics: Known for producing basidiospores on a club-shaped structure called a basidium during sexual reproduction.
• Thallus Structure: Basidiomycetes have a well-developed, septate, and branched mycelium. The mycelium is differentiated into two mating types (+ve and -ve), and there are two types of mycelium: primary and secondary.
• Asexual Reproduction: Asexual reproduction can occur through fragmentation, budding, oidia, conidia, or chlamydospores.
• Sexual Reproduction: Sexual reproduction is marked by the formation of a dikaryotic cell, with no motile cells present throughout the life cycle.
• Examples: Mushrooms, Puccinia, and Ustilago.

Deuteromycetes (Imperfect Fungi)

• Characteristics: Deuteromycetes are a diverse group of fungi with over 17,000 species. They lack a defined sexual reproductive stage, which is why they are known as “Fungi Imperfecti.”
• Thallus Structure: These fungi can be unicellular or multicellular and are found in various habitats as saprophytes or parasites.
• Asexual Reproduction: Asexual reproduction occurs through the formation of conidia and other types of spores.
• Sexual Reproduction: No sexual reproduction occurs, and the sexual or perfect stage is absent from their life cycle. This makes them distinct from other fungal divisions.
• Examples: Alternaria, Fusarium, and Helminthosporium.

This classification of fungi is primarily based on the characteristics of their spore formation and life cycle, providing an organized approach to understanding their biological and ecological roles.

Classification of Medically Important Fungi

Fungal infections are categorized based on the site of infection, which determines the extent and type of tissue involvement. These infections are broadly divided into superficial, cutaneous, subcutaneous, and systemic (deep) types. Understanding the classification of these infections helps in identifying the causative agents and managing the diseases.

1. Superficial Mycoses (Tineas)
   • These infections affect only the outermost layers of the hair and skin.
   • Piedraia hortae, a filamentous Ascomycota, causes black piedra, a condition marked by brown to black nodules on the scalp hair, which are actually the ascostromata of the fungus.
   • Trichosporon cutaneum, a yeast from the Basidiomycota, is found in the environment and is part of the normal flora of humans and animals. It causes white piedra, affecting the scalp, skin, and pubic hair. This fungus can also become an opportunistic pathogen, particularly in immunocompromised individuals.
2. Cutaneous Mycoses
   • These mycoses are caused by fungi that infect the skin, hair, or nails, growing in areas where keratin is deposited.
   • The primary genera responsible are Microsporum, Trichophyton, and Epidermophyton, all of which are filamentous Ascomycota. These fungi are commonly known as dermatophytes, causing dermatomycosis (often referred to as “dermatophytosis”).
   • These fungi degrade keratin, growing as saprotrophs on skin and appendages. Their growth results in inflammation and irritation of the underlying tissue, which can cause allergic reactions and cell death in affected areas.
3. Subcutaneous Mycoses
   • These infections typically occur after fungi, which are usually saprotrophic soil inhabitants, enter the body through skin wounds, particularly in tropical and subtropical regions.
   • Madurella mycetomatis and M. grisea, both filamentous Ascomycota, cause mycetoma (commonly known as “madura foot”). This infection involves localized, tumor-like abscesses that can become chronically inflamed, resulting in swelling, ulceration, and tissue damage. Mycetomas are notoriously difficult to treat with chemotherapy, often requiring surgical intervention, including amputation in severe cases.
   • Sporothrix schenckii, a thermally dimorphic Ascomycota, causes sporotrichosis, also known as “rose handler’s disease.” The fungus enters the body through minor skin injuries and can spread via the lymphatic system. It forms conidiophores and conidia at cooler temperatures and budding yeast cells at body temperature. In severe cases, sporotrichosis can lead to systemic infections affecting the lungs, bones, joints, and eyes.
4. Systemic Mycoses
   • These infections affect the entire body, with two main categories: infections caused by primary pathogens (often dimorphic fungi) and those caused by opportunistic pathogens.
   • Primary systemic mycoses are typically due to fungi with inherent virulence and the ability to affect healthy individuals. These fungi are often dimorphic, meaning they can exist in both yeast and mold forms, depending on environmental conditions.
   • Opportunistic pathogens are generally non-virulent in healthy hosts but can cause severe infections in immunocompromised individuals, such as those with HIV/AIDS or organ transplant recipients.

Classification Based on Route of Acquisition

Fungal infections can be classified based on how the infection is acquired: exogenous or endogenous. This classification focuses on whether the infection originates from outside or inside the body.

1. Exogenous Fungal Infections
   • These infections are caused by fungi that come from external sources.
   • Transmission Routes:
     • Airborne: Fungi are inhaled through the respiratory tract. This is common in environments where fungal spores are present in high concentrations, such as in soil or decaying organic matter.
     • Cutaneous: Fungi infect the skin directly, typically through contact with contaminated surfaces or infected individuals.
     • Percutaneous: Fungi enter through breaks or wounds in the skin. This can occur when individuals come into contact with fungi from the environment, such as during soil exposure or animal interactions.
2. Endogenous Fungal Infections
   • These infections are caused by fungi already present in the body.
   • Fungi in this category either:
     • Colonize: The fungus exists in a dormant state within the body without causing harm until specific conditions trigger its growth.
     • Reactivate: The infection may stem from a latent fungal infection that resurfaces due to factors like immune suppression or other predisposing conditions.

Classification Based on Virulence

Fungal infections can be classified according to the virulence of the pathogen. This classification distinguishes between primary pathogens and opportunistic pathogens based on their ability to infect hosts with normal or compromised immune defenses.

1. Primary Pathogens:
   • These fungi can cause disease in individuals with healthy immune systems.
   • They are capable of establishing infections even in the absence of any predisposing factors.
   • Examples:
     • Coccidioides immitis
     • Histoplasma capsulatum
     • Blastomyces dermatitidis
     • Paracoccidioides brasiliensis
   • Primary pathogens typically enter the host through the respiratory tract, where they can evade the immune system and establish deep infections.
2. Opportunistic Pathogens:
   • These fungi only cause infections in hosts with weakened or compromised immune systems.
   • Conditions like cancer, organ transplantation, surgery, and diseases like AIDS increase the susceptibility of individuals to opportunistic infections.
   • Common Pathogens:
     • Cryptococcus neoformans
     • Candida species
     • Aspergillus spp.
     • Penicillium marneffei
     • Zygomycetes
     • Trichosporon beigelii
     • Fusarium spp.
   • Opportunistic pathogens can infect through a variety of routes, including:
     • Respiratory tract
     • Alimentary tract
     • Intravascular devices