Fungal Classification: Phycomycetes

• Part 4: General Mycology

General Characteristics of Fungi

• Mycology studies fungi, classified under the kingdom myceteae (kingdom fungi).
• Medical mycology is the study of medically important fungi and the mycotic diseases they cause.
• Mycoses is any disease caused by a fungus.
• Fungi are eukaryotic, possessing a true nucleus.
• They are heterotrophic, unable to produce their own food.
• Fungi can be saprobic, symbiotic, or parasitic.
• They lack chlorophyll and are thus achlorophyllous.
• The cell wall is composed of chitin and complex carbohydrates like mannan and glucan.
• The cell membrane contains sterols, specifically ergosterol.
• Saprobic fungi feed on dead tissues or organic waste, functioning as decomposers.
• Symbiotic fungi engage in mutually beneficial relationships with other organisms.
• Parasitic fungi feed on the living tissue of a host, causing disease.

General Properties of Fungi

• Food Storage: Fungi store food as lipids and glycogen.
• Eukaryotic Cells: Fungi have cells with nuclei and mitochondria.
• Nutrient Acquisition: They can be saprophytes or parasites.
• Reproductive Methods: Fungi reproduce asexually and sexually.
• Cell Wall Composition: Cell walls are made of chitin.
• Cell Membrane Components: Cell membrane components are ergosterols and 80S ribosomes.
• Nutritional Mode: They are chemoheterotrophs or non-autotrophic.
• Growth Patterns: Fungi grow via budding or hyphal tip elongation.

Classification of Fungi

• Fungal classification serves practical applications and reflects phylogenetic relationships.
• Nomenclature is binomial, consisting of a generic and a specific name (e.g., Aspergillus niger).
• Species are grouped into genera, genera into families (suffix -aceae), families into orders (suffix-ales), and orders into classes (suffix-mycetes).
• Mycota (fungi and molds) includes true slime molds (Myxomycetes), lower fungi (Phycomycetes), and higher fungi (Eumycetes).
• Alexopolous and Mims proposed a fungal classification in 1979, placing fungi and slime molds in the kingdom Mycetae within the super kingdom Eukaryota, alongside four other kingdoms.

Fungi Sexual Reproduction

• Zygomycetes produce zygospores, using a simple reproductive strategy.
• Ascomycetes produce ascospores in asci, showing complex cellular structures.
• Basidiomycetes produce basidiospores in basidia, highlighting external spore formation.
• Deuteromycetes have no known sexual reproduction, indicating reproductive uncertainty.

Classification Based on Spore Production

• The kingdom Mycota is classified based on the organization of vegetative thallus, morphology of reproductive structures, spore production, and life cycle.
• Phycomycetes include the simplest fungi, also known as Algae-Fungi due to similarities with algae like Vaucheria.
• They have simple unicellular, coenocytic, or aseptate filaments.
• Asexual reproduction occurs through zoospores or non-motile spores.
• Sexual reproduction is isogamous or heterogamous via gametangial contact.
• The diploid phase is represented by a zygote.
• Phycomycetes are divided into subclasses; oomycetes and zygomycetes

Divisions of Phycomycetes

• Oomycetes range from unicellular thallus to profusely branched filamentous mycelium.
• Many are terrestrial and obligate parasites.
• Asexual reproduction occurs through biflagellate zoospores.
• Sexual reproduction is oogamy involving the fusion of male and female gametes to form oospores.
• Oospores undergo meiosis to produce haploid biflagellate zoospores.
• Phytophthora infestans causes potato blight.
• Zygomycetes form a diploid resting spore called the zygospore during their life cycle.
• Mostly saprophytic, with some parasites on plants and animals.
• The vegetative body is a well-developed, profusely branched, and coenocytic mycelium.
• Sexual reproduction occurs by conjugation of gametangia, resulting in zygospore formation.
• Rhizopus and Mucor are examples.
• Ascomycetes are called sac fungi because they produce sexual pores within a sac-like ascus.
• They are mostly terrestrial, occurring as saprophytes or parasites.
• They possess a well-developed, branched, septate mycelium, except for yeast, which is unicellular.
• Asexual reproduction occurs through non-motile spores, conidia, oidia, or chlamydospores.
• Sexual reproduction involves the fusion of gametangia of opposite mating types.
• There is an absence of motile cells.
• Examples : Saccharomyces cerevisiae, Penicillium, and Aspergillus.
• Ascomycetes are central in land-based ecosystems.
• Basidiomycetes produce basidiospores at the club-shaped basidium during sexual reproduction.
• Mycelium is highly developed, profusely branched, and septate, differentiated into two mating types (+ve and -ve).
• Primary and secondary mycelium exist.
• Asexual reproduction occurs through fragmentation, budding, oidia, conidia, or chlamydospore.
• A dikaryotic cell forms during sexual reproduction, without motile cells.
• These are advanced fungi with often large and prominent fructifications.
• Mushrooms, Puccinia, and Ustilago are examples.
• Basidiomycota significantly impact ecosystem functioning as detritivores, absorbing nutrients from decaying matter.

Deuteromycetes

• Deuteromycetes, traditionally known as fungi imperfecti, have no acknowledged sexual stage.
• They comprise over 17,000 species with diverse habits and habitats and represent an artificial class of fungi.
• They are saprophytes and parasites, with parasitic fungi causing serious diseases in plants and animals.
• Some are unicellular, while others are multicellular.
• They reproduce asexually by conidia along with other types of spores.
• Sexual reproduction is entirely absent.
• The asexual (imperfect) stage is well defined, but the sexual (perfect) stage is absent, thus the name 'Fungi Imperfecti'.
• Alternaria, Fusarium, and Helminthosporium are examples.

Fungal Morphology

• Yeasts are unicellular organisms and round-like oval cells.
• Colony morphology is assessed through culture.
• Candida albicans (found in normal flora) and Saccharomyces cerevisiae (found in baking powder) are examples.
• Yeast is around 5 times larger than bacteria.
• Filamentous fungi exhibit a mold form with hyphae and mycelium.
• Hyphae are multicellular filamentous structures constituted by tubular cells with cell walls.
• Budding yeast cells and pseudohyphae are seen in clinical samples.
• Dimorphic fungi exist between yeast and filamentous forms: yeast (parasitic, tissue form, cultured at 37°C) and filamentous [saprophytic] (cultured at 25°C).
• Dimorphic forms depend on changes in environmental factors.
• Molds are filamentous fungi like Aspergillus species.
• Yeast-like fungi produce pseudohyphae, e.g., Candida.
• Dimorphic fungi change forms based on the environment, like Histoplasma; they exist in two different morphological forms under differing conditions.
• Dimorphic fungi grow as either a mold or a yeast, with most pathogenic fungi being dimorphic.
• They grow as yeast at 37°C and as mold at 25°C and are affected by changes in CO
• Their growth differs in tissue versus nature/culture, influenced by temperature.
• Examples include Histoplasma capsulatum and Blastomyces dermatitidis.

Morphology of Filamentous Fungi

• A hypha (plural hyphae) is a long, branching filamentous cell that is the main mode of vegetative growth.
• Mycelium is the intertwined mass of hyphae that forms the fungal colony.
• Conidia/spores (singular=conidium) are asexual spores borne externally on hyphae or on a conidiophore.
• Aspergillus is a common pathogenic filamentous fungi.
• Example; Penicillium synthesizes penicillin.
• Example; Rhizopus causes black bread mold.
• Moniliaceous molds have hyaline or lightly pigmented conidia or hyphae and appear colorless.
• Dematiaceous molds are pigmented, and the colonies appear dark, brown, or black.
• Septate-hypha has cross-walls (septa) that divide hyphae into segments.
• Non-septate lacks cross-walls.

Reproduction in Fungi

• Asexual reproduction occurs through mitotic cell division.

• Somatic production of daughter cells means that cells are genetically the same
• Spore formation;
  • Sporangiospores germinate in sporangia
  • Chlamydospores germinate in or on hyphea
  • Conidia germinate on a hyphea
• Spores are small airborne particles by which fungi reproduce, produced by mitosis, and readily disseminate in the air.
• Sexual reproduction occurs through fusion, mitosis, and meiosis.
• Spores germinate and produce filaments that grow to mycelium.
• Fungi reproduce both sexually and asexually.
• Sexual mode of reproduction is referred to as teleomorph, and asexual mode of reproduction is referred to as anamorph.
• Vegetative reproduction occurs by budding and fission.
• Asexual reproduction takes place with spores (conidia, zoospores, or sporangiospores).
• Sexual reproduction produces ascospores, basidiospores, and oospores.
• In some fungi, the fusion of two haploid hyphae does not immediately result in diploid cells so there appears an intermediate dikaryophase.

Explore the characteristics, taxonomy, and classification of Phycomycetes fungi. Learn about key features like coenocytic hyphae and reproduction methods, and how these differentiate subgroups like Oomycetes and Zygomycetes. Test your knowledge of fungal taxonomy.