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Evolution of Fungi

Overview of Phylogeny

The evolution of fungi recognizes 9 distinct phyla:

  • Opisthosporidia

  • Chytridiomycota

  • Neocallimastigomycota

  • Blastocladiomycota

  • Zoopagomycota

  • Glomeromycota

  • Mucoromycota

  • Basidiomycota

  • Ascomycota

Taxonomic Hierarchy

Fungi are classified within the following taxonomic hierarchy:

  • Domain

  • Kingdom

  • Phylum

  • Class

  • Order

  • Family

  • Genus

  • Species
    This classification system is outlined in the publication by Hibbett et al. (2007), which provides a higher-level phylogenetic classification of the fungi, found in Mycological Research, 111:509-547.

Fungal Evolution Research

Further study on fungal evolution encompassing diversity, taxonomy, and phylogeny can be found in Biological Reviews, Volume 94, Issue 6, Pages 2101-2137. This research was first published on 29 October 2019 and can be accessed via DOI: (10.1111/brv.12550).

Eumycota Classification

The remaining Eumycota are categorized into:

  • Zygomycota

  • Glomeromycota

  • Chytridiomycota

Evolutionary Transition in Fungi

The transition into Zygomycota and Glomeromycota marks a significant shift towards terrestrial habitats. This led to:

  • Increased Niches/Substrates: Expansion into a wider array of environmental niches.

  • Unique Challenges: New challenges arose related to survival and dispersal.

  • Motility Representation: The Chytridiomycota are the only true fungi characterized by having motile stages, whereas aquatic higher fungi (Basidiomycetes and Ascomycetes) lack motility organs, which suggests a derivation from terrestrial forms.

Zygomycota

Characteristics of Zygomycota

Zygomycota represents about 1% of all known fungi. They are distinct, common, and fast-growing. Their primary ecological role involves colonizing substrates that are rich in accessible carbon sources such as sugars and starch. The name Zygomycota is derived from their reproductive process, which involves the conjugation (fusion) of morphologically similar gametangia, leading to the formation of a structure known as the Zygosporangium (teleomorph).

Reproductive Characteristics

  • Heterothallic and Homothallic Forms: Both reproductive forms are known within this classification.

  • Zygosporangia: Typically characterized by thick walls.

  • Coenocytic Mycelium: This feature denotes that the mycelium lacks separate cells across its structure.

  • Asexual Reproduction: Achieved through the formation of sporangiospores.

  • Absence of Flagellated Cells: This sets them apart significantly from groups like the Ascomycetes where asexual characteristics may differ.

Sub-classes in Zygomycota

Classes of Fungi

  • Zygosporangia

  • Trichomycetes

Detailed Subclasses

Zygomycetes consist of at least three distinct evolutionary lines:

  1. Mucorales: Characterized by their well-developed mycelium and their status as common saprobes and pathogens.

  2. Entomophthorales: Includes predominantly homothallic species which have less differentiated asexual reproductive structures and limited growth.

  3. Zoopagales: Known for their heterothallic species with complex asexual reproduction mechanisms.

Mucorales Characteristics

  • Include over 300 species, many of which are saprobes and pathogens.

  • Mycelium: Well-developed and septate (coenocytic).

  • Rarely form anastomoses (interconnecting networks typical in most fungi).

  • Presence of rhizoids or stolons is common.

Morphological Diversity in Zygomycetes

The variation among zygosporangia is minimal among species, yet the anamorphs (asexual forms) exhibit a multitude of bizarre adaptations presumably resulting from varied evolutionary pressures. This variation in morphology relates to two main functions:

  • Survival: The longevity of these structures is enhanced by minimal surface area and robust, thick walls.

  • Dispersal: Achieved through diverse reproductive strategies.

Unique Explosion Mechanism

Notably, subsporangial vesicles with light-sensitive retinas, when exposed to osmotically active compounds, can build up pressure exceeding 100 lbs/in². This pressure triggers an explosive dispersal mechanism that can propel a black sporangium up to 2 meters, facilitating dispersal while also adhering to nearby vegetation due to mucilaginous contents that accompany it.

Glomeromycota

Classification and Characteristics

Glomeromycota was only recently described in 2001 based on small subunit rDNA sequences. It comprises about 200 species primarily associated with the roots of a vast majority of plants, which cannot be cultured independently. The fungi grow mostly between root cortical cells, often forming large vesicles meant for food storage. They penetrate cells to create extensively branched structures known as arbuscules.

Mycorrhizal Associations

Prevalence and Importance

Approximately 5-10% of plant species do not form mycorrhizal associations, while about 70% form arbuscular mycorrhizae (AM). The prevalence of this association suggests it originated very early in the evolutionary history of land plants, further supported by fossil evidence.

Functional Mechanism of AM Symbiosis

The primary function of the AM symbiosis is the exchange of carbon for phosphorus (P). The benefit to the plant may not be as evident as that to the fungus:

  • Phosphorus Mobility: P is noted for its immobility in the soil due to its tendency to create insoluble complexes with many soil cations, resulting in extremely slow diffusion rates.

  • Zone of Depletion: As roots extract phosphorus, a depletion zone forms.

  • Fungal Advantage: The fungi extend their hyphal network beyond this depletion zone, allowing for effective phosphorus uptake.

  • Cost-Efficiency: The cost to the plant for constructing hyphae is about 100 times less than that of root construction.

Evolution of the Symbiosis

The potential evolution of this beneficial relationship could stem from an ancestral fungus that functioned as a pathogen—invading plant tissues while facing phosphorus deficiencies. It is hypothesized that these fungi might have initially imported phosphorus through their hyphal networks, eventually presenting some leakage of phosphorus to the plant partner.

Chytridiomycota

General Characteristics

Chytridiomycota are distinguished as the only members of the Kingdom Fungi that possess motile cells, characterized specifically by:

  • Motile Cells: These cells include zoospores and gametes that bear a single, posterior, whiplash flagellum.

  • Thallus Form: Their thallus structure, if present, is coenocytic.

  • Chitinous Cell Walls: Typical of fungal structure.

Morphological Reliability

Morphological traits within this phylum can be highly variable and not considered reliable for classification purposes due to differences observed in zoospore ultrastructure.

Ecology of Chytridiomycota

Chytridiomycota are found in both aquatic and terrestrial environments. They function as saprobes, parasites (of protists, invertebrates, fungi, plants, etc.), with some known to inhabit anaerobic conditions. Although they are widely distributed, there remains a lack of comprehensive data concerning their ecological significance, although they potentially serve as primary invaders and decomposers of organic materials.

Economic Impact of Chytridiomycota

Chytridiomycota can act as vectors for damaging viruses and can be parasitic on pollen grains. A notable example is Synchytrium endobioticum, which causes ‘wart disease’ in potatoes; while their overall economic impact is often regarded as negligible, additional studies detail their complex interactions and implications in ecosystems.