Ascos

Introduction to Ascomycetes

  • Overview of Ascomycetes as a division (Phylum) in fungi.

Ascomycetes and Basidiomycetes (Dikarya)

  • Dikarya Characteristics:

    • Both groups feature compartmentalized mycelium.

    • Have a sexual cycle that includes hyphal fusion independent of meiosis.

    • Dikaryotic stage present in their life cycle.

    • Ability to produce asexual spores known as conidia.

    • Can exhibit complex dispersal systems.

    • Emerging evidence suggests both groups diverged from a common ancestor.

    • Primary morphological character distinguishing Ascomycetes is yet to be determined.

Mycelial Ascomycetes

  • Characteristics of mycelial Ascomycetes:

    • Possess compartmentalized mycelium with septa featuring simple pores (contrast with dolipore septa in Basidiomycetes).

    • Have Woronin bodies present in their structures.

Occurrence and Importance of Ascomycetes

  • Incredible diversity of Ascomycetes:

    • Includes yeasts, filamentous ascomycetes, asexual taxa, and lichen-forming taxa.

    • Extremely broad ecological amplitude.

    • Many species are microscopic, leading to them being undiscovered.

    • Certain species fruiting underground.

    • Variety of ecological roles: mycorrhizae, endophytic, mycoparasitic, etc.

Importance of Ascomycetes in Human Affairs

  • Pathogens:

    • Affect crops and trees (e.g., Apple scab, Powdery mildews, Chestnut blight - Cryphonectria parasitica, Dutch elm disease - Ophiostoma).

    • Interactions with animals: Ringworm, Athlete’s foot, Pneumocystis.

  • Industrial Mycology:

    • Involved in fermentation processes.

  • Medical Mycology:

    • Production of antibiotics.

Case Study: Chestnut Blight

  • History:

    • First recorded in 1905 in the Bronx, New York.

    • The chestnut tree was previously dominant in eastern hardwood forests (accounting for 25% of stand, diameter of 3-4 ft, height of 100-150 ft).

    • Geographic range: from Southern Maine to Georgia, extending westward to Ohio and Kentucky.

    • Chestnut was the most utilized hardwood for lumber.

  • Advantages of Chestnut:

    • Features straight grain, ease of handling, rapid growth rate, resistance to decay.

    • Was the exclusive tree for telegraph poles and railroad ties.

    • Significant source of food for humans and wildlife.

  • Introduction of Cryphonectria:

    • Likely introduced from Japan between 1876-1904.

    • In a span of 50 years, it devastated approximately 3.6 million hectares.

    • The once-dominant tree is now reduced to a minor understory shrub.

    • Current attempts being made for restoration projects.

Somatic Structures in Ascomycetes

  • Structures may be single-celled, mycelial (filamentous), or dimorphic.

  • Possess simple pores that may be coenocytic.

  • Woronin bodies:

    • Membrane-bound structures associated with septum; function is currently undetermined.

  • Concentric bodies:

    • Found in lichenized Ascomycetes; origin and function remain unknown.

Asexual Reproduction in Ascomycetes

  • Methods of Asexual Reproduction:

    • Fission (budding): Typical in yeasts and dimorphic Ascomycetes.

    • Fragmentation: Separation of parts that reproduce.

    • Production of asexual propagules: Includes conidia and/or chlamydospores; also involves soredia (small clusters of algal cells and fungal hyphae).

Sexual Reproduction in Ascomycetes

  • Involves bringing two compatible nuclei together within the same cell, initiated by:

    • Contact between two morphologically similar gametangia, creating a fusion cell leading to ascus (no long-lived dikaryotic phase).

    • Morphologically differentiated gametangia: antheridia (male gametangia) and ascogonia (female gametangia) are connected via a structure called trichogyne (in certain cases leading to persistent dikaryotic stages).

    • Somatogamy: Fusion of somatic hyphae leading to a long-lived dikaryotic phase.

Ascomycete Life Cycle

  • Life Cycle Stages:

    • Formation of ascus; mitotic division occurs here, leading to ascospore formation.

    • Further meiotic divisions yield viable ascospores.

    • Asexual reproduction occurs through spore production (conidia) as well.

  • In relation to the diagram from Biology of Plants, the primary events include:

    • Development of an ascus containing asci and ascogenous hyphae, there exists an initial karyogamy phase followed by meiosis leading to ascospores, illustrating both sexual and asexual methods of propagation.

Filamentous Ascomycetes (Pezizomycotina)

  • Characterized by:

    • High diversity within this group.

    • Development of functional sex organs (e.g., ascogonium, ascogenous hyphae, croziers) encapsulated within ascocarps (structures housing the asci).

    • Types of ascocarps include cleistothecium, perithecium, apothecium, and ascostroma.

    • Generally, mycelial or ascospore stages endure through overwintering.

Relationships and Phylogenetic Status

  • The phylogenetic relationships within the group remain insufficiently resolved, with indications that traditionally recognized groups might be polyphyletic.

  • Previous concepts distinguishing primitive from derived statuses may be incorrect.

  • However, the entire group is determined to be monophyletic based on rDNA sequencing distinctions from both Archiascomycetes and Sakcharomycetales.

Ascosphaerales

  • Sometimes associated with yeasts due to the absence of developed ascocarps resulting from hyphal outgrowths.

Onygenales

  • Comprises many species of medical importance, characterized by:

    • Asci being free on mycelium or encapsulated within ascocarps.

    • Fusion occurring without trichogyne, but with spherical to ovoid deliquescent asci.

    • Formation of single-celled ascospores and dry conidia; many have the capability to degrade keratin.

Eurotiales and Relatives (Plectomycetes)

  • Notable characteristics include:

    • Typically thin-walled, evanescent asci that do not form hymenium.

    • One-celled ascospores.

    • Ascocarp represented by cleistothecium.

    • Anamorphs in this group are diverse.

  • Specializations of Eurotiales include:

    • Overall diminishment of ascocarp and ascogenous systems.

    • Ability to tolerate or necessitate very low water potential.

  • Economically important roles include:

    • Pathogenesis (animal and plant pathogens), producers of antibiotics and mycotoxins, and fermentation processes in foods.

Positive Contributions of Conidial States (Anamorphs)

  • Includes beneficial genera such as Aspergillus and Penicillium (providers of penicillin).

    • Penicillin V Capsules:

    • Each contains: Penicillin V las potassium.

    • Indications for pediatric and adult dosing with specifics on frequency and quantity.

Negative Implications - Toxicity

  • Some fungal species are categorized as highly toxic, demonstrating harmful effects on humans and ecosystems.

Pyrenomycetes

  • Defines ascocarps that are generally perithecial, with ovoid to cylindrical unitunicate asci present, usually occurring in hymenium.

    • Persistent or evanescent asci engaging in forcible spore discharge.

    • Development of one to several celled ascospores.

    • Frequently associated with complex anamorphs and diverse ecological roles.

Plant Pathogens

  • Examples include Cryphonectria, Nectria, and Ophiostoma (notorious for their pathogenic effects).

Discomycetes

  • Characterized by their incredible diversity with ascocarps forming apothecia.

    • Traditionally classified into operculate and inoperculate categories, with recent inclusions of lichenized forms.

    • Exhibit saprobic, predatory, and mycorrhizal lifestyles.

Apothecium Characteristics

  • Structure includes:

    • Hymenium: Layer of asci lining the disk surface, comprised of clavate or cylindrical asci interspersed with paraphyses.

    • Hypothecium: A thin layer of intertwining hyphae beneath the hymenium.

    • Excipulum: Represents the fleshy component of the ascocarp.

Classification of Ascus Types

  • Unitunicate-operculate: Found only within apothecial ascocarps.

  • Unitunicate-inoperculate: No operculum but adapted with a unique elastic ring mechanism, present in perithecial and some apothecial ascocarps.

  • Prototunicate: Lacks an active spore discharge mechanism, mainly residing in cleistothecial ascocarps.

  • Bitunicate: Presents a double wall structure; diverged long ago from unitunicate forms.

Loculoascomycetes

  • Characterized by asci forming within locules in a pre-formed stroma, often correlated with the evolution of bitunicate asci.

  • Includes a significant number of both plant and animal pathogens as well as endophytes, potentially being one of the most species-rich groups of Ascomycetes.

Problematic Taxa

  • Certain taxa like Erysiphales, Laboulbeniales, Spathulosporales are not closely related but are poorly resolved phylogenetically.

    • Traditionally classified among the orders of cleistothecial or perithecial ascomycetes.

Deuteromycetes

  • Exhibit a diversity of life cycle patterns across the Fungi Kingdom; capability to reproduce both sexually and asexually among species from all phyla.

  • Possess an ability to propagate various types of spores, subjecting them to increased dispersal and survival options.

Deuteromycetes Life Forms

  • Holomorph: Combination of anamorph (asexual form) and teleomorph (sexual form).

  • Differentiations include “imperfect” vs. “perfect” forms based on spore types (conidia vs. ascospores).

  • The entire organism is encapsulated as a holomorph.

Pleomorphy in Deuteromycetes

  • Tulasne brothers (1861-1865) described the pleomorphy—capacity of fungi to manifest multiple forms or spore types within the lifecycle.

  • Variability in the production of urinary patterns (synanamorphs) allows for flexible reproductive strategies.

  • The International Code of Botanical Nomenclature allows for differentiating names for teleomorphs and anamorphs.

Deuteromycetes Taxonomy Challenges

  • Approximately 30,000 known Ascomycetes with about 5,000 connected to anamorphs; the fate of thousands that are “orphans” remains unanswered.

    • Exploration of genetic flexibility through heterokaryosis and parasexual cycles.

Classification Challenges in Deuteromycetes

  • Minimal reference to teleomorphs.

  • Teleomorphs identified only in 10-15% of anamorphic species.

  • Single anamorph genus may house various teleomorphs across distinct holomorph genera, leading to convergent evolution among taxa.

  • Mixed associations where several different anamorph genera may yield sexual cycles within a singular holomorph genus, indicating a more complex evolutionary process.

Relevance and Ecological Importance of Conidial Anamorphs

  • Understanding conidial fungi is critical as many species present significant health risks (e.g., conidial fungi often responsible for human mycoses like ringworm, jock itch, athlete's foot).

    • Key plant pathogens and contributors to wood rot and mycotoxin production (e.g., Aspergillus flavus).

  • Vital roles in ecological processes: decomposition, recycling, and engagement within carbon and nitrogen cycles, stream ecology functions, food chain intermediary roles, biocontrol applications (e.g., Beauveria, Arthrobotrys), and human medicinal uses (e.g., cyclosporin, penicillin).