Q2. (a) Enumerate the types of fruiting bodies of Ascomycota and Basidiomycota. Write the various steps of ascus formation in Ascomycetous members with suitable illustrations.

Types of Fruiting Bodies

Fruiting bodies are macroscopic, spore-producing structures that arise from the vegetative mycelium after sexual reproduction. They are highly diverse and critical for fungal survival and dispersal. We will categorize them based on the two major phyla: Ascomycota and Basidiomycota.

Fruiting Bodies of Ascomycota (Ascocarps or Ascomata)

Ascocarps are the reproductive structures of fungi in the phylum Ascomycota. They contain sac-like structures called asci, which typically bear four to eight ascospores. The classification of ascocarps is primarily based on their structure and the manner in which asci are exposed or enclosed. The major types include:

• Apothecium: This is an open, cup-shaped, or saucer-shaped fruiting body where the hymenium (spore-producing layer with asci and paraphyses) is freely exposed at maturity. They can be fleshy and sessile or stalked.
  • Examples: Morels (Morchella spp.), cup fungi (e.g., Peziza spp.).
• Cleistothecium: This is a spherical, completely closed fruiting body without a pre-formed opening. The asci are typically globose and scattered irregularly within the ascocarp. Spores are released only upon the disintegration or rupture of the ascocarp wall.
  • Examples: Aspergillus spp., Penicillium spp., powdery mildew fungi.
• Perithecium: This is a flask-shaped or spherical fruiting body with a small, pore-like opening called an ostiole at the apex, through which ascospores are released. The asci are typically cylindrical and arranged in a hymenial layer at the base of the perithecium.
  • Examples: Neurospora spp., Sordaria spp., Xylaria polymorpha (dead man's fingers).
• Pseudothecium (Ascostroma): Similar in appearance to a perithecium, but it is formed directly within a stroma (a mass of fungal tissue) rather than being a distinct, separate structure. The asci are bitunicate (double-walled) and scattered or arranged in irregular locules (cavities) within the stroma.
  • Examples: Apple scab (Venturia inaequalis).
• Gymnothecium: A pearl-shaped, completely enclosed structure similar to a cleistothecium, but its wall consists of loosely interwoven hyphae, often ornamented with coils or spines.
  • Examples: Talaromyces, Gymnoascus.

Fruiting Bodies of Basidiomycota (Basidiocarps or Basidiomata)

Basidiocarps are the multicellular structures of basidiomycetes that bear spore-producing basidia. These are the familiar "mushrooms" and exhibit an incredible variety of forms. Basidiocarps are classified based on their morphology, presence of a stipe (stalk), pileus (cap), and hymenophore (spore-producing surface).

• Agaricoid (Gilled Mushrooms): Characterized by a central stipe (stalk) supporting a pileus (cap) with lamellate (gilled) hymenophores on the underside. Basidia line the surface of the gills.
  • Examples: Common button mushroom (Agaricus bisporus), Amanita muscaria.
• Boletes: Fleshy fruiting bodies with a cap, a stalk, and a hymenophore composed of tubes (pores) on the underside of the cap, rather than gills.
  • Examples: Boletus edulis.
• Polypores (Bracket or Shelf Fungi): Often hard, woody, and perennial, growing shelf-like on trees or logs. They typically have a pileus and a poroid hymenophore (tubes on the underside). They may or may not have a distinct stipe.
  • Examples: Trametes versicolor (Turkey Tail).
• Puffballs and Earthstars (Gasteromycetes): Fruiting bodies with a ball-like or enclosed shape where the hymenophore is entirely enclosed within the fruiting body. Spores are released through an opening or by the breakdown of the outer wall.
  • Examples: Lycoperdon spp. (Puffballs), Geastrum spp. (Earthstars).
• Club and Coral Fungi: Erect fruiting bodies that are unbranched (club fungi) or profusely branched (coral fungi), without a distinct stalk and cap. The basidia are borne on the surface of these structures.
  • Examples: Clavaria spp. (Club fungi), Ramaria spp. (Coral fungi).
• Jelly Fungi: Fruiting bodies with a gelatinous, often brain-like or lobed texture. They lack distinct stipe and cap structures, and basidia are embedded within the jelly-like mass.
  • Examples: Tremella mesenterica (Witch's Butter), Auricularia auricula-judae (Wood Ear).
• Tooth/Hydnoid Fungi: Fruiting bodies where the hymenophore consists of spines or teeth.
  • Examples: Hydnum repandum (Hedgehog mushroom).

Various Steps of Ascus Formation in Ascomycetous Members

Ascus formation is a crucial sexual reproductive process in Ascomycetes, leading to the production of ascospores. It typically involves a series of precisely orchestrated events following plasmogamy (fusion of cytoplasm) and preceding karyogamy (fusion of nuclei) and meiosis. The most common pathway involves the formation of a crozier.

1. Plasmogamy and Dikaryon Formation:
   • In sexual reproduction, two compatible hyphae (or gametangia, such as an ascogonium and an antheridium) undergo plasmogamy, where their cytoplasms fuse.
   • However, their nuclei do not fuse immediately. Instead, the haploid nuclei from both parents pair up within the same cell, forming a dikaryon (n+n).
   • The female gametangium, the ascogonium, develops outgrowths called ascogenous hyphae, which are dikaryotic. These hyphae are supported by sterile vegetative hyphae.
2. Crozier Formation:
   • The tip of an ascogenous hypha bends to form a characteristic hook-like structure called a crozier.
   • Within the crozier, the two dikaryotic nuclei (one from each parent) undergo simultaneous mitotic division. The mitotic spindle aligns so that one pair of daughter nuclei is positioned at the bend of the crozier, another single nucleus at the tip, and a third single nucleus near the basal septum.

   Illustration 1: Crozier formation with mitotic division of dikaryotic nuclei.

   [Imagine a diagram here showing a bent hypha (crozier) with two nuclei at the bend, one nucleus at the tip, and one at the base, separated by septa.]

3. Septum Formation and Ascus Mother Cell:
   • Two septa (cross-walls) then form in the crozier. One septum forms at the base of the hook, separating the terminal cell (which contains a single haploid nucleus, referred to as the apical cell) from the penultimate cell.
   • Another septum forms near the apex, separating the penultimate cell (which is now dikaryotic, containing one nucleus from each mitotic division) from the basal cell (which also contains a single haploid nucleus).
   • The dikaryotic penultimate cell is the ascus mother cell. This is the cell that will develop into the ascus.

   Illustration 2: Septum formation in crozier, leading to ascus mother cell.

   [Imagine a diagram here showing the crozier with two septa, delineating three cells: an apical cell (n), a penultimate cell (n+n, ascus mother cell), and a basal cell (n).]

4. Karyogamy:
   • Inside the ascus mother cell, the two haploid nuclei (n+n) fuse to form a single diploid nucleus (2n). This process is called karyogamy. This is the only diploid stage in the life cycle of most Ascomycetes.

   Illustration 3: Karyogamy in the ascus mother cell.

   [Imagine a diagram here showing the penultimate cell (ascus mother cell) with the two haploid nuclei fusing to form a single diploid nucleus.]

5. Meiosis:
   • The diploid nucleus within the young ascus then undergoes meiosis, a reduction division. This results in the formation of four haploid nuclei (n).

   Illustration 4: Meiosis of the diploid nucleus within the ascus.

   [Imagine a diagram here showing the developing ascus with the diploid nucleus undergoing meiosis to produce four haploid nuclei.]

6. Mitosis and Ascospore Formation:
   • Typically, each of the four haploid nuclei then undergoes one round of mitotic division, resulting in eight haploid nuclei.
   • Each of these eight nuclei, along with a portion of the cytoplasm, gets surrounded by a new cell wall, forming an ascospore. This process is often called "free cell formation."
   • The ascus, now containing eight ascospores, matures. The remaining cytoplasm is known as epiplasm.

   Illustration 5: Mitosis and ascospore formation within the ascus.

   [Imagine a diagram here showing the mature ascus containing eight distinct ascospores.]

7. Ascospore Release:
   • Upon maturity, the ascospores are released from the ascus through various mechanisms, such as an operculum (lid) in apothecia or an ostiole in perithecia, or by the breakdown of the ascocarp wall in cleistothecia. These ascospores can then germinate to form new haploid mycelia, completing the life cycle.

The morphology and development of the ascus and ascocarps are key features used in the classification of Ascomycota.