Sexual Sporulation in the Ascomycota

Formation of ascospores:
Diagram illustrating formation of ascospores.
  • The apical compartment of a DIKARYOTIC ASCOGENOUS HYPHA elongates and bends over to form a hook (CROZIER).
  • The two compatible nuclei in the apical compartment then undergo MITOSIS SIMULTANEOUSLY.
  • Two SEPTA develop in such a way that the crozier becomes divided into THREE COMPARTMENTS - the tip and basal compartments are uninucleate; the middle compartment is binucleate and is called the ASCUS MOTHER CELL (since it is destined to become an ascus).
Diagram illustrating next stages in formation of ascospores.
  • The nuclei in the ascus mother cell fuse to form a DIPLOID NUCLEUS, which then undergoes MEIOSIS to form FOUR HAPLID NUCLEI.
  • Each haploid nucleus then DIVIDES MITOTICALLY - resulting in EIGHT HAPLOID NUCLEI.
  • A portion of protoplasm surrounds each nucleus - this becomes enveloped by a WALL and matures into an ASCOSPORE.
  • Meanwhile, another ascus mother cell will have been developing alongside the first.
  • In most (not all) fungi belonging to the Ascomycota the asci don't occur singly - they form in groups, surrounded by hyphae and are enclosed in fruiting bodies (ASCOCARPS).
Use of ascocarps in the classification of fungi belonging to the Ascomycota:

1. Hemiascomycetes (includes yeasts)

  • In this case the asci are NOT ENCLOSED in an asocarp.
  • We saw in a previous section how in the yeasts the diploid cell (zygote) is transformed directly into an ascus containing eight ascospores.

2. Plectomycetes

Diagram of a cleistothecium.
  • Fungi belonging to this group form CLEISTOTHECIA.
  • These are round, completely closed ascocarps, possessing no natural opening.
  • The asci are arranged irregularly within them.
  • When mature the cleistothecia burst open to release their asci and ascospores.

3. Pyrenomycetes

Diagram illustrating a perithecium.
  • Fungi belonging to this group form PERITHECIA.
  • Perithecia are spherical or flask-shaped ascocarps.
  • They open via a neck-like OSTIOLE with a terminal pore through which the ascospores are liberated.
  • The asci are arranged in an orderly layer at the base of the cavity .
  • WARNING! - don't confuse perithecia with pycnidia which contain conidia.

4. Loculoascomycetes

Diagram illustrating an ascostroma.
  • Fungi belonging to this group form ASCOSTROMATA (or PSEUDOTHECIA).
  • Ascostromata resemble perithecia but in the former there is no wall surrounding the central region of the ascocarp - only a cavity within the mass of hyphal tissue (STROMA) in which the asci are located.

5. Discomycetes

Diagram illustrating an apothecium.
  • Fungi belonging to this group form APOTHECIA.
  • An apothecium is an OPEN- or CUP-SHAPED ascocarp.
  • The asci are arranged on the exposed surface (HYMENIUM).

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