Reproduction in the dikarya: Ascomycota

The  vegetative stage  of the  Ascomycota life cycle  is accompanied or followed by asexual sporulation by the  production of single  spores called  conidia from  the  tips  of aerial hyphae called conidiophores. The spores can be delimited by a complete trans- verse wall formation followed by spore differentiation termed thallic spore formation, or more usually by the extrusion of the wall from the hyphal tip, termed blastic spore formation. These spores can be single-celled and contain one haploid nucleus, or they can be multicellular and contain several haploid nuclei produced by mitosis.

Spores can be produced from single, unprotected conidiophores or they can be produced from aggregations that are large enough to be seen with the naked eye. The conidiophores can aggregate into stalked structures where the spores produced are exposed at the top (synnema or coremia). Alternatively, varying amounts of sterile fungal tissue can protect the conidia, as in the flask-shaped pycnidia. Some species produce conidia in plant tissue and the conidial aggregations erupt through the plant epidermis as a cup-shaped acervulus or a cushion-shaped sporodochium.

Sexual reproduction in this group occurs after somatic fusion of different mating-type mycelia. A transient diploid phase is rapidly followed by the formation of ascospores within sac-shaped asci differentiated from modified hyphal tips.  In the initial stages of ascal development hooked hyphal tips form, called croziers or shepherds’ crooks because of their shape. They have distinctive septae at their base, which insure that two different mating-type nuclei are maintained in the terminal cell. Formation of the septae is coordinated with nuclear division. In yeasts all these events occur within one cell, after fusion of two mating-type cells, the whole cell being converted into an ascus.

In more complex Ascomycota many asci form together creating a fertile tissue called a hymenium. In some groups the hymenium can be supported or even enclosed by large amounts of vegetative mycelium. The overall structure is called a fruit body or sporocarp and is used as a major taxonomic feature. It can become big enough to be seen with the naked eye. The Flask-shaped sexual reproductive bodies are called perithecia cup-shaped bodies are called apothecia and closed bodies are called cleistothecia.

Figure:Asexual reproduction in the  Ascomycetes. (a) Thallic spore formation; (b) blastic spore formation; (c) aggregations of conidiophores

                                                            Figure:  Sexual reproduction in the  Ascomycetes

Figure.  Structures of sexual  sporocarps in the  Ascomycetes. (a) Perithecium; (b) apothecium; (c) cleistothecium.

These structures have evolved to protect the assist and asci in spore dispersal, but the hymenium itself is unaffected by the presence of water.