Ascomycetes

  

There are approximately 65,000 described species of ascomycetes. It is the largest known group of fungi. Ascomycetes are found in marine, terrestrial, and freshwater environments. 

Members of the group are also known as sac fungi because of the presence of asci, the sac-like structures that contain spores during sexual reproduction.
 

Ascus

 

They are also characterized by the formation of fleshy "fruiting bodies" called ascocarps, which contain the asci. Ascocarps may be microscopic, but others are large, including many edible species such as morels (Morchella esculenta) and truffles (Tuber melanosporum).   

   

Ascocarps

Although the asci inside them are sac-like, the ascocarps themselves have many shapes. They may look like cups, clubs, potatoes, sponges, corals, golf-balls, cushions, or plates. They may be solitary or in clusters. Their texture can be fleshy, leathery, rubbery, gelatinous, slimy, powdery, or cob-web-like. Ascocarps also come in multiple colors, including red, orange, yellow, brown, black, and occasionally even green or blue. 

Morchella esculenta

  

Some (such as Saccharomyces cerevisiae) grow as single-celled yeasts. During sexual reproduction, the yeasts develop into an ascus but do not form ascocarps.
  

Budding yeast Saccharomyces cerevisiae

The ascomycetes are very important ecologically, nutritionally, and medically. Ecologically, they are the fungal symbionts present in most fungal-algal (lichen) associations and are extremely important decomposers of plant matter, which in turn makes them extremely important in the global carbon cycle.

  

Crustose lichens 

 Over their long evolution, they have developed enzymes capable of breaking down almost every organic molecule, including cellulose, lignin, and collagen.

 

 Serpula lacrymans breaking down cellulose 

Nutritionally, ascomycetes include a variety of edible species. The yeasts are also important constituents of nutritional supplements. 

Medically, several ascomycetes (including Penicillium) are important antibiotics, and several species serve as biological models for laboratory research in genetics and cell biology studies. 
  

Neurospora crassa, the common bread mold, has been particularly important as a model organism for biological research. Its genome was sequenced in 2003 and it was found to have a small genome of approximately 10,000 genes (about half as many as humans). The genome appears to have natural defenses against mutations such as the accumulations of transposons.
  

A rosette of Neurospora asci

Asexual reproduction is dominant in ascomycetes and is responsible for their ability to spread rapidly into new areas. In the asexual portion of the cycle, mycelia germinate and form conidiophores (specialized hyphae). 

  

Conidiosphore reproducing asexually via production of conidia

They do not form sporangia to contain the spores, as zygomycetes do. The conidiophores contain numerous haploid spores called conidia (sing. conidium), or "naked" spores, at their tips. The conidiophores may form long chains, and spores are dispersed from them into the air. They may also be dispersed by water or by animals. 

Once released, the spores can settle and germinate to form asexually reproducing mycelia, continuing the cycle. They can also enter a sexual cycle in which different mating types fuse by plasmogamy and produce dikaryotic asci.
    

AscomyceteLife Cycle

 Inside each sac-like ascus, the nuclei of the mating types fuse by karyogamy, and meiosis occurs, producing four haploid nuclei. This is followed by mitosis, which produces eight nuclei.

Cell membranes and walls form around the nuclei, turning them into ascospores. Ascospores are ejected forcibly and settle to form mycelia, which can enter either the sexual or the asexual cycle.