L. 12-G. Biology Mycology D. Ebtihal Muiz Division: Deuteromycota




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L.12-G.Biology Mycology D.Ebtihal Muiz

Division:Deuteromycota 

The division Deuteromycota is also called the Fungi Imperfecti or Imperfect Fungi referring to our "imperfect" knowledge of their complete life cycles. The Deuteromycota are characterized by production of septate mycelium and/or yeasts, and a sexual life cycle that is either unknown or absent. Asexual reproduction is by means of conidia (sing.=conidium) or may be lacking. A conidium may be defined as an asexual spore that is not produced in a sporangium. Where sexual reproduction has been determined for species in this taxon, the sexual stage is usually referrable to the Ascomycota or Basidiomycota. Ideally, once the sexual stage has been determined, that species should be reclassified and placed in the appropriate subdivision. However, this did not prove to be practical since many species are known best by their asexual stage. Thus, a compromise was reached and both the asexual and sexual stage are recognized. As previously discussed in the Ascomycota, when both sexual and asexual stages are known to occur in a life cycle, they are referred to as telomorph and anamorph, respectively. The Deuteromycota is a heterogeneous group of unrelated species in which sexual reproduction has never been observed. Since mycologists refer to the "perfect phase" of a life cycle as the phase in which sexual reproduction occurs, these fungi are often referred to as imperfect fungi. These fungi may have lost their sexual phase through the course of evolution. Alternatively, biologists simply may not have found the appropriate environmental conditions to observe development of the sexual phase of their life cycle.

The Deuteromycota are classified as fungi for two main reasons. First, their multicellular tissue is similar to the hyphae of sac fungi and club fungi. Second, they have erect hyphae with asexual spores, called conidiophores, which are similar to those of the sac fungi and club fungi.

Most imperfect fungi are believed to be related to the sac fungi because their conidiophores closely resemble those produced by the sac fungi during their sexual phase. The imperfect fungi are not placed in the Ascomycota phylum because classification of that group is based on the morphology of sexual structures which the Deuteromycota do not have.

The best known fungus in this phylum is Penicillium. Some species in this genus appear as pathogenic, blue-green molds on fruits, vegetables, and cheeses. Several other species are important for the making of cheeses, such as blue cheese, Roquefort, and Camembert. Certainly the best known product from this genus is penicillin, the first widely-used antibiotic. Penicillin was first discovered in Penicillium notatum over 50 years ago, but is now known to be produced by many other species in this genus. There are a number of different classification schemes for this group of fungi. However, keep in mind that since we are not working with sexual stages here that the classification schemes used to classify the Deuteromycota is artificial and is not intended to show relationship between the taxa.

These groups are no longer formally accepted because they do not adhere to the principle of monophyly. The taxon names are sometimes used informally. In particular, the term 'hyphomycetes' is often used to refer to molds, and the term 'coelomycetes' is used to refer to many asexually reproducing plant pathogens that form discrete fruiting bodies. Other systems of classification are reviewed by Kendrick (1981).



  • Class Hyphomycetes lacking fruiting bodies

    • Order Moniliales (producing spores on simple conidiophores)

    • Order Stilbellales (producing spores on synnemata)

    • Order Tuberculariales (producing spores in sporodochia)

  • Class Coelomycetes spores produced in fruiting bodies

    • Order Melanconiales (producing spores in acervuli)

    • Order Sphaeropsidales (producing spores in pycnidia)

  • Class Agonomycetales lacking spoes

. Some example of orders:

1-Order: Moniliales

Conidia and conidiophore produced on mycelium (Fig. 1-2)








Figure 1: Conidiophores of Ulocladium and a single conidium. Conidia in this order are produced directly on hyhal cell or specialized hyphal cells called conidiophores.



Figure 2: Conidia of Alternaria tenuis are borne in chains







 

2- Order: Sphaeropsidales

Conidia and conidiophore produced in pycnidia (sing.=pycnidium): A fruiting body of variable shape and size, e.g., globose, flask-shaped, disk-shaped, etc., in which conidia and conidiophore are borne.









Figure 4: Pycnidium of Chaetomella. Unlike most pycnidium, this genus does not have a closed, flasked-shaped pycnidium. It is a bowl-shaped structure with many setae: dark, thick-walled hairs.









Figure 5: Conidia of Chaetomella are bean-shaped.







3- Order: Melanconiales

Conidia and conidiohore produced in acervuli (sing.=acervulus): A plate-like stroma on which conidia and conidiophore are borne (Figs. 6-8).











Figure 6: Acervulus of Pestalotia sp. The acervulus is covered by dark conidia









Figure 7: Acervulus at a slightly higher magnification, with fewer conidia. The acervulus appears to be almost cellular because of its tightly interwoven hyphae.









Figure 8: Conidia of Pestalotia are very distinctive.The end cells with the single appendage is where the conidia were attached and the other end of the conidium has two to three appendages. Micrographs were taken under phase optics.

4-Order: Mycelia Sterlia

Mycelium sterile, conidia not produced. Thus, in order to identify these fungi, other characteristics must be utilized. For example, sclerotia (sing.= sclerotium) may be produced (Fig. 9). A sclerotium is a usually rounded structure composed of mass of hyphae, which is normally sterile. Such a structure serves as a "resistant" stage which may give rise to mycelium, fruitbodies or stromata. Some genera may also have distinctive mycelia characteristics that allow them to be identified (Fig. 10).











Figure 9: A sclerotium of the genus Sclerotium   the genus Sclerotium sp. is pictured. Taxa producing sclerotia differ in their apperance and the differences in their morphology is the basis by which their genera are defined.



Figure 10: The mycelium pictured at the left is typical of the genus Rhizoctonia. The hypha is relatively broad and has a characteristic branching pattern. Hyphal branches are oriented perpendicular from their point of origin and are noticeably constricted at their

base. Immediately above the constriction, a septum is formed.

Once again, keep in mind that the classification schemes used to categorize the Deuteromycota is artificial and has no meaning, with respect to phylogeny.





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