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. 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. We will recognize a single class: Deuteromycetes and four orders:
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|
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 (Fig. 3).
|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.|
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.|
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.
There are many species of Deuteromycota in which a sexual stage is not known. Of these, there are, undoubtedly, species in which sexual reproduction occurs only in a restricted set of environmental conditions so that the occurrence of the sexual stage is infrequent. However, it is also apparent that some species have lost the ability to reproduce, sexually. Yet, many of the Deuteromycota are highly successful in their environment. Since sexual reproduction is the means by which genetic diversity is maintained in eukaryotic organisms, and diversity is the the key to survival in species, how would a species that has apparently lost the ability to reproduce, sexually, survive? A possible mechanism that provides an answer to this question is the parasexual cycle. This is a process in which plasmogamy, karyogamy and haploidization takes place, but not in any particular place in the thallus nor at any specific period during its lifecycle.
Parasexuality was first discovered by Pontecorvo and Roper (1952) in Aspergillus nidulans. During the parasexual cycle, the following events take place:
Figure 1. Heterokaryon formation refers to the condition by which genetically different nuclei are associated in a common cytoplasm. The most common way in which this can occur is by anastomosis (fusion) of genetically different hyphae (see Fig. 1a on left). Another means by which genetically different nuclei may enter a common protoplasm is by mutation of one or several nuclei. We will refer to the former in this description of parasexuality.
Following initial fusion of hyphal cells, to form a genetically different cell, mitotic division perpetuates the cell and mycelium that is made up of genetically, different nuclei is formed.
|Figure 2. Karyogamy and mitotic division of diploid nuclei: Following heterokaryon formation, fusion of some haploid nuclei that are genetically the same will fuse as well as those that are genetically different. The latter will result in heterozygous diploid nuclei. It is estimated|
|that there is one heterozygous diploid nucleus will occur per one million haploid nuclei (Pontecorvo, 1958).|
|Figure 3. Mitotic Crossing Over: Figs. 3a-b. During prophase of mitosis, mitotic crossing over can occur between chromatids of homolous chromosomes and may produce a unique genetic recombinant. Fig 3c. Recombinant chromosomes separate, during anaphase, and give rise to nuclei that are genetically different from existing nuclei in protoplasm. This is also a rare event, occurring in diploid nuclei, once, in 500 mitoses.|
|Figure 4. Haploidization (not meiosis) of diploid nuclei. During mitosis, errors are common. Diploid nuclei often form one nucleus with three copies of one chromosome|
|(2N+1) and the other with one copy of one chromosome (2N-1). In the latter nucleus, the continual, sequential loss of chromosomes with two copies can occur to eventeually give rise to a haploid nucleus. When haploidization occurs in heterozygous diploids, the resulting haploid will result in a new genetic combination.|
While the parasexual cycle appears to be a viable mechanism by which genetic recombination occurs, many mycologist believe that it does not play a role in maintaining genetic diversity in fungi that have lost their ability to reproduce, sexually. Instead, this has been looked upon as a laboratory phenomenon and that heterokaryon formation, in nature, is not a common event. Thus, the parasexual cycle must also be a rare event.
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