The last group of fungi we will be discussing is the Deuteromycota. Because members of this group lack a sexual stage, they are often referred to as imperfect fungi ( or formally the Fungi Imperfecti). Deuteromycota is referred to as form-phylum because the divisions within the group are based on morphology and not on common phylogenetic background. It is thus considered an artificial grouping. There are approximately 17,000 species in this form-phylum but this number is continuously decreasing as more of the teleomorphic stages are being associated with the anamorphic stages. In addition, molecular methods are being used to elucidate the relatedness of the different fungal species in this group. Most Deuteromycota have a well-developed, septate mycelium with distinct conidiophores but some have a unicellular thallus. With the exception of one group, all members reproduce by means of special spores known as conidia. A few imperfect fungi lack conidia and form only sclerotia. There is a tremendous variety of morphologically different conidia produced in the form- phylum Deuteromycota. Conidia may be spherical, ovoid, elongated , star-shaped and so on. They may be one-celled to many-celled, with either transverse septa or both transverse and longitudinal septa. In addition, conidia may be hyaline or colored. These conidial characteristics are the basis for the artificial classification. Members of this group are mostly saprobes, but some are parasitic on plants and animals, including man.
1. Ascomycetes with no sexual stage. Exceptions:
2. Ascomycetes for which the sexual stage is unknown
3. Ascomycetes for which sexual stage and asexual stages are not associated
I. FORM-CLASS BLASTOMYCETES
Members of this form-class are characterized by yeast-like cells which propagate by budding. A pseudomycelium may or may not be formed. True mycelium is either lacking or is not well developed.
A. FORM-ORDER SPOROBOLOMYCETALES
This form-order is characterized by the presence of forcibly discharged ballistospores. There is only one form-family, the Sporobolomycetaceae.
Sporobolomyces - The thallus of this form-genus is unicellular and it forms ballistospores. Mount a small portion of a colony in water and observe the thallus cell and the spore attachment. Ballistospore discharge can be observed in culture by using the low power objective of the compound microscope.
B. FORM-ORDER CRYPTOCOCCALES
In this form-order, the spores are not forcibly discharged. They reproduce by budding and a few produce pseudomycelium or true mycelium. Some also produce arthrospores. Some members are thought to be the imperfect stages of ascomycetous and basidiomycetous yeasts. There is only one form-family, the Cryptococcaceae.
Rhodotorula - Members of this form-genus produce pink colonies of unicellular cells which reproduce by budding. Mount a small portion of a colony of Rhodotorula rubra in water and examine under the microscope. You should be able to observe blastogenous conidium formation or budding.
Candida - This is a very heterogenous form-genus containing members that can be pathogenic to humans. Candida albicans is often isolated from warm-blooded animals, including humans, where it exists as part of the normal flora of mucous membrane. In the presence of predisposition factors like the presence of other diseases, obesity, use of broad spectrum antibiotics and steroids, this fungus can cause a disease known as candidiasis. Examine cultures of Candida spp.
II. FORM-CLASS COELOMYCETES
In this form-class, the conidiophores are borne in a fruiting body. There are two form-orders in this class, depending on what kind of fruiting body is produced. Those forms producing pycnidia are placed in the form-order Sphaeropsidales; those producing acervuli are placed in the Melanconiales.
A. FORM-ORDER MELANCONIALES
This order includes members which have conidiophores produced in an acervulus. Many members are parasitic on plants and cause a group of diseases called anthracnoses. Since an acervulus forms by eruption of the fungus through the host epidermis, it does not form in culture. There is a single family, the Melanconiaceae.
Colletotrichum - This genus typically forms hyphal clumps from which stiff, tapering, dark brown setae arise. The conidia are hyaline amerospores, and are either curved or straight, depending upon the species. Mount a portion of a colony of Colletotrichum sp. in water and look for setae, conidiophores, and conidia under the microscope.
Pestalotia-the conidia are brown phragmospores with hyaline end cells bearing appendages. Mount a portion of a colony in water and observe conidiophores and conidia under the microscope.
B. FORM-ORDER SPHAEROPSIDALES
The distinctive structure of the Sphaeropsidales is the pycnidium. Pycnidia may be quite different in appearance from one genus to another. Depending on the type of pycnidia formed, the Sphaeropsidales are divided into four form-families: (1) Sphaeropsidaceae - dark-colored pycnidia which are globose, leathery to carbonous, usually with an opening; (2) Nectrioidaceae - light- colored pycnidia which are soft or waxy instead of hard; (3) Leptostromataceae - shield-shaped or flattened pycnidia; and (4) Excipulaceae - cup- or saucer-shaped mature pycnidia. We will just examine the form-family Sphaeropsidaceae.
This is a large form-family containing saprobic as well as plant parasitic species.
Phoma -the small, one-celled, hyaline conidia are borne inside an ostiolate, dark-brown pycnidia. The conidia are often extruded from an ostiole in a cirrhus. Examine a colony under the dissecting microscope and look for cirrhi. Mount several pycnidia in water and examine under the microscope.
Septoria -the conidia are hyaline scolecospores. Examine culture under the dissecting scope and look for pycnidia. Mount some pycnidia in water, crush gently, and examine scolecosporous conidia under the microscope. Examine leaves infected with S. nodorum.
Ascochyta - This form genus is characterized by two-celled hyaline conidia which are ovoid to oblong. There are several species parasitic on plants causing leaf spots. Examine cultures of Ascochyta and mount a portion of colony in water. Look for the characteristic conidia.
III. FORM-CLASS HYPHOMYCETES
In the Hyphomycetes the mycelium is well-developed and budding cells are absent. The conidia are borne on conidiophores which are not in a fruting body. Some species form only sclerotia.
A. FORM-ORDER MONILIALES
The form-order Moniliales is a very large group consisting of important species that are plant pathogens, human pathogens and fungi which are widely used in various industry. We will consider four form-families in this exercise.
1. FORM-FAMILY MONILIACEAE
Members of this form-family have hyaline or light-colored conidia. Two important form- genera are Aspergillus and Penicillium which we will consider in the next exercise.
Geotrichum - The hyaline mycelium in Geotrichum fragments into arthrospores with truncate ends. Mount a small portion of a colony in water and observe fragmenting hyphae and arthrospores. (Most of you should have seen this fungal species before).
Gliocladium - In Gliocladium, the phialides are arranged in a compact 'brush' or penicillus and the conidia are held together in a slime droplet. Observe a colony under the dissecting microscope and look for slime droplets. Mount a portion of a colony in water and observe the arrangement of the phialide and metulae.
Trichoderma - This is a ubiquitous soil fungus producing white, yellowish or greenish colonies when cultured. The conidia are ovoid, hyaline, one-celled and are borne in small terminal clusters. Observe cultures of Trichoderma koningi. Mount in water and observe under the microscope.
2. FORM-FAMILY DEMATIACEAE
These fungi have brown or black conidia or hyphae. The majority of genera are saprobic but there are some plant parasites and a few animal and human parasites.
Alternaria - In this form-genus, both conidia and hyphae are dark brown. The conidia are dictyospores. Observe conidia in prepared slides.
Curvularia - The conidia of Curvularia are dark brown phragmospores. The central cells are swollen and the conidia are often curved; the end cells are usually lighter in color. Mount some young conidiophores and observe the conidium development. Mount and observe some mature conidia.
Thielaviopsis - This form-genus produces both dark-brown aleuriospores and hyaline endoconidia with truncate ends. Mount a portion of a colony of Thielaviopsis basicola in water and look for brown phialides with endoconidia and phragmoporous aleuriospores.
Orbimyces - This genus produces staurospores; the conidium has an inflated central cells with four radiating arms. Examine conidia under the microscope from cultures provided.
3. FORM-FAMILY STILBELLACEAE
Members of this family have conidiophores that are united into synnemata.
Dendrostilbella - This form-genus produce green amerospores that are formed in a slime droplet at the apex of a synnema in Dendrostilbella. Observe synnema in culture under the dissecting microscope, then mount some synnemata in water and observe under the microscope.
4. FORM-FAMILY TUBERCULARIACEAE
The Tuberculariaceae are characterized by production of a sporodochium.
Fusarium - Members of this large form-genus produce two types of conidia termed macroconidia and microconidia because of their respective sizes. Observe cultures of Fusarium and mount some macroconidia and microconidia.
Epicoccum - The conidia in Epicoccum are dark-brown dictyospores that are borne in clusters. Mount and examine mature conidia under the microscope.
Starkeyomyces - In this form-genus, the conidia form dark-green, slimy masses on the pure white mycelium of the sporodochium. Mount a portion of a sporulating area of a sporodochium in water and look for conidiophores and conidia. The conidia are one-celled and have tapered ends, with a pair of hair-like appendages at one end.
B. FORM-ORDER AGONOMYCETALES
Members of this form-order do not form conidia, instead sclerotia or sclerotial bodies are produced. For this reason, this form-order is also referred to as Mycelia Sterilia.
Papulospora - In this genus, the sclerotia consist of compact clusters of small cells. Mount a small portion of a colony in water and observe the sclerotia or bulbuls under the microscope.
Sclerotium - In this form-genus the sclerotia are usually relatively large and hard and are composed of very compact hyphae. Observe sclerotia of Sclerotium rolfsii under the dissecting microscope. Mount and crush some of the sclerotia and observe under the compound microscope.
Rhizoctonia-This is also a widely distributed genus found in soils and causing root disease in many host plants. Observe cultures and sclerotia under the microscope. Also look for clamp connections.
IV. TERMS TO KNOW
blastic conidium - a conidium arising from only a portion of a pre-existing conidiogenous cell
thallic conidium - a conidium formed by the transformation of an entire pre-existing hyphal cell
chlamydospores - a thick-walled thallic conidium that generally functions as a resting spore
acervulus - an erumpent, open, saucer-shaped fruit body, bearing conidiophores and conidia, characteristic of Melanconiales
pycnidium - a closed or nearly closed asexual fruit body bearing conidiophores and conidia internally, characterstic of the Sphaeropsidales
setae - sterile hyphae associated with various fruiting stuctures
amerospore - a one-celled spore
conidiophore - a simple or branched hypha arising from a somatic hypha and bearing at its tip or side one or more conidiogenous cells.
phragmospore - a several-celled conidium with transverse (horizontal) septa only
scolecospore - a long slender conidium
phialide - a specialized sporogenous cell producing conidia from an open end in basipetal succession
metulae - cells giving rise to the sterigmata or phialides
dictyospore - conidium having both horizontal and vertical septa
aleuriospore - usually single and apical conidium, thick-walled and pigmented but may also be hyaline
endoconidia - conidia formed from within a conidiogenous cell or phialide
staurospore - a branched or star-shaped conidium
synnema (pl. synnemata) - a cylindrical compact group of conidiophores, charcteristic of Stilbaceae
sporodochium - a cushion-shaped structure made up of loosely grouped conidiophores, characteristic of the Tuberculariaceae
blastospore - conidium that develops as buds from simple or branched conidiophores, or directly from vegetative cells, or previous conidia; often forming simple or branched chains