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The Basics of  Mycology & The Fungi

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The Slime Molds:  Amoebozoa, Mycetozoa1

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          Slime molds were considered by DeBary as a separate group of organisms and he placed them in the Mycetozoa   Bessey included Mycetozoa with the Protozoa, and Martin placed them in Myxomycetacea with the “True Fungi.”  Here the group will be treated in the following four classes: Myxogastria: (Myxomycetes; Mycetozoa), Acrasieae, Plasmodiophoreae (Phytomyxineae) and Labyrinthalae

 

Class:  Myxogastria (Myxomycetae) – True Slime Molds

 

          The true slime molds are by far the largest group in the Amoebozoa, with several hundred species having been described.  These organisms are frequently encountered on the forest floor, although sometimes they may be found on lawns, in gardens or other situations.  The vegetative phase is rarely seen by the casual observer because it develops in a concealed position beneath fallen leaves, underneath bark of old fallen tree trunks, or even inside the spongy wood mass of a decaying log.  The plasmodium will crawl out to an exposed and drier situation just before the organism passes into the fruiting stage.  Occasionally such migrating plasmodia can be found creeping over the outside of a stump or a log, or over the outside of a pile of dead leaves.  However, most often it is the fruiting bodies that are found, and they bear a resemblance to the fructifications of some true fungi. Two principal sub-classes of the Myxomycetae are the Exosporeae and Endosporae.  In the Exosporeae the spores are borne on the outside of the fruiting body.  Ceratiomyxa, a typical example, has an erect, branched fruiting body on the surface of which are spines.  At the tip of each spine multinucleate spores without a peridium are formed.

 

 

          In the Endosporeae the spores are borne on the inside of the fruiting body and a true plasmodium is present.  It is an unwalled, multinucleate mass of protoplasm, capable of ingesting solid food particles and absorbing water soluble nutrients.  It represents the vegetative stage of the Mycetozoa. This group is commonly found in cool, shady and moist places.  Their shape is irregular and a membrane binds the outer surface.  They are multinucleate with vacuoles and food granules.  A characteristic streaming action can be seen in these fungi. 

 

 

          They may contain various pigments, and as the plasmodium moves it is capable of absorbing soluble nutrients and also it can ingest relatively large particles.  Wastes are excreted in the path of movement as “ghosts” left in distinctive patterns.  Two plasmodia of similar “race” will fuse and the protoplasm of one will be engulfed in the other.  When incompatible races approach each other, they will not fuse.  No actual contact is necessary as chemical signals are transmitted across the gap.

 

 

          A fruiting stage is formed when unsuitable environmental conditions prevail (See PLATE 8 below).

 

 

          Progressive cleavage is where uninucleate spores are formed beginning at the peridium.  Then membranes begin at the peridium and capillitia.  At maturity the peridium breaks and flakes off and the spores are distributed through the air.  Spores may give rise to 1 or 3 protoplasts, which emerge flagellated or non-flagellated.   The non-flagellated protoplast, or myxamoebae, is haploid.  The flagellated protoplast (zoospore, planospore or swarm spore) bears two anterior flagella (one short and one long).

 

          Two zoospores may fuse at their posterior ends, which give rise to a zygote (diploid).

 

 

          Several zygotes may fuse to give a multinucleate diploid zygote.  Mitosis follows.  Or, in a rare case one zygote will undergo mitosis to form a multinucleate structure.  The inner core may form and ascend through the sporangium to form a columella.

 

 

          In Stemonitis and Hemitrichia the vacuoles may anastomoze to form a network capillitium, or, there may no vacuoles formed.  The stalk length may vary to sessile to form a aethalium (= a fusion of many sporangia), as in Lycogala and Fuligo)

 

 

          The inside of the plasmodium may disrupt to form a plasmodiocarp:

 

 

          A sclerotium is a mass of multinucleate cells in the plasmodium called spherules.  These are very resistant to extremes in temperature and desiccation.  They may contain 1, 7 or 8 nuclei.  When optimum conditions are reestablished, the walls of the spherules break-up and a plasmodium is reconstituted.  Sclerotia will form if the plasmodium is subjected to slow desiccation, reduced temperatures, or low pH.  They are very durable structures, capable of withstanding the most adverse weather conditions.  If formed naturally outdoors they develop in the location where the plasmodium was growing.

 

 

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Comparison of Expsporae with Endosporae

 

          In the Exosporae the spores are borne on strigmata that cover the outer surface of the fruiting body.  Ceratiomyxa is the main example of this group.  In the Endosporae spores are borne within the fructification.  Generally, three morphological types of fruiting bodies are apparent:  (1) small, discrete sporangia, (2) plasmodiocarps and (3) aethalia.

 

[Please see PLATE 7 for life cycle of Physarum polycephalum.]

 

Class: Acrasieae – Cellular Slime Molds

 

          Acrasieae have a characteristic slimy appearance during the vegetative stage.  They are of little economic importance and the emphasis is rather on their morphogenesis.  They are usually isolated from humus soil where they utilize bacteria.  They are not known to use soluble nutrients and they may be cultured in “pure-mixed” cultures (growing with a single species of bacteria).

 

          The vegetative stage is the myxamoeba, which does not have a cell wall but rather a cell membrane.  They are uninucleate with food and waste vacuoles.  They show a creeping action, engulfing bacteria by extension of pseudopodia.  They may also exist on dead bacteria and have been grown on the protein fraction extracted from bacteria.

 

          Adverse conditions are passed in a Microcyst, which will round up and build a wall around itself.  They have the capacity to reduce the density of bacteria in the soil.  When a minimum density is reached on a substrate, the myxamoebae will tend to aggregate around certain centers.  These centers are formed by initiator cells, which secrete Acrasin that provides a positive chemotrophic attraction to other cells in the mass.

 

 

          The Fruiting Stage of the Acrasieae begins with a pseudoplasmodium, which is an aggregation of amoeboid cells that constitute the initial stage of fruiting.  In some species the pseudoplasmodium will not tip.  But if it does tip it shows a short migratory motion.  The pseudoplasmodium then migrates and leaves a mucous sheath behind.  Sometimes certain species will leave a stalk in their trail.  While sclerotia may be interpreted as an interruption in the vegetative development, the formation of fruiting bodies completes the life cycle of the organism.  The spores of most species are very durable and, similar to sclerotia, they can carry the slime mold over periods of unfavorable weather.  Some species require a relatively long rest period before they will germinate.

 

 

          During the migratory phase, the pseudoplasmodium is sensitive to light and temperature, and some feeding occurs.

 

 

          The Culmination Stage is reached as the apex portions of the pseudoplasmodium come to the top.

 

 

          The Sorocarpic Stage varies among species.  In Dictyostelium a single sphere occurs at the apex of a cellular stalk, while in Polyspondylium each section separates into individual sori and each sends out a stalk in a whorl.

 

 

Comparison of the Myxogastria and the Acrasieae

 

        The Myxomycetae have gametes, a single-celled plasmodium and flagellated spores.  The Acrasieae do not have gametes, a pseudoplasmodium that gives rise to many cells and no flagellated spores.

 

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          Also view the following Plates for Example Structures of the Amoebozoa:

 

Plate 9 =  Amoebozoa: Myxogastria, Endosporeae-1

 

 Plate 10 = Amoebozoa:  Myxogastria, Endosporeae-2 

 

 Plate 11 = Amoebozoa:  Myxogastria, Exosporeae and Amoebozoa:  Acrasieae 

 

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Class:  Plasmodiophoreae (Phytomyxineae)

 

          The Plasmodiophoreae are the parasitic slime molds, a group that seemes to be allied to the Myxomycetes.  Some species are pathogens of economic concern.  Important genera are Plasmodiophora, Spongospora, Sorosphaera and Sorodiscus All are obligate parasites and intracellular in their hosts.  None form fruiting bodies and they attack angiosperms primarily.  The genera are grouped by the manner in which spores are held together.  The vegetative phase is an unwalled amoeboid mass, which is probably diploid and develops within the cells of the host attached.  No fruiting body is formed, and the entire thallus is ultimately cleaved into resting spores, which in some cases become firmly joined into a ball (cystosorus) of definite form.  The genera have been defined on the basis of whether the spores are united or free, and on the manner of grouping if the spores adhere with one another or are cemented together.

 

          There are over 70 genera with the life cycle of Plasmodiophora brassicae, the club root of crucifers, being worked on thoroughly.  In this species the plasmodia reside inside the living cell of the host root.  They are picked up as small, multinucleate plasmodia.  These may pass from one cell to another and although they do not kill the host cell they cause it to become large and watery (hypertrophy).  The trend is to move toward the cambial layer where they stimulate the host to divide its cells (hyperplasia).  Each daughter cell of the host then carries over a portion of the plasmodium with it.  The plasmodium may fragment into a meront (parent plasmodium = shizont; fragment = meront).  Eventually the plasmodium will fill the host cell almost entirely.  As it enlarges, the nuclei increase in number by protomitosis.

 

 

          At midsummer the nuclei will lose all karysomes to form the Akaryote State.  The plasmodium becomes vacuolated at this stage that may precede meiosis.  Then the plasmodium splits up into uninucleate material and finally forms resting spores, which can remain viable from 7 or 8 years.  Under optimum conditions, the spores break up and release a single anteriorly flagellated zoospore each.  These swim in free water of the soil until reaching a host.  Zoospores penetrate the epidermis of the host, discard their flagella and become myxamoeba inside the host.  The plasmodium that forms becomes multinucleate and is thought to be haploid.  The plasmodium will break up into parts that form up to 8 nucleate bodies (gametangium).  The gametangia will release gametes, each with two flagella that fuse later on, although there is some doubt as to whether or not two gametes will fuse from the same gametangium.  The fusion of gametes results in a zygote, which usually takes place near the host epidermis.  The zygote divides, becomes uninucleate and migrates to the host cambium. 

 

          Plasmodium brassicae is considered a “High Type” parasite, as it does not readily kill the host cell.  This is in contrast to a “Low Type” parasite, which either kills the host cell on entering or shortly thereafter.  The nucleus of the host is the most highly resistant to the pathogen, but it will be greatly enlarged.  Islands of infected cells surrounded by healthy cells are known as “Krankheitsheid  The hypertrophy initiated by the pathogen results in a rather soft host tissue, which is easily attacked by secondary pathogens.  A wilt also results due to the disruption of the vascular system.

 

     Other species and genera differ in their development from P. brassicae.  In Spongospora subterranean the gametangia may release zoospores that may infect other hosts.  Here the zoospore-producing bodies are referred to as “sporangia.”   Spongospo. subterranean causes “Powdery Scab” disease of potato.   Sorosphaera veronicae is a parasite on Veronica and causes tumors on petioles, stems and leaf midribs.  If there were a sporangium produced in the order Plasmodiophorales, it would be best to place it with the Zygomycota.  In any case, the spores tend to be held together in sponge-like masses or cystosori.  [In P. brassicae the spores are in no definite mass pattern].].  In the Genus Sorosphaera the spores are held together in a sphere; in Tetramyxa they are in groups of four and in Octomyxa the spores are in groups of eight.

 

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          Also view the following Plates for Example Structures & Life Cycles of the Amoebozoa:

 

Plate 6 = Cultures on Agar -- Actinomycetales

Plate 7 = Life Cycle -- Physarum polycephalum

Plate 8 = Fruiting State -- Myxomycetae, Endosporae

Plate 9 = Example Structures -- Amoebozoa: Myxogastria, Endosporeae 1

Plate 10 = Example Structures -- Amoebozoa:  Myxogastria, Endosporeae 2 

Plate 11 = Example Structures -- Amoebozoa:  Myxogastria, Exosporeae  and Amoebozoa:  Acrasieae 

Plate 12 = Life Cycle -- Plasmodiophora brassicae

Plate 13 = Life Cycle -- Spongospora subterranea

Plate 14 = Example Structures -- Amoebozoa: Plasmodiophoreae

Plate 68 = Life Cycle -- Physarum polycephalum

Plate 69 = Four types of resting spores:  Smooth, Spiny, Reticulate, Warty.

Plate 70 = Sporangia types in Myxogastres, Stemonitis, Dictydium, Physarum & Arcyria.

Plate 71 = Three types of capillitium in Myxogastres

Plate 72 = Life Cycle -- Plasmodiophora brassicae.

Plate 73 = Life Cycle -- Spongospora subterranea.

 

Bibliography

 

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