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The Basics of  Mycology & The Fungi

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True Fungi (Eumycophyta) 1

Zygomycota (Phycomycetes) -- Zygote fungi





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          The Zygomycota includes a group of organisms that show a tremendous variation in structure.  There is some question as to whether these are advanced organisms that have degenerated, or whether they are truly primitive.  Subclasses are Monoflagellatae, Biflagellatae and Aflagellatae.  It is suspected that these three groups might have evolved independently.




          The order Chytridiales has been considered at the evolutionary bottom of the Monoflagellatae and are thus among the lowest of the True Fungi.  There are over 40 genera and several hundred described species, all distributed among over ten different families.  The majority of species are aquatic, but some species are common in some soils and may also be found in other terrestrial habitats.  A considerable number are known to live as saprophytes on submerged organic debris; others parasitize algae, especially freshwater algae or aquatic filamentous fungi.  Some species grow on microscopic animals, or on pollen grains that have fallen into the water.  Finally some attack terrestrial angiosperms, and among these there are incitants of some important diseases of food plants.


         Among the simplest members of the Chytridiales the body is a single globular cell, and the entire structures is eventually used up in the formation of reproductive bodies.  Such organisms are often called holocarpic.  Among the more advanced species the thallus is polycentric, a number of nucleated centers being connected by a system of generally enucleate rhizomycelial threads.  The nucleated areas are transformed into or serve as points of origin for reproductive organs.


          The Genus Olpidium parasitizes algae, fungi, pollen, small aquatic animals and some higher plants.  The life cycle has been worked out on cabbage, where the organism is unicellular, uninucleate and intracellular. 


          An exit papilla dissolves a hole in the sporangium wall and later through the host cell wall. The sporangium develops uniflagellated zoospores of the “whip lash” type.  The organism is Holocarpic where the entire vegetative thallus is used up in reproduction (Eucarpic = only a portion of the thallus is used in reproduction).



          The zoospore (or planospore, swarm spore) lands on another cabbage root epidermal cell and loses its flagellum and builds a wall around itself.  The wall remains on the epidermis and the protoplast is discharged into the cell.  It may go in for a few layers of cells before building up another wall around itself.  Germination follows.



          In some cases the zoospores may act as facultative gametes.  Two gametes may fuse to form a zygote (diploid).  Rather than the protoplast penetrating the host in this case only, the entire zygote will enter, no portion being left outside.  A wall is secreted around the zygote and the overwintering stage is formed.  In the spring meiotic division occurs and produces a multinucleate structure (haploid nuclei).  The zygote then turns loose many uniflagellated zoospores.  When gametes are produced, the “sporangium” may best be referred to as a gametangium).  [See PLATE 15 for Life Cycle of Olpidium viciae].


          The Genus Synchytrium has species that parasitize higher plants.   In the reproductive phase there is a cluster of sporangia involved.  The parasite induces hypertrophy of the epidermal cells of the “hog peanut” host.



          The life cycle of Synchytrium decipiens, is shown as follows in PLATE 16  & PLATE 19:




          The life cycle of Synchytrium endobioticum is shown in PLATE 17 and  PLATE 18:




          Another species, Synchytrium cellulare, parasitizes false wood nettle leaves.  It is similar to S. endobioticum in having a sexual and asexual stage, but there are 9-16 sporangia in a sorus (S. endobioticum has 7-9).         


          When septa occur in the Zygomycota, which may happen in an older mycelium, they are complete and without a septal pore.  The flagellation of motile cells is the basis of the Subclass classification:



          The vegetative thallus of Chytridiales is single-celled (holocarpic).  There is a cell wall, typical of a true fungus, and single-flagellated motile cells.  Synchytrium is one step ahead of Olpidium in that it possesses a character where each thallus goes into several sporangia.


          Podochytrium cornutum parasitizes diatoms of the Genus Stephanodiscus.  There is a single uniflagellated zoospore that comes to rest on Stephanodiscus.  It then sends out rhizoids, which penetrate diatoms, presumably through the pores in the walls.  Rhizoids may be called haustoria, but they are not “hyphae” because there is no nucleus present.  More than one parasite (zoospore) may infect one diatom cell at any given time.  P. cornutum is a monocentric and eucarpic organism.



          The Asexual Stage in the Genus Rhizophidium has zoospores with a single posterior flagellum that land on a host cell (an alga).



The spore sends out rhizoids and the protoplasm divides into more nuclei and then cleaves into uninucleate divisions.  These then become zoospores.  They are eucarpic. 


          The Sexual stage of Rhizophidium has two zoospores, which land side by side on an alga.  One will enlarge and the wall between the two cells will rupture.  The protoplasm of the small one erupts into the larger one.  A resting sporangium with a hard surrounding wall is formed and the zoospores are released thereafter.


        The Genus Entophlyctis has motile zoospores that land on algal filaments and send germ tubes through the host cell wall.



Nuclear division precedes cleavage.  The swollen section is abscised from the germ tube, and an exit papilla is sent up to the surface where zoospores are released.  There is no sexual stage and the organism is Eucarpic and Monocentric).



          Urophlyctis alfalfae causes “Crown Wart” disease.  It differs from Physoderma zeae-maydis by inducing hypertrophy.  A single zoospore alights on a host epidermal cell and sends out a germ tube, causing the hypertrophy.  The tube may extend to more than one cell (= Polyphagous).



The nucleus of the zoospore migrates to a turbinate cell where it will divide into 5-8 nuclei, which cleave into as many cells.  Each of the cells of a turbinate cell may send out a network of germ tubes with their respective turbinate cells.  The organism is polycentric.


          The turbinate cells mature and a system of sparsely branched rhizoids develop at the end of each turbinate cell.  Eventually one of the cells at the end of a turbinate, instead of giving rise to a rhizo-mycelial system, balloons out to form a structure larger than the turbinate cell itself.



          The nucleus divides; a thick wall is laid down and forms a resting sporangium.  A halo of rhizoids is also present.  Often the host tissue may wall-off a cluster of resting sporangia with corky tissue.  Eventually the host disintegrates, releasing the resting sporangia.  These will split up into many multinucleate sporangia.  Each then cleaves into uninucleate portions that give rise to a zoospore.  The zoospore may land on a host’s epidermal cell where it sends out some rhizoids.  This structure then forms a sporangium (ephemeral sporangium), which in turn releases zoospores that begin the vegetative mycelium over again.  There is no evidence of a sexual stage.



          Physoderma zeae-maydis causes “Brown Spot” disease of maize.  The sporangium is slipper-shaped.  As soon as the sporangium becomes empty an internal proliferation occurs and a new sporangium develops from a remaining nucleus of the old sporangium.  The new sporangium releases zoospores.  This process may be repeated several times.  The slipper-shaped structure may be considered a gametangium.


          As the germ tube goes through a maize leaf, there is no hypertrophy.  The resting sporangium migrates into the upper sorus portion and cleavage occurs forming the sporangium.  Each entity gives rise to one zoospore.





       Please see following plates for Life Cycles and Structural characteristics in the Chytridiales:


Zygomycota:  Monoflagellatae:  Chytridiales


     Plate 15 = Life Cycle -- Olpidium viciae

     Plate 16 = Life Cycle -- Synchytrium decipiens #1

     Plate 19 = Life Cycle -- Synchytrium decipiens #2

     Plate 17 = Life Cycle -- Synchytrium endobioticum

     Plate 18 = Life Cycle -- Synchytrium endobioticum #2

     Plate 20 = Life Cycle    Podochytrium cornutum

     Plate 21 = Life Cycle -- Chytridiales:  Endophlyctis

     Plate 22 = Life Cycle -- Chytridiales:  Rhizophidium couchii

     Plate 78 = Life Cycle2 -- Rhizophidium couchii.

     Plate 23 = Life Cycle -- Chytridiales:  Urophlyctis alfalfae

     Plate 24 = Example Structures -- Chytridiales:  Sexual Reproduction

     Plate 25 = Example Structures -- Chytridiales:  Synchytrium spp.

     Plate 26 = Example Structures -- Chytridiales:  Physoderma, Podochytrium, Urophlyctis.

     Plate 74 = Thallus of Chytriomyces aureus.

     Plate 75 = Flagella of Zygomycota:  Whip lash, Tinsel and combination.

     Plate 76 = Life Cycle -- Olpidium viciae.

     Plate 77 = Life Cycle -- Synchytrium endobioticum.




          The order Blastocladiales has an arbusculate mycelium.  Species are usually aquatic, but some may occur in moist soil.  The zygotes do not have a hard wall.


          Allomyces arbusculus is a typical representative of this order.  It lives in fresh water or moist soil and the thallus is visible with the naked eye.  The base of the mycelium is attached to a plant thallus by rhizoids.  It has a coenocytic mycelium.


          During the life cycle the trunk will branch and the tips of branches differentiate into hairs of sex organs.  The bottom differentiation is reddish and small (male) while the top one is clear and larger (female).



          The thallus is multinucleate and has N chromosomes (gametophyte generation on N thallus).  Smaller differentiation cleaves into many protoplasts that form gametes with a single posterior flagellum (male).  The larger one cleaves into a smaller number of larger gametes, which are more sluggish in their movement (female).  Two gametes eventually fuse, giving a zygote.  The zygote is biflagellated for a short period of time.  A system of rhizoids is sent out from the basal end while aerial hyphae are sent out from the anterior end.  This thallus is 2N.



          The tips of the 2N thallus can differentiate into either of two structures.  The first is a thin-walled sporangium that may occur singly or in a chain.  It gives rise to a 2N zoospore, which in turn gives rise to another 2N thallus.



          The second structure is a thick-walled sporangium that possesses characteristic pits.  It can survive for years in the dry state.  In a moist environment the multinucleate protoplast will divide and release uniflagellated motile cells.  Meiosis is believed to take place in the thick-walled sporangium.  Spores are 1N and either fuse to form a 2N thallus or they may simply give rise to a 1N thallus without fusing.  The gametes involved are called anisogametes that only differ in size.  An Alternation of Generations is shown by the genus Allomyces.



        In the Genus Blastocladia there is an extensive system of rhizoids.  They are an aquatic genus with an arbusculate mycelium that usually branches dichotomously.  During the life cycle the tips of the mycelium branches giving rise to clavate or elongated, thin-walled sporangia (think-walled = 2N).  Or they may develop spheroid, thick-walled sporangia, which are also 2N.  There is no sexual stage or resting spore and no Alternation of Generations.






          The order Monoblepharidales has a species that parasitizes nematodes and another the larvae of mosquitoes.  Their zygotes develop a hard wall.  Monoblepharis is a representative species that will develop a coenocytic mycelium with a few rhizoids at the base and the indication of an arbusculate form.  The protoplasm in the stalk becomes highly evacuolated, which is a good diagnostic character.



          One or more sporangia develop at the tip, and these release zoospores.  There is an internal proliferation where a new sporangium arises from an old sporangial wall, an event that may reoccur 3-4 times.  At the ends of other branches male and female gametangia will differentiate.



          The female gamete is uninucleate and referred to as an oösphere.  At maturity a papilla will jet out to one side of the oösphere, which allows an entry point to male gametes.  The male gametangium is multinucleate and cleaves out into many single posterior-flagellated gametes (antherozoid).  These swim toward the papilla of the female gametangium.



          Oögamy is exhibited in this genus.  Karyogamy has not yet occurred, and the binucleate “zygote-to-be” crawls out to the end of the papilla and drops off.  The nuclei fuse and then the structure germinates by sending out a germ tube.  The mycelium may be either 1N or 2N, depending on when meiosis occurred.  The 1N mycelium is probably most common.  There is no Alternation of Generations.



          In another genus, Monoblepharella, the “zygote-to-be” swims around a bit before forming the resting spore.




       Please see following plates for life cycles in the Blastocladiales and Monoblephariales:


Zygomycota:  Monoflagellatae:  Blastocladiales


     Plate 27 = Life Cycle – Blastocladiales:   Allomyces arbusculus

     Plate 28 = Life Cycle -- Blastocladiales:  Blastocladia

     Plate 79 = Life Cycle -- Allomyces javanicus var. macrogynus.


Zygomycota:  Monoflagellatae:  Monoblepharidales


     Plate 29 = Life Cycle -- Monoblephariales:  Monoblepharis

     Plate 80 = Life Cycle -- Monoblepharis polymorpha.






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