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Mycology 4th Biology geology English Assoc Prof. Shimaa M Abdelsalam 1- Myxomycetes Division :-Myxomycota (plasmodial slime molds) The organisms included in this division are commonly known as the plasmodial or acellular...

Mycology 4th Biology geology English Assoc Prof. Shimaa M Abdelsalam 1- Myxomycetes Division :-Myxomycota (plasmodial slime molds) The organisms included in this division are commonly known as the plasmodial or acellular slime molds. The exact systemic position of the Myxomycota is notclear. Ainsworth (1973) classification is most accepted in which Division Myxomycota belong to Kingdom Mycetae (=Fungi). Changed and Olive (1975) classified hem in the Kingdom Protista (why?).  Vegetative phase is naked unicellular, without a cell wall, amultinucleate mass of protoplasm called a plasmodium.  They engulf bacteria, protozoa, and other foodparticles byphagocytosis.During this modeof ingestion, the food particles,usually bacteria, beceomesurrounded by thepseudopodia of the myxamoeba. Once the food has been engulfed in this matter, it is surrounded by a membrane or food vacuole wherehydrolytic enzymes are secreted that will digest the food. The vegetative stage in slime molds is morphologically similar to that of an amoeba, because of that known as myxamoeba. Plasmodium produces one or more sporangia where meiosis takes place. Within the Myxomycota, the class Myxomycetes, known as the acellular slime molds. Class: Myxomycetes There are approximately 71genera and 500species of Myxomycetes. Habitats (Occurrence) There are commonly occur in damp placese specially on decaying wood, all of which are terrestrial (found on land). Reproduction  Asexual reproduction occurs by binary fission of myxamoeba or fragmentation of the plasmodium.  Sexual reproduction takes place by the plasmogamy (fusion of compatible gametes) occurs between myxamoebae or swarm cells (some species are heterothallic). Karyogamy (fusion of nucleus) occurs shortly after plasmogamy to form zygote (2n). Zygote forms plasmodium – longest lived vegetative stage. Life Cycle of Myxomycetes  Bi-phasic life cycle with a haploid and a diploid phase. The diploid stage is a naked coenocytic protoplast (that is, it consists of a multinucleate mass of cytoplasm that is enclosed only by a plasma membrane and does not have a cell wall).  When the sporangia matured and turned grey. Normally globose, with a definite, rather thick cell wall, unicellular, uninucelate and haploid spores are released by rupture of the 11 Mycology 4th Biology geology English Assoc Prof. Shimaa M Abdelsalam sporangial coat. The spore surface may range from almost smooth to reticulate. Spores of myxomycetes are small (4-20 μm) and are easily picked up by air currents, arthropods and other animals.  On germination a spore produces one or four myxamoebae or flagellate cells known as swarm cells- that have one or two anterior flagella, all of which are of the whiplash type. In most cases swarm cells possess one long prominent flagellum and a second shorter, inconspicuous flagellum that is directed backward and appressed to the cell surface.  Myxamoebae and swarm cells can be interconverted. When food is abundant and environmental conditions are favorable, myxamoebae divide repeatedly, giving rise to a large population of cells.  The nuclear divisions are centric and open (the nuclear membrane breaks down during prophase and is reconstituted after telophase.)  Under unfavorable conditions myxamoebae roundup and secrete a galactosamine wall to form microcysts (sclerotia).  When favorable conditions return, the microcysts germinate and either amyxamoeba or swarm cell emerges from each wall.  When free water is available myxamoeba can differentiate into flagellated swarm cells Swarm cells and myxamoeba may function as gametes (both in homothallic or heterothallic strains).Thecompatible gametes fuse in pairs (two swarm cells, two myxamoebae) to form a diploid zygote.As the zygote grows, its nucleus undergoes successive synchronous mitotic divisions withoutcytokinesis. And the cell becomes transformed into a multinucleate, amoeboid structure, the plasmodium. Plasmodium:- The plasmodium is a diploid structure. Plasmodium is a naked, multinucleate, motile mass of protoplasm; no cell wall around it. However, in most species, the plasmodium is enveloped by a gelatinous slime sheath that contains microfibrils. Just inside the slime sheath is the plasma membrane that surrounds and confines the cytoplasm Plasmodia are of various colors. Examples- Physarum polycephalum it is a bright yellow, slimy structure and Didymium iridis the plasmodium is colorless. In nature plasmodia probably feed on bacteria, spores of fungi and plants, and possibly on protozoa and even on bits of nonliving organic matter. Growth is accompanied by successive mitotic divisions of the nuclei embedded in the cytoplasm.. There are three types of plasmodia :- 1) protoplasmodium -microscopic throughout its existence; gives rise to only a single sporangium when it fruits. 2)aphanoplasmodium - resembles a protoplasmodiumin its initial stages, but soon elongates, branches, and becomes a network of very fine, transparent strands;3) phaneroplasmodium - characteristic of Physarales, also resembles a 12 Mycology 4th Biology geology English Assoc Prof. Shimaa M Abdelsalam protoplasmodium at first; it grows larger and becomes more massive. Its protoplasm is very granular, and the plasmodium is visible even at an early stage of development. Sporulation and Sporophores :- Under favorable conditions, the plasmodium will migrate and feed for a period of time before being converted to one or more sporophores. Sporophores posses brightly coloured sporangia.  Entire plasmodium of a myxomycete usually is converted into one or more sporophores so that the somatic and reproductive phases seldom coexist in the same individual.  Sporangium consists of the following parts: - 1- Peridium (persistent or evanescent):- The fragile, outer layer of the sporangium is the peridium (pl.=peridia), which may be persistent or degenerate by the time the sporangium is ready to disperse its spores. 2- Hypothallus - The hypothallus is a plasmodial remnant forming the base for one or more fruiting bodies. The hypothallus connects the stalk or stipe to the substrate. It may be dull or brightly colored, thin and delicate or coarse. The hypothallus may be composed of calcium carbonate 3- Stalk :- Basal portion of sporangium, may or may not be present, may be hollow or filled with material Stalks formed from secretions of plasmodium and are acellular (in contrast to cellular slime molds 4- Columella and Pseudocolumella :-The columella appears as an extension of the stalk into the spore mass, although it may not resemble the stalk. In a sessile fruiting body, the columella may be an area on the inside of the peridium where it contacts the substrate orappears as a dome-shaped structure. A pseudocolumella (pseudo=false) is a columella that does not attach to the stalk. The pseudocolumella is found only in the order Physarales, existing as a lime mass within the spore mass. Capillitial elements may be attached to the columella or pseudocolumella. 5- Capillitium and Pseudocapillitium :-The capillitium consists of threadlike elements inside the sporophores, intermingled with the spores. Some elements of capillitium may be elastic, allowing for expansion when the peridium opens, while other types are hygroscopic and capable of dispersing spores by a twisting motion. A pseudocapillitium is present in some aethalia and pseudoaethalia producing species. Pseudocapillitial elements are highly variable in size and shape, and may appear as bristles, threads or perforated plates. 6- Spores. 13 Mycology 4th Biology geology English Assoc Prof. Shimaa M Abdelsalam 2- Mastigomycotina General characteristics  One large group of the Mastigomycotina is aquatic. While another group of the Mastigomycotina are primarily terrestrial, although the organisms still form motile zoospores when open water is available  The members of Mastigomycotina produce flagellated zoospores in their life cycle.  Most of them are filamentous and have coenocytic mycelium. However, unicellular form are present, and some genera show the pseudosepta (false cross wall) formation.  Rhizoids are present in some of unicellular forms.  They show centric nuclear division. Their centrioles remain functional during nuclear division.  Live either as saprophytes or parasites. Due to presence of haustoria in a majority of Mastigomycotina , the mode of nutrition is typically absorptive.  The sexual reproduction takes place by different methods , oospores formation are common in almost all Mastigomycotina  Three classes are included in this sub-division, on the basis of zoospore and oospore and comprise 204 genera and 1160 species:- 1- Chytridiomycetes: produces posteriorly uniflagellate zoospores Chytridiomycetous fungi occur as saprobes on plants and animal remains in water while other members occur as parasites on algae and aquatic animals. 2- Hyphochytridiomycetes:- Zoospores are anteriorly uniflagellate. The Hyphochytridiomycetes are those aquatic fungi whose thallus is holocarpic or eucarpic, monocentric or polycentric and their vegetative system is rhizoidal or hypha-like with intercalary swellings. 3- Oomycetes :- The Oomycetes contain 74 genera and 580 species, which are mostly aquatic, though some are terrestrial and live as parasites or saprophytes. Includes classic “water molds” in the Order Saprolegniales and the “downy mildews” in the Order Peronosporales. Oomycetes General characteristics of Class  Vegetative body is filamentous and coenocytic except the unicellular Lagenidiales.  Holocarpic or Eucarpic 1.Holocarpic - Entire thallus converted into reproductive structures 14 Mycology 4th Biology geology English Assoc Prof. Shimaa M Abdelsalam 2.Eucarpic - Reproductive organs arise from only a portion of the thallus and remainder continues as somatic Majority of species are eucarpic.  Cell wall contains cellulose and glucans. Chitin is absent. Asexual reproduction is by biflagellate heterokont (different) and anisokont (unequal) zoospores that are produced in zoosporangia.  Zoosporangia- Modified hyphae that are usually terminal and delimited by a septum  Zoospores are diploid formed by mitosis  Anteriorly directed flagellum is tinsel type and posteriorly directed is whiplash type.Depending on genera single type-monomorphic or two types of zoospores are formed-dimorphic. Two types of zoospores are formed in the life cycle are :- 1- Primary zoospores :- First formed, pip-shaped, and the flagella are located anteriorly. Primary zoospore is released from the zoosporangium, encyst and germinates to form the secondary zoospore. 2. Secondary zoospores :- The secondary zoospore which is reniform or bean-shaped and laterally flagellated. Zooporangium and zoospores are the major dispersal agents for most species. Sexual reproduction:-  Sexual reproduction is heterogamous (oogamous) by oogonia (female) and antheridia (male).  Female gamete(oosphere) produced by an oogonium. Depending on taxon, there may be one to many oospheres per oogonium.  Male gamete is produced by antheridium and transferred to the oogonium by gametangial contact and migration of male nuclei into oogonia and fertilize oospheres.  Homothallic– self-fertile or Heterothallic– opposite mating types required for sexual reproduction.  A swimming sperm is absent in the Oomycetes. This type of sexual reproduction is referred to as gametangial copulation. In antheridia and oogonia meiosis take place. The eggs and sperms are products of meiosis and the only parts of the life cycle that are haploid. Diploid zygote develops into thick-walled resistant oospore that germinates and give rise to vegetative diploid hyphae that reproduce asexually by production of zoospores. According to the previous characteristics, in which kingdom Do you Classify Oomycetes? Saprolignia – Potato late blight: Phytophthora- Whit rust Albugo candida- Dawny mildew of lattice Brimia lactiuca 15 Mycology 4th Biology geology English Assoc Prof. Shimaa M Abdelsalam Glossary Endobiotic: Living within the tissue of host. Antheridia: Male reproductive body. Oogonia: Female reproductive body. Stomata: Pores which are present in the epidermal surface of leaf. Facultative saprophyte: Those which are normally parasitic but are capable of being saprophytic. Coenocytic: Multinucleate protoplasmic mass. Appresorium: Flattened tip of hyphal branch that enable penetration of host plant. Planogametic copulation: Fusion of naked motile gametes. Meiosis: A kind of cell division that results in four daughter cells each with half the number of genetic material of the parent cell. Mitosis: A kind of cell division resulting in formation of two daughter cells each having same no. of chromosomes as the parent nucleus. Parasite: Those which live and draw nourishment from living organism. Homothallic: Presence of reproductive structures (male and female) on the same thallus. Heterothallic: Presence of reproductive structures (male and female) on separate thallus. 16