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Oomycota and Zygomycota

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Oomycota and Zygomycota Phylum: Oomycota Phylum is similar to Chytridiomycota in that there are unicellular, short hyphal to mycelial members and in the latter mycelium is nonseptate except where reproductive structures occur. Differ from Chytridiomycota in that cell wall component is usually cellulose and life cycles are always gametic. Oogamous: Oogonium, with immobile egg, and sperms in antheridium are naked nuclei. Lecture 02 Phylum: Oomycota Zoospores Phylum was once thought to represent the transition from aquatic fungi to terrestrial fungi, but is no longer thought to be related to fungi. In one classification scheme they are placed in the Kingdom Stramenopila with Phaeophyta, Chrysophyta, etc. All have same cell wall composition, zoospore morphology and food storage material is mycolaminarin. Chytridiomycota Oomycota primary and It is believed Oomycota had lost their zoospore secondary zoospore chloroplast (not most parsimonious event). Order: Saprolegniales Order: Saprolegniales We will go over two orders in this Life cycle is quite different from phylum: Chytridiomycota. Will use the genus Saprolegniales: Both primary and Saprolegnia as representative of this secondary zoospores may occur in order. the same life cycle. Each oogonium Gametic life cycle: Only phylum to always produce several eggs. have this type of life cycle. Peronsporales: Only secondary Sperms are naked, nonmotile zoospore is produced. Each oogonium gametes. Only flagellated fungi with always produce one egg. this characteristic. 1 Order: Saprolegniales Order: Saprolegniales Zoosporangia Summary of Zoosporangia of Saprolegnia Saprolegnia life with primary Asexual are not well zoospores.. cycle. Asexual Cycle differentiated Go cover asexual from hyphae. cycle first. Zoosporangia delimited by septum at base and dense protoplasm. Order: Saprolegniales Order: Saprolegniales Dense Protoplasm Encyst Basal Septum Primary zoospores Encyst (posterior flagella) are released from Cyst zoosporangium. Zoospore swims for a Primary period of time and zoospore encysts: rounds up and Terminal Zoosporangium of Saprolegnia cell wall forms around itself. Order: Saprolegniales Order: Saprolegniales Cyst will germinate Secondary zoospore to give rise to encyst. secondary zoospore Cyst germinates to (laterally give rise to diploid flagellated). mycelium. Secondary zoospore 2 Order: Saprolegniales Order: Saprolegniales Formation of Summary of Typically, oogonia gametangia Saprolegnia life and antheridia form cycle: Sexual stage. on same mycelium. The sexual stage is Nuclei migrate into more uniform than gametangia, followed the asexual. Most Sexual by formation of species are Cycle septa to “trap” homothallic as nuclei. Meiosis shown here. occurs at this time. Antheridium Oogonium Order: Saprolegniales Order: Saprolegniales Fertilization Tubes Plasmogamy Oogonia with several eggs is characteristic of this order. Antheridia Typically, oogonia and antheridia form on are always difficult to see in live or same mycelium. prepared material. Order: Saprolegniales Order: Saprolegniales Karyogamy occurs in Oospore germinates oogonium to form to form the diploid zygotes. mycelium. The mature zygotes The mycelium will are referred to as give rise to both the oospores. zoosporangia and gametangia, i.e., antheridia and oogonia. 3 Order: Saprolegniales Order: Saprolegniales Variations occur in zoosporangia. Two common genera: Saprolegnia: Following release of zoospores, an internal proliferation occurs to produce another zoosporangium. Proliferation of zoosporangium Previous through previous zoosporangium zoosporangium Order: Saprolegniales Order: Saprolegniales Encysted zoospores Achyla: Primary zoospores are released and encyst, immediately. Following release of Encysted zoospores, a new zoospores zoosporangium forms below the septum and adjacent to Achyla empty zoosporangia with encysted old zoosporangium. zoospores at tip. Note formation of new, lateral zoosporangium. Order: Peronosporales Order: Peronosporales This order has some of the most well known pathogens. They include the genera Pythium and Phytophthora. Differs from the Saprolegniales in producing only secondary zoospores in a zoosporangium that is differentiated from hyphae, and one egg per oogonium. Zoosporangium of Phytophthora tropicalis releasing secondary zoospores. Note that the zoosporangium can readily be distinguished from hyphae. 4 Phylum: Microsporidia Phylum: Microsporidia Until the 1990’s this genus was Until the 1990’s this genus was classified as a very early eukaryote: classified as a very early eukaryote: Anaerobic, intracellular parasite of Placement among other phyla of arthropods. fungi uncertain at this point. Genus lacks mitochondria. Was originally assumed to have evolved before acquisition of mitochondria. Based on sequencing of protein tree of beta-tubulins is a fungus! Phylum: Microsporidia Phylum: Zygomycota Spore germination: This phylum represents the first group PiercePierce hosthost of terrestrial fungi, but still has some cellcell ifif nearbynearby characteristics in common with the Anchoring Polar tube flagellate fungi: disk ruptured Mycelium is coenocytic. Although asexual spores are not motile, they are still produced in a sporangium. Above characteristics not found in other terrestrial fungi Phylum: Zygomycota Phylum: Zygomycota Once thought to be more closely Characteristic that defines phylum is related to flagellated fungi and formation of the zygospore during classified in Phycomycetes (literally sexual reproduction. algal fungi) with flagellated fungi. Zygospore formation usually results from the fusion of isogametangia: Cell wall material, however, is composed gametangia that are morphologically mostly of chitin. This and other fungal identical. characters places this taxon with the The life cycle of Rhizopus stolonifer fungi, in the strict sense. will be used as the representative of this phylum. 5 Rhizopus stolonifer Lifecycle Rhizopus stolonifer Lifecycle Rhizopus stolonifer life cycle includes both sexual and asexual reproduction. Sporangium development Nuclei migration, Stolon up sporangiophore Rhizoids Rhizoids form with contact Will cover asexual cycle first. with substrate Rhizopus stolonifer Lifecycle Rhizopus stolonifer Lifecycle Cell wall forms Nuclei migrate Developing around each into swollen tip Mature Sporangium nucleus to form Sporangium sporangiospores Columella forms Columella or just spores trapping nuclei in future Columella sporangium Rhizopus stolonifer Lifecycle Rhizopus stolonifer Lifecycle Sporangium ruptures, releasing Asexual reproduction will continue as air-borne spores long as there is available food. Sexual reproduction will not take place unless a different mating strain is present. The two mating strains are designated Germination as “+” (plus) and “-” (minus). Mycelium 6 Rhizopus stolonifer Lifecycle Rhizopus stolonifer Lifecycle Two mating strains are allowed to grow Suspensors together. Plus and minus Progametangia grow together due to phermonal response. “-” (Minus) “+” (Plus) Nuclei migrate to Mycelium Gametangia Mycelium tips of progametangia. Sexual reproduction will take place where mating strains meet in center. Rhizopus stolonifer Lifecycle Rhizopus stolonifer Lifecycle Gametangia fuse, Zygote develops a followed by thick, pitted wall plasmogamy and referred to as the karyogamy to form zygospore. Zygote Zygospore multinucleate Zygospore has a long zygote. dormancy period before meiosis and germination occurs. Does not disperse! Rhizopus stolonifer Lifecycle Zygorhynchus molleri Developing Germination of sporangium zygospore forms a Sporangiophore sporangium containing “+” and “-” sporangiospores. Germinating zygospore A species that is homothallic and produces anisogametangia. 7 Phylum: Zygomycota Phylum: Zygomycota There is a great deal of variation in the asexual stage of the Zygomycota: Rhizopus is representative of simplest and what is believed to be the earliest sporangia from which other variations are derived. Syncephalastrum produces spores in cylindrical sporangiole. Phylum: Zygomycota Cunninghamella produces a one spored sporangiole and is believed to be the most recently derived condition. 8.
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