oomycota stramenopila. two phyla in the straminopila oomycota – zoospores have two flagella, one...
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Oomycota
Stramenopila
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Two phyla in the Straminopila
• Oomycota – zoospores have two flagella, one whiplash and one tinsel flagellum, 580 spp.
• Hyphochytridiomycota – zoospores have one anterior tinsel flagellum, 16 spp.
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Oomycota• This phylum of organisms differs in
fundamental ways from the organisms in the Kingdom Fungi – cell wall chemistry, lysine biosynthesis, rDNA sequences
• Cell wall composition – most fungi contain chitin as the major structural component (microfibrils) – not cellulose as in plants
• Oomycota – contain cellulose but no chitin
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Lysine biosynthesis• Two biosynthetic pathways exist for the
amino acid, lysine– DAP pathway – diaminopimelic acid (DAP) is an
intermediate - found in bacteria, algae, higher plants and Oomycota
• Pyruvate + aspartate DAP lysine
– AAA pathway - α-amino adipate (AAA)is an intermediate - found in fungi and euglenoids
• Acetate + α-ketoglutarate AAA lysine
– Animals do not synthesize lysine. It is an essential amino acid
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Membrane sterols• Fungi differ from most other organisms in that they
contain the sterol ergosterol• Other organisms contain cholesterol (animals) and
cholesterol-like phytosterols (plants and Oomycota) • Important in practice – some antifungal antibiotics &
fungicides act on ergosterol or its biosynthesis
• Major differences between Oomycota and true fungi
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Oomycota• Major characterisitics
– Biflagellate zoospore – 1 whiplash and 1 tinsel type flagellum
– Sexual reproduction results in production of oospore
– Large aseptate hyphae
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Zoospores in Oomycota
• Two types of flagella are found on zoospores
– Whiplash flagellum – smooth & tapered
– Tinsel flagellum – has lateral filaments perpendicular to the main axis
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Flagella• Both types beat with a
sinusoidal wave, but cause the zoospore to move in opposite directions
• Filaments on tinsel flagella have a large surface area relative to flagellum – cause water movement in opposite direction
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Evolutionary trends in Oomycota• Vegetative thallus – in some species, it is
holocarpic, endobiotic – for most - well developed aseptate hyphae (eucarpic, polycentric)
• Nutrition – some species are aquatic (both saprotrophic and parasitic). Others are highly specialized biotrophic parasites of higher plants
• Asexual reproduction – In many, zoospores are produced in a zoosporangium, in some the number of zoospores/sporangium is reduced. In a few species, the sporangium functions as a conidium (germinates with a germ tube)
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Oomycota• Classification subject to some dispute
• We will discuss three orders:– Saprolegniales - aquatic– Pythiales – aquatic & terrestrial– Peronosporales – terrestrial plant parasites
• Basic form of thallus in most is the mycelium with hyphae, some form holocarpic thalli
• Significant economic group – contains species that are important plant pathogens
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Saprolegniales• Vegetative thallus varies from relatively simple
holocarpic thallus (parasitic on algae and fungi) to well developed mycelium
• Hyphae are aseptate• Commonly called “water molds”• Most are saprotrophs, some parasitize fish and
fish eggs – can cause large economic losses to fish hatcheries
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Asexual reproduction
• Zoospores produced by long cylindrical zoosporangia typically formed at the hyphal tips (formed when immersed in water)
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Zoospores• Two types of zoospores produced sequentially =
dimorphic• If they produce only one type of zoospore =
monomorphic• Primary zoospores are pear shaped, poor
swimmers• Secondary zoospores are oval to kidney shaped
and better swimmers
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Primary & secondary zoospores
• Species vary in sequence of events in formation of primary and secondary zoospores
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Zoospores• Saprolegnia – primary zoospore swims away before
encysting, then forms secondary zoospore• Achlya – 1º zoospore encysts right outside
zoosporangium, then forms 2º zoospore• Dichtyuchus – No 1º flagellated zoospores, they
encyst in the zoosporangium, cysts germinate to form 2º zoospores or they may germinate with a germ tube
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Zoospores
• Saprolegnia
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Zoospores
• Achlya
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Zoospores
• Dichtyuchus
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Asexual reproduction
• May also produce gemmae – irregularly shaped hyphal segments that separate from the thallus and can germinate (germ tube or zoospores) to form a new thallus
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Sexual life cycle• For a long time the Oomycetes were
thought to have a haploid vegetative phase like most fungi
• Since nuclei are small, it is difficult to determine where meiosis takes place
• Microspectrophotometry – allowed concentration of DNA in nuclei to be measured in situ – indicated that vegetative mycelium is diploid, meiosis takes place in gametangia
• Exhibit a diploid life cycle (unusual for fungi
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Life cycle
• Vegetative hyphae are diploid
• Undergo asexual reproduction by formation of zoospores in zoosporangia
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Life cycle
• Gametangia formed & meiosis occurs–
• Homothallic & heterothallic species
• Male gametangium – antheridium
• Female gametangium – oogonium that contains multiple gametes – oospheres in the Saprolegniales
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Life cycle• Antheridium grows to
oogonium
• Forms fertilization tubes to oospheres
• Nuclei from antheridia migrate through fertilization tubes
• Plasmogamy and karyogamy take place
• Oosphere is now diploid
• Develops thick wall and becomes an oospore
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Oogonium• Oospore can remain dormant• Germinates to produce
zoosporangium or 2n mycelium
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Life cycle
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Pythiales & Peronosporales• Most highly evolved
members of the Oomycota
• Include aquatic, amphibious and terrestrial forms
• Saprotrophs and parasites
• Oogonia produce only one oosphere
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Oogonium
• In Saprolegniales – multiple oospheres in oogonium – formed from a large central vacuole that produces furrows that cleave out oospheres before fertilization
• In Pythiales & Peronosporales – single oosphere in oogonium – oosphere is not delimited by membrane until plasmogamy occurs, no central vacuole, periplasm is present (cytoplasm in oogonium that is not incorporated into oosphere)
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Pythiales & Peronosporales
• Sporangia may produce zoospores but only secondary zoospores or,
• Trend in these organisms is for the zoosporangia to produce smaller number of zoospores
• In some species, sporangia may germinate with a germ tube (called conidia)
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Pythiales
• Pythium – a large genus that includes saprotrophs (soil and water) and facultative parasites of algae, fungi and plants
• One species causes “damping off” of seedlings – grows intercellularly in stems and roots of seedlings and rots tissue
• Asexual reproduction by zoosporangia and zoospores (secondary zoospores only)
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Pythium
• Contents of zoosporangium empty into a vesicle, cleave and form zoospores outside zoosporangium
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Pythiales• Phytophthora – facultative
plant parasites • In plant host, form
intercellular – between cells - and intracellular – penetrate cells - hyphae
• Intracellular hyphae produce haustoria – exchange of nutrients from host to parasite
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Phytophthora
• Forms lemon shaped sporangia that detach from sporangiophore in asexual reproduction – sporangia can germinate by forming zoospores or forming a germ tube directly
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Phytophthora
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Phytophthora
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Peronosporales
• Albugo – obligate plant parasite, causes white rust
• Sporangia detach, dispersed by wind
• If moisture available, sporangia produce 4-12 zoospores
• Can germinate directly with a germ tube
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Albugo
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Albugo
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Peronosporales
• Peronospora and related genera – obligate parasites of plants – cause downy mildews
• In some species, sporangia always germinate with a germ tube, never form zoospores – called conidia
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Fungicide development• In 19th century, grapes in the
Bordeaux wine region infected with downy mildew, Plasmopara viticola
• Vines by the road were sprayed with a mixture of copper sulfate and lime to keep people from eating the grapes
• French botanist, Millardet, noticed that these vines did not have downy mildew
• First fungicide – Bordeaux mixture (CuSO4 and lime)
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Peronosporales
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Peronosporales