Oomycota Stramenopila

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.

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

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

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

Oomycota Major characterisitics Biflagellate zoospore – 1 whiplash and 1 tinsel type flagellum Sexual reproduction results in production of oospore Large aseptate hyphae

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

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

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)

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

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

Asexual reproduction Zoospores produced by long cylindrical zoosporangia typically formed at the hyphal tips (formed when immersed in water)

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

Primary & secondary zoospores Species vary in sequence of events in formation of primary and secondary zoospores

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

Zoospores Saprolegnia

Zoospores Achlya

Zoospores Dichtyuchus

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

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

Life cycle Vegetative hyphae are diploid Undergo asexual reproduction by formation of zoospores in zoosporangia

Life cycle Gametangia formed & meiosis occurs– Homothallic & heterothallic species Male gametangium – antheridium Female gametangium – oogonium that contains multiple gametes – oospheres in the Saprolegniales

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

Oogonium Oospore can remain dormant Germinates to produce zoosporangium or 2n mycelium

Life cycle

Pythiales & Peronosporales Most highly evolved members of the Oomycota Include aquatic, amphibious and terrestrial forms Saprotrophs and parasites Oogonia produce only one oosphere

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)

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)

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)

Pythium Contents of zoosporangium empty into a vesicle, cleave and form zoospores outside zoosporangium

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

Phytophthora Forms lemon shaped sporangia that detach from sporangiophore in asexual reproduction – sporangia can germinate by forming zoospores or forming a germ tube directly

Phytophthora

Phytophthora

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

Albugo

Albugo

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

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)

Peronosporales

Peronosporales