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