1. Oomycota
Stramenopila

2. 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.

3. 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

4. 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

5. 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

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

7. 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

8. 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

9. 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)

10. 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

11. 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

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

13. 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

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

15. 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

16. Zoospores
Saprolegnia

17. Zoospores
Achlya

18. Zoospores
Dichtyuchus

19. 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

20. 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

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

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

23. 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

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

25. Life cycle

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

27. 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)

28. 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)

29. 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)

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

31. 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

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

33. Phytophthora

34. Phytophthora

35. 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

36. Albugo

37. Albugo

38. 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

39. 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)

40. Peronosporales

41. Peronosporales