Oomycota

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