Phylum Oomycota

1. Phylum Oomycota General Mycology Pl P 42/1521 Lecture 21

2. Stramenopilous Fungi • Tinsel flagellum • Tubular mitochondrial cristae • Cellulose in cell walls • Lysine synthesis by diaminiopimelic acid pathway (DAP)

3. Phylum Oomycota • 1 class (Oomycetes), 8 orders, 92 genera and 800 species • Orders to cover: • Leptomitales • Filamentous thallus with cellulin plugs • Rhipidiales • Inflated, branched thallus with cellulin plugs • Saprolegniales • Holocarpic or eucarpic/filamentous • Lagenidiales • Holocarpic • Pythiales • Filamentous thallus • Peronosporales • Filamentous thallus

4. Characters • Predominantly diploid life cycle with gametangial meiosis • Oogamous sexual reproduction: • Gametangia = oogonia and antheridia • Thick-walled, resistant oospore formed inside oogonium from oosphere • Asexual reproduction by heterokont zoospores with tinsel + whiplash flagella

6. Characters • Cell walls composed primarily of beta-glucans with small amounts of cellulose • Filamentous species generally produce coenocytic hyphae • Principle storage compounds are water-soluble mycolaminarins (beta-glucans) • Haustoria formed by plant parasitic species

7. Zoospores • Primary zoospores • First formed • Pip-shaped • Flagella emerging from anterior • Poor swimmers, encyst readily • Secondary zoospores • Reniform or bean-shaped • Flagella emerging from lateral groove

8. Zoospores • Dimorphic • Primary and secondary zoospores present in life cycle • Primary zoospores formed in zoosporangium • Secondary zoospores formed upon germination of primary zoospores • Monomorphic • Secondary-type of zoospores formed inside zoosporangium

9. Sexual Reproduction • Mostly heterogametic (morphologically differentiated gametangia): • Hyphal-like antheridia • Attracted to oogonium by hormones and develop fertilization tubes • Large, globose oogonia • One or more oospores in oogonium • Simultaneous meiotic divisions occur in antheridia and oogonia prior to fertilization • Karyogamy occurs in oosphere to form oospore that matures inside oogonium

10. Oospore from APSnet

11. Oogonia & Oospores • Oospore wall consists of 3 parts: • Exospore • Epispore • Endospore • Periplasm = residual protoplasm • Plerotic = periplasm present • Aplerotic = lacking periplasm

12. Oogonium & Antheridium • Hypogynous • Antheridium formed in oogonial stalk • Paragynous • Antheridium attaches laterally to oogonium • Monoclinous—branch off oogonial stalk • Diclinous—branch off separate hypha • Amphigynous • Oogonium grows through antheridium

13. Hypogynous Paragynous/monoclinous Paragynous/diclinous Amphigynous Oosphere Oogonium Fertilization tube antheridium

14. Order Leptomitales • Mostly aquatic, saprotrophic • Filamentous thallus with regular constrictions plugged by chitinous material called cellulin granules • Asexual reproduction by elongated, hyphal-like zoosporangia • Some species form primary and secondary zoospores

15. Apodachlya http://plantbio.berkeley.edu/~taylor/pmb110/labpages/8leptopythi/index.htm

16. A, B. Leptomitus C-K. Apodachlya From Sparrow, 1960. Aquatic Phycomycetes.

17. Order Rhipidiales • Strictly aquatic, saprotrophic, commonly in stagnant and polluted water • Thallus: • Inflated and branched • Attached to substrate by rhizoids • Oogonia, antheridia and zoosporangia formed terminally • Branches and reproductive organs constricted at base and plugged with cellulin granules • Only secondary zoospores formed • Strong tendency towards fermentative metabolism

18. Zoosporangia • C. Sapromyces • Araiospora • Rhipidium • Mindeniella • Aqualinderella Thalli From Emerson and Weston, American Journal of Botany 54 (1967)

19. Order Lagenidiales • Mostly aquatic and parasitic on fungi, algae and marine crustacean larvae • Endobiotic, mostly monocentric • Examples: • Olpidiopsis—obligate endoparasite of Saprolegniales • Lagenidium—endoparasites of algae and marine crustacean larvae

20. Lagenidium 1-4. Infection stages in Spirogyra 5-10. Asexual reproduction 11-17. Sexual reproduction Karling, 1942. Simple Holocarpic Biflagellate Phycomycetes.

21. Order Saprolegniales • Saprotrophs (fresh water and soil), or parasites (plants and animals) • Aphanomyces--sugar beets, peas, radishes, aquarium plants • Achlya, Aphanomyces and Saprolegnia—fish, crustaceans • “Water mold” applies to this group • Primary and secondary zoospores formed • Filamentous thallus, abundantly branched • Stout, coenocytic hyphae able to increase in diameter as they age • Gemmae = modified segments of hyphae with dense cytoplasm

22. Saprolegniales—sexual reproduction • Oogonia with multiple oospores • Oospores with prominent, granular, fluid ooplast (formed by coalescence of dense vesicles) • Extent of lipid coalescence used as taxonomic character • Centric—small lipid drops uniformly surrounding ooplast • Eccentric—ooplast to one side of oospore, not completely surrounded by lipid globules

23. Sexual Reproduction • Most members are self-fertile • Sexual reproduction in both self-fertile and self-sterile forms involves pheromones • Diffusible substances produced by both partners • First demonstrated by John Raper (1940s)

24. Role of pheromones • Female produces and secretes antheridiol (hormone A; sterol pheromone) which induces formation of antheridial branches in male strain • Male strain is stimulated to secrete oogonial (hormone B) which causes formation of oogonial initials in female • Antheridiol causes chemotropic growth of mail structures toward female

25. Asexual Reproduction • Primary zoospore stage formed in most species, but may be reduced: • Achlya and Aphanomyces • Uninucleate protoplasts emerge from zoosporangium and encyst at opening or immediately after release • Dictyuchus • Cysts formed inside zoosporangium • Saprolegnia • Motile, primary zoospores released from zoosporangium

26. Saprolegnia Proliferation of zoosporangium Secondary zoospore Primary zoospores

27. Achlya Uninucleate protoplasts encyst in ball at mouth of zoosporangium Lateral proliferation of zoosporangium Secondary zoospores emerge from cysts From Webster, 1980

28. Dictyuchus Primary cysts inside zoosporangium germinate to release secondary zoospores From Webster 1980