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OOMYCETES I INTRODUCTION & SAPROLEGNIALES

OOMYCETES I INTRODUCTION & SAPROLEGNIALES. IB 371 – GENERAL MYCOLOGY LECTURE 13 THURSDAY, OCTOBER 9, 2003. OOMYCETES DEFINING CHARACTERISTICS. Vegetative thallus filamentous, coenocytic, in some groups unicellular Cell walls lacking chitin (except for a few taxa), made of glucans & cellulose

Samuel
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OOMYCETES I INTRODUCTION & SAPROLEGNIALES

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  1. OOMYCETES IINTRODUCTION &SAPROLEGNIALES IB 371 – GENERAL MYCOLOGY LECTURE 13 THURSDAY, OCTOBER 9, 2003

  2. OOMYCETESDEFINING CHARACTERISTICS • Vegetative thallus filamentous, coenocytic, in some groups unicellular • Cell walls lacking chitin (except for a few taxa), made of glucans & cellulose • Asexual reproduction by biflagellate zoospores • Sexual reproduction by oogonia (female) & antheridia (male) - no motile gametes • Sexual spore is the oospore • Diploid life cycle

  3. OOMYCETESDEFINING CHARACTERISTICS • Sexual reproduction is oogamous • Female gamete (oosphere) produced by an oogonium • Depending on taxon, there may be one to many oospheres per oogonium • Male gamete is produced by antheridium and transferred to the oogonium by gametangial contact and migration of male nuclei

  4. Oogonium and antheridium (From Tom Volk)

  5. OOMYCETESDEFINING CHARACTERISTICS • Asexual reproduction is by zoospores • Zoospores have flagella that are anisokont (unequal) and heterokont (different) (straminipilous) • The anteriorly directed flagellum bears two rows of tubular tripartite hairs • The posteriorly directed flagellum is a whiplash type

  6. From Lower Fungi in the Laboratory

  7. DIPLOID LIFE CYCLE (From Alexopoulos et al.)

  8. OOMYCETES • Phylogenetically unrelated to the fungi but similar in morphology & physiology • Now in Chromophyta with brown algae • 9 orders, 25 families, 95 genera • Between 500-800 species • Called Peronosporomycetes by Dick

  9. ORDERS OF OOMYCETES TO BE COVERED • Saprolegniales • Leptomitales • Lagenidiales (Salilagenidiales) • Peronosporales

  10. SAPROLEGNIALES • Found in water and soil. • Captured by baiting soil and water samples with hemp seeds or dead flies. • Saprobic on both dead plants and animals. • A few species are parasitic on fish.

  11. SAPROLEGNIALES • Thallus is filamentous. • Hyphae is hyaline, broad & coenocytic. • Asexual reproduction is by biflagellatezoospores formed in large, cylindrical sporangia. • Sexual reproduction is by oogonia containing several oospheres that receive nuclei directly from antheridia. • A fertilized oosphere is called an oospore.

  12. From Introductory Mycology by J. Webster

  13. ZOOSPORES • Primary – oval with two flagella at apex, one is tinsel and the other is whiplash. • Secondary – kidney shaped with two lateral flagella, one anteriorly directed tinsel and one posteriorly directed whiplash

  14. From Introductory Mycology by J. Webster

  15. ZOOSPORES • Monomorphic – having only one type of zoospore. • Dimorphic – having both types of zoospores.

  16. ZOOSPORES • Monoplanetic – having only one swimming period followed by encystment. • Diplanetic – having one swimming period followed by encystment, emergence and a second swimming period followed by encystment. • Polyplanetic – Having multiple swimming and encystment periods.

  17. SAPROLEGNIALES • Saprolegnia – Dimorphic, diplanetic with swimming periods of equal duration. • Achlya – Dimorphic, diplanetic but primary zoospores encyst just outside the sporangium. • Dictyuchus – Monomorphic, polyplanetic, no primary spores are liberated. Primary spore encysts in the sporangium, each cyst releases a secondary zoospore that can swim and encyst repeatedly.

  18. From Money & Webster Trans. Br. Mycol. Soc 88: 341-348 (1987)

  19. Sporangium of Dictyuchus sp.

  20. Secondary zoospores of Dictyuchus sp. emerging

  21. Primary zoospore cyst walls of Dictyuchus sp.

  22. SAPROLEGNIALES • Thraustotheca – Monomorphic, monoplanetic, primary zoospores encyst within sporangium, secondary zoospores swim only once. • Geolegnia – Aplanetic, both swimming periods have been repressed, aplanospores germinate directly from sporangia.

  23. SEXUAL REPRODUCTION • Most species are monoecious and self-fertile. • A few species are self-sterile and heterothallic (Achlya ambisexualis, Achlya bisexualis).

  24. SEXUAL REPRODUCTION • Hormone A (antheridiol) - produced by the vegetative hyphae of female, causes the vegetative hyphae of the male strain to form antheridial branches and grow toward the oogonia along a concentration gradient (chemotropic). • Hormone B (oogonial) - produced by the antheridial branches, causes female vegetative hyphae to produce oogonial initials.

  25. Sexual reproduction in Achlya sp.

  26. From Lower Fungi in the Laboratory

  27. LEPTOMITALES • Small group of about 30 species. • Differs from Saprolegniales in having hyphae that is constricted at regular intervals. • Cytoplasm of coenocytic hyphae contains conspicuous granules. • Oogonia contain a single oosphere (except in Apodachlyella completa)

  28. From Lower Fungi in the Laboratory

  29. From Lower Fungi in the Laboratory

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