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

2. OOMYCETES DEFINING 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. OOMYCETES DEFINING 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. OOMYCETES DEFINING 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 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 biflagellate zoospores 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.

13. From Introductory Mycology by J. Webster

14. 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

15. From
Introductory
Mycology by
J. Webster

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

17. 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.

18. 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.

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

20. Sporangium of Dictyuchus sp.

22. Secondary zoospores of Dictyuchus sp. emerging

23. Primary zoospore cyst walls of Dictyuchus sp.

24. 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.

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

26. 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.

30. Sexual reproduction in Achlya sp.

31. From
Lower Fungi in
the Laboratory

33. 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)

34. From Lower Fungi in the Laboratory

36. From
Lower Fungi in the Laboratory