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
• Asexual reproduction by biflagellate
zoospores
• Sexual reproduction by oogonia (female) &
antheridia (male) - no motile gametes
• Sexual spore is the oospore
• Diploid life cycle
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
Oogonium and antheridium (From Tom Volk)
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
From Lower
Fungi in the
Laboratory
DIPLOID
LIFE
CYCLE
(From
Alexopoulos
et al.)
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
ORDERS OF OOMYCETES
TO BE COVERED
• Saprolegniales
• Leptomitales
• Lagenidiales
(Salilagenidiales)
• Peronosporales
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.
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.
From Introductory Mycology by J. Webster
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
From
Introductory
Mycology
by
J. Webster
ZOOSPORES
• Monomorphic – having only one type
of zoospore.
• Dimorphic – having both types of
zoospores.
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.
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.
From Money & Webster Trans. Br. Mycol. Soc 88: 341348 (1987)
Sporangium of Dictyuchus sp.
Secondary zoospores of Dictyuchus sp. emerging
Primary zoospore cyst walls of Dictyuchus sp.
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.
SEXUAL REPRODUCTION
• Most species are monoecious and
self-fertile.
• A few species are self-sterile and
heterothallic (Achlya ambisexualis,
Achlya bisexualis).
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.
Sexual reproduction in Achlya sp.
From
Lower Fungi in
the Laboratory
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)
From Lower Fungi in the Laboratory
From
Lower
Fungi in
the
Laboratory