Zoosporic fungi: Oomycota Kingdom Chromista

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Zoosporic fungi Oomycota Kingdom Chromista
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Body plan

• thallus - mostly filamentous
• holocarpic
• - entire thallus converted into reproductive
  structures
• eucarpic
• - reproductive organs arise from only a
  portion of the thallus
• - remainder continues as somatic
• - majority of species
• vegetative thallus is diploid
• gametangial
• - isogametangic or heterogametangic
• - most species heterogametangic

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Organelles and cell biology

• mitochondria with tubular cristae
• golgi bodies with multiple cisternal elements
• finger print vacuoles for storage of
  polysaccharides
• no Spitzenkorper associated with growing hyphal
• tips
• nuclear division is intranuclear and centric
• vegetative thallus is diploid

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Biochemistry

• lysine biosynthesis
• - diaminopimelic acid pathway (DAP)
• - same as in plants
• sterols
• - fucosterol
• - required for sexual and asexual reproduction
• storage compounds
• - beta 1-3 glucans called mycolaminarins

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Cell Wall

• primarily composed of beta-glucans cellulose
• rare exceptions of chitin
• beta-glucan beta-1,31,6 D-glucose
• cellulose beta-1,4 glucose
• chitin beta-1,4 of N-acetylglucosamine

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Sexual Reproduction

• oogamous by gametangial contact (gametangy)
• monoecious or dioecious
• homothallic or heterothallic
• no flagellated gametes
• oogonium - female gametangium
• antheridium - male gametangium

antheridium
oogonium
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• female gametes (oospheres) produced by oogonium
  
• one to many oospheres
• male gamete produced by antheridium
• male gametes are donor nuclei that migrate from
• antheridium into oogonia and fertilize oospheres
• developing antheridia attracted to oogonium and
  upon
• contact with oogonial wall produce a
  fertilization tube

fertilization tube
oospheres
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Meiosis

• meiosis is gametangial (vs. zygotic)
• somatic nuclei diploid
• antheridia oogonia are sites of meiosis
• oosphere fertilized by male gamete
• fertilized oosphere is the oospore or diploid
  zygote
• oospore germinates and give rise to vegatative
  diploid
• hyphae

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Asexual reproduction - zoospores

• biflagellate, heterokont zoospores
• tinsel whiplash flagella
• tinsel flagella possesses crosshairs
• or mastigonemes
• mono- or dimorphic
• mono-, di- or polyplanetic
• zoospores are not gametes diploid products of
  mitosis

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• primary zoospores - the flagella are attached
  anteriorly
• Saproleginales
• secondary zoospores - the flagella are attached
  laterally
• all orders
• produce in zoosporangia modified hyphae that
  are usually
• terminal and delimited by a septum
• zoosporangia can be persistent or deciduous

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zooporangium and zoospores are the major
dispersal agents for most species
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Two large groups of Oomycota
Saproleginalean Saproleginales,
Leptomitales produce primary and secondary
zoospores spores always form in the
sporangium large hyphae considered
more primitive Peronosporalean
Peronosporales spores may form in the
sporangium or in evanescent vesicle
produce only secondary zoospores
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Saproleginales

• the water molds
• most abundant in fresh, clear water
• mostly saprobes, some are parasites
• (e.g. Saprolegnia parasitica on salmon)
• large diameter hyphae profusely branched
• coenocytic except at the base of reproductive
  structures
• mostly eucarpic
• mostly hermaphroditic homothallic
• several oospores/oogonium
• long cylindrical terminal zoosporangia
• zoospores primary secondary
• Review life cycle of Saprolgenia (page 705)

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Peronosporales

• considered most advanced group of Oomycota
• aquatic, amphibious, terrestrial
• some of the most destructive plant pathogens
• Phytophthora infestans - late blight of potato
• P. ramorum - sudden oak death
• zoosporangia often deciduous
• zoospores often formed in vesicle
• zoospores only secondary
• oogonium with usually one oosphere
• eucarpic
• Review life cycles of Pythium (p 714)
• and Phytophthora (p 720)