Zoosporic Fungi Phylum Chytridiomycota

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Zoosporic FungiPhylum Chytridiomycota

• General Mycology Pl P 421/521

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Zoospore

• Microscopic
  (2-14 x 2-6 micron),
  uninucleate, unicellular,
  flagellated spore
  lacking a
  cell wall
• Formed in a zoosporangium by a process involving
  mitosis and cytoplasmic cleavage
• Zoospores do not feed, and rely on endogenous
  energy reserves

Catenaria zoospore, from George Barrons website
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Zoospores
Olpidium zoospores/D. J. S. Barr
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Flagella (sing. flagellum)

• 0.25 microns wide, up to 50 microns long
• Composed of a 9(2) 2 arrangement of
  microtubules enclosed in a plasma membrane

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Flagella (sing. flagellum)

• Attached to a kinetosome (basal body)
• Flagellated centriole highly conserved structure
  composed of 9 triplets of microtubules arranged
  in a cartwheel manner
• All zoospores have two kinetosomes, but if only
  one flagellum is formed, the second kinetosome is
  non-functional

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Prokaryote vs. Eukaryote flagellum
Bacterial flagellum from www.arn.org
http//fig.cox.miami.edu/Faculty/Dana/flagellum.jp
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Centriole

• Nuclear-associated organelle (NAO) involved in
  nuclear divisioninvolved in formation of spindle
  fibers that separate chromosomes during division
• All eukaryotes with a flagellated stage in their
  lifecycle have a pair of centrioles that
  replicate during cell division

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Flagella

• One to many flagella depending on the taxonomic
  group
• Two types of flagella
• Whiplash
• Smooth, usually directed backwards, propels the
  zoospore
• Tinsel
• With tripartite hairs (mastigonemes) directed
  forward, pulls the zoospore
• May have only whiplash, whiplash tinsel, or
  only tinsel
• Flagellum may be of unequal length ( heterokont)

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Zoospore types from Dick, 2001. Straminipilous
Fungi
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Zoospore ultrastructure
Plasma membrane
Lipid globules
microbodies
nucleus
ribosomes
Endoplasmic reticulum
Microtubules (cytoskeleton
mitochondria
Kinetosome and non-functioning kinetosome
flagellum
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Motile Phase

• Requires water
• Free-swimming phase is influenced by
• Endogenous energy reserves (lipids)
• Environmental conditions

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Encystment

• Prior to germination, zoospores must
• shed or retract flagella
• Form a cell wall

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Germination

• Direct
• Formation of germ tube
• Indirect
• Formation of another zoospore

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Zoosporangium

• A typically multinucleate structure that produces
  zoospores by a process call zoosporogenesis
• Zoosporogenesis involves mitosis and cleavage of
  zoospores from zoosporangium cytoplasm
• Zoospores release
• Breakdown of zoosporangial wall
• Opening of cap-like cover called operculum
• Discharge papillae plugged with gelatinous
  material

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Thallus Types

• Holocarpic
• Conversion of entire thallus into one
  (monocentric) or more (polycentric) zoosporangia
• Eucarpic
• Entire thallus not converted into zoosporangium,
  and other structures may be formed
• Rhizomyceliumhyphal-like structures connecting
  sporangia, lack nuclei
• Rhizoidsroot-like structures, lack nuclei
• Mycelium

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sporangium
rhizoids
Eucarpic thallus of Spizellomyces Photo by D. J.
S. Barr
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Thallus types relative to substrate

• Endobiotic
• Thallus produced inside host or substrate
• Epibiotic
• Thallus produced outside host or substrate
  rhizoids anchor thallus to substrate

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Zoosporic fungi

• Kingdom Fungi
• Phylum Chytridiomycota
• Kingdom Straminipila
• Phylum Oomycota
• Phylum Hyphochytriomycota
• Phylum Labyrinthulomycota
• Protista
• Phylum Plasmodiophoromycota
• Phylum Myxomycota

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Phylum Chytridiomycota

• True Fungi based on
• Chitinous walls
• Flattened mitochondrial cristae
• Lysine synthesis by the alpha aminioadipic acid
  (AAA) pathway characteristic of all true Fungi
  and some protists
• compare to diaminopimelic acid pathway found in
  bacteria, plants, and some protists

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Classification

• Two phyla, Blastocladiomycota and Chytridiomycota
• 123 genera, 900 species in 5 orders
• Chytridiales
• Spizellomycetales
• Monoblepharidales
• Neocallimasticales
• Blastocladiales

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Thallus types

• Chytridiales, Spizellomycetales and
  Neocallimasticales
• Relatively simple thalli, holocarpic or eucarpic
  with rhizoids or rhizomycelium
• Monoblepharidales
• Filamentous thalli (mycelium)
• Blastocladiales
• Stalked thalli with rhizoids

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Asexual Reproduction

• Uniflagellate zoospores
• One whiplash flagellum inserted in posterior part
  of zoospore
• Zoospores formed in zoosporangia and are released
  through an operculum or discharge papilla

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Zoospore release in Chytrium
Photos by D. J. S. Barr
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Sexual Reproduction

• Plasmogamy can involve one of five different
  structures depending on species
• Isogamous planogametes
• Anisogamous planogametes
• Nonmotile female gamete and motile male gamete
• Gametangial copulation
• Somatogamy

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Sexual Reproduction

• Plasmogamy and karyogamy results in formation of
  resting sporangium
• Thickened, often pigmented and/or ornamented wall
• Germination of resting sporangium occurs after
  meiosis by cleavage of cytoplasm into zoospores

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Order Spizellomycetales

• Monocentric thalli
• Blunt rhizoid tips (1-2 microns diam)
• Inoperculate, multipapillate zoosporangia
• Amoeboid-like zoospores
• Mostly soil-inhabiting

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Order Chytridiales

• Monocentric or polycentric thalli
• Slender rhizoid tips (lt 0.5 micron)
• Inoperculate or operculate if inoperculate, then
  single or multipapillate
• Regular-shaped zoospores
• Mostly aquatic

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Zoospore ultrastructure
Spizellomycetales
Chytridiales
Barr, 1990
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Batrachochytrium dendrobatidis

• Responsible for decline of amphibian populations,
  particularly frogs, in six continents (Africa,
  South, Central and North America, Europe,
  Australia and Oceania)
• First reported in 1993
• Only member of Chytridiomycota to parasitize
  vertebrates

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How does it kill amphibians?

• Sporangia restricted to keratinized skin of adult
  frogs and keratinized mouth parts of tadpoles
• Causes widespread, fatal epidermal infection only
  in adults
• Epidermal hyperplasia that results may seriously
  impair cutaneous respiration and osmoregulation
• Toxin production has not been demonstrated

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Order Neocallimasticales

• Also spelled Neocallimastigales
• Rumen fungifirst discovered in 1977
• Obligately anaerobic chytrids that live in
  digestive tract of herbivores (ruminants and
  hind-gut fermenters
• Some taxa produce polyflagellated zoospores
• Zoospores lack mitochondria

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Biology of rumen fungi

• Zoospores encyst on plant material in rumen and
  intestine
• Form thallus with well-developed rhizoidal system
  that penetrates plant material
• Passed from mother to offspring, probably through
  licking or feces
• No known sexual stage

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Numbers represent hours after encystment
Life cycle animation
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Thallus of rumen fungus
Thallus (zoosporangium) of rumen fungus
Polyflagellated zoospore
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Cow Facts

• Each day, the average cow drinks 120 liters and
  eats 44 kilos (95 lb) of feed
• The rumen has a volume of 100-150 liters
• A cow produces 98-190 liters of saliva each day
• A cow produces an average of 25 kilos (55 lbs) of
  manure/day

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Phylum Blastocladiomycota

• Zoospores with tightly organized organelles and
  characterized by nuclear cap
• Most species are saprotrophs in soil, water, mud,
  plant and animal debris exceptions
• Coelomomyces, is an obligate endoparasite of
  insects
• Catenaria species parasitize small animals
• Physoderma species are plant parasites
• Separate gametophytic and sporophytic thalli in
  several genera, including Coelomomyces, Allomyces
  and Blastocladiella

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ZOOSPORE ULTRASTRUCTURE
Nuclear cap
Blastocladiales
Monoblepharidales
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Coelomomyces

• Alternating sporophytic and gametophytic stages
  in mosquito larvae and copepod (fish lice) hosts,
  respectively
• Wall-less hyphal bodies (hyphagens) formed in
  coelom of host

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Life Cycle of Coelomomyces
Motile zygote encysts, infects mosquito larvae
Conjugation of gametes
Gametophytic thallus lacking cell wall forms in
copepod
Sporothallus develops in host, resting sporangia
formed
Zoospores (meiospores) infect copepod
Resting sporangia
Germination of resting spore
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Coelomomyces resting spore
Photograph by CC López lastra, JJ García
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Catenaria

• Polycentric and eucarpic, with catenate
  zoosporangia and resting spores that are
  connected by rhizomycelium rhizoids present
• Catenaria anguillulae is a parasite of nematodes,
  copepods and other small animals, but also grows
  in organic debris

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Catenaria anguillulae
http//helios.bto.ed.ac.uk/bto/microbes/catenar.ht
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Allomyces

• Branched thalli with marked bipolarity (basal
  rhizoids and apical sporangia)
• Sexual reproduction by anisoplanogametes
• Female gametes are hyaline and twice the size of
  the orange male gametes
• Two different thalli are produced
• Haploid gametophytes
• Diploid sporophytes

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Top and bottom gametothallus orange male
gametangium, larger hyaline female gametangium
Sporothallus
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Rozella

• Rozella spp. are holocarpic parasites of chytrids
  and oomycetes, grow inside host as a wall-less
  trophic form, until they produce resting spores
  or zoosporangia
• May represent earliest diverging lineage of
  fungi lineage may also include Microsporidia

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Olpidium

• Currently placed in Spizellomycetales
• Forms resting spores, zoosporangia in host cells
• Includes Olpidium brassicae, vector of Lettuce
  Big Vein varicosavirus

www.biltek.tubitak.gov.tr
vegetablemdonline.ppath.cornell.edu