Lecture #3

Chytridiomycetes - Chytridiomycota

Chytrids & Zygomycetes (lower fungi)

• formerly refered to as Phycomycetes
• algal fungi; thought to have evolved from algae via the loss of chloroplast; no longer excepted theory
• chytrids and the rest of Fungi are hypothesized to be more closely related to animals via protozoan ancester

Chytridiomycetes

General characteristics

• only flagellated members of the Kingdom Fungi
• chitin cell walls
• flattened mitochondrial cristae
• AAA lysine biosynthesis

Ecology

• aquatic & terrestrial
• saprobes, some parasites of protists, inverts., fungi & plants; a few anaerobic species in the rumen of herbivores

Thallus and life history diversity

• unicellular, holocarpic - may produce rhizoids that mainly serve to anchor thallus; rhizoids lack nuclei
• filamentous, eucarpic - coenocytic mycelium; septa may form at base of reproductive strucs.
• mono- or polycentric
• endobiotic - living entirely within the cells of their hosts
• epibiotic - producing reprod. organs on the surface of either a living host or dead organic matter with rhizoids or mycelium remaining inside

Flagellated stages of life cycle

• zoospores - asexual reproduction
• planogametes - sexual reproduction
• both zoospores & planogametes possess a single, posterior whiplash flagellum

Asexual reproduction

General characteristics of asexual reproduction in chytrids

• initiates with zoosporangium filled with protoplasm and many nuclei
• protoplasm of zoosporangium is then cleaved into numerous minute section which develop into zoospores
• zoospores are released, swim, encyst, then germinate producing a thallus
• operculate zoosporangia always form a well-defined circular cap, operculum, through which the zoospores emerge
• inoperculate zoosporangia discharge their zoospores through a pore in the wall of the sporangium or discharge tube, formed when the discharge papilla dissolves
• most known species possess inoperculate zoosporangia

Nuclear dynamics of zoosporogenesis

• endogenous
  • nucleus remains in zoospore cyst where it multiplies
  • encysted zoospore converted into sporangium
• exogenous
  • nucleus migrates into germ tube & rhizoid
  • sporangium develops following nuclear division
• exogenous-endogenous

Endo-exogenous, endo-epibiontic, mono-polycentric are not phylogenetically conserved

Sexual reproduction

Modes of sexual reproduction

• Planogametic copulation (three forms)
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  • isogamous
  • anisogamous
  • nonmotile egg fert. by a motile antherozoid
• Gametangial copulation
  • tranfer of protoplast from one gametangium to another
• Somatogamy
  • fusion between rhizoidal filaments

Major problems in interpreting sex in chytrids

• difficult to distinguish between resting spores and thick-walled resistent sporangia
• biflagellate zoospores may be result of plasmogamy or incomplete cleavage in the zoosporangium

Life cycles

• haplobiontic - one free living thallus; haploid (haplobiontic-haploid), diploid (haplobiontic-diploid)
• diplobiontic - alternation of generation; haploid thallus alternates with a diploid thallus
• Oomycetes - haplobiontic-diploid; gametangial meiosis
• Chytridiomycetes - haplobiontic or diplobiontic-haploid; zygotic meiosis

Zoospores

General characteristics

• cornerstone of classification
  • five zoospore types
  • basis for ordinal groupings
  • single whiplash flagellum
  • rarely polyflagellate cells
  • usually attached posteriorly but occassionally attached anteriorly or laterally but always directs posteriorly
• organelles
  • mitochondria, microbodies, endoplasmic reticulum and one to many lipid bodies located in specific regions of the zoospore
• some possess microbody-lipid globule complex (MLC)
  • appear to be involved in the utilization of stored lipid during zoospore motility and in the regulation of calcium
• gamma particle
  • organelle unique to chytrid zoospores
  • small membrane-bound organelle containing an electron-opaque, cup-shaped inclusion
  • protein storage

Flagellar apparatus

• flagellum
• kinetosome (=basal body, flagellated centriole), dormant kinetosome (nonfunctional centriole or nfc)
• transition zone; area of the flagellar shaft between the flagellum proper & the kinetosome; kinetosome props, transitional fibers, transitional plate
• flagellar roots

Microbody-lipid globule complex

• assemblage of organelles
• lipid bodies in close association with microbodies, mitochondria and membrane cisternae or rumposomes
• not all chytrids have well organized MLC

Orders of Chytridiomycota

Chytridiales

• true mycelium lacking
• rhizoids or rhizomycelium present in some species
• zoospores
  • globose
  • elongated when swimming
  • ameboid prior to encyst
  • single large lipid globule
  • core area of compacted ribosomes, partially enclosed by ER
  • nucleus is to one side and not conspicuously attached to kinetosome
  • lipid globule is on the opposite side of the zoospore; partially surrounded by a microbody, membrane cisternae, rumposome, & mitochondria which surround the ribosome core
  • 2-16 microtubules run from the kinetosome to the rumposome surface dormant kinetosome is parallel to kinetosome;connected by fibers

Spizellomycetales

• mainly isolated from soil
• zoospores
  • irregular morphology & undergo ameboid movement while actively swimming
  • nucleus in connected to the kinetosome
  • ribosomes dispersed throughout the zoospore
  • MLC is simple with mitochondria spaced apart from the lipid
  • number of lipid bodies is highly variable
  • no rumposome
  • dk is at angle to kinetosome
  • no electron opaque material in transition zone

Blastocladiales

• thought to be the most advanced group
• zoospores
  • elongated morphology
  • nucleus is cone-shaped with long end towards kinetosome
  • nuclear cap on anterior end of nucleus; densely packed with ribosomes
  • MLC is posterior to side and partially surrounds the nucleus
  • intimate association of microbody, several lipid globules, single backing membrane on one side & usually a large mitochondrion on the other
  • root consists of 27 microtubules ensheath nucleus
  • no rumposome
  • dk is at angle; typically diminutive or maybe absent

Monoblepharidales

• oogamous & anisogamous reproduction
• zoospores
  • somewhat elongate
  • nucleus centrally located & apparently unconnected to kinetosome
  • ribosomes are loosely enclosed by ER about nucleus
  • rumposome is posterior; interconnected to with microbodies & ER to lipid globules in the anterior
  • mitochondria vary in position & are not associated with MLC
  • flagellar root consists of two parts, striated discs & microtubules
  • dk parallel to the kinetosome
  • electron opaque plate in transition zone

Neocallimastigales

• obligate anaerobes in rumen & hindgut of herbivores
• zoospores
  • uni- to polyflagellated
  • ribosome aggregates mostly anterior
  • nucleus in the center
  • hydrogenosome; modified microbodies
  • dk absent in all species
  • no kinetosome props; saddle-like structure surrounding kinetosome
  • irregular microtubules
  • posterior dome- specialized area of plasmalemma; funx. unknown; composed of peripheral granules and parallel megatubules
• resting spore stage is diploid

Life Cycles to Review

Blastocladiales

• Allomyces macrogynus
  • alternate generations, anisogamy
• Coelomomyces
  • lacks nfc

Monoblepharidales

• Monoblepharis polymorpha
  • nonmotile egg

Chytridiales

• Chytriomyces hyalinus
  • rhizoidal somatangy; operculate
• Rhizophidium couchii
  • gametangial; inoperculate