L7-9 fungi

• characteristics of a fungi:

  • eukaryotes
  • cell wall of chitin and polysaccharides
  • reproduce sexually and asexually
  • heterotrophs
• 3 main types of fungi

  1. saprophytes (derive from dead remains)
  2. necrophytes (derive from organisms they have killed)
  3. biotrophs (derive nutrients from living host)
• parasitic fungi can be:

  necrophytes or biotrophs
• fungi are usually filamentous
• nuclear mitosis in a fungi takes place _

  within the nucleus
• fungi and animals are each others closest relatives
• the 2 main forms of growth of fungi are:

  Yeast and filamentous form
• Yeast model organisms:

  1. Saccharomyces cerevisiae (Baker's yeast)
  2. Schizosaccaromyces pombe (model laboratory organism)
• yeast are single celled, oval/spherical, asexually reproduce, 3-5 microns in diameter
• filamentous fungi are multicellular, long thread like filaments (hyphae), grow from the tip, mycelium (mass of hyphae), reproduce via spores
• dimorphic-> environment cases to be in yeast/filamentous form
• yeast produce _


  daughter cell (identical to parent)
• filamentous fungi produce spores which are light for effective _

  dispersion
• filamentous fungi produce spores in food rich environments
• sexual reproduction of fungi:

  • union of compatible nuclei
  • when nutrients are in poor supply
  • sexual spores are more hardy structures allowing survival in adverse conditions
• heterothallism:


  results in exchange of genetic material and requires 2 different mating types (plus and minus)
• homothallism:


  self-fertilisation, produce sexual spores
• sexual reproduction in fungi:

  • 2 haploids fuse
  • forms specialised structure (dikaryon)
  • fusion of nuclei
  • then produces diploid sexual spores or haploid sexual spores
• 3 major types of fungi:

  1. zygomycota
  2. ascomycota
  3. basidomycota
• zygomycetes produce sporangiospores in sac structure which bursts to release spores which germinate to produce aseptate hyphae
• sexual reproduction of zygomycetes:

  • form zygosporangium then
  • forms diploid zygospore
  • undergoes meiosis to produce haploid spores which germinate to produce hyphae
• aseptate means


  have no cross-cell wall and the long filamentous cell is a continuous large cell
• septate hyphae means


  filamentous cells are divided into smaller cells
• zygomycete characteristics:

  • smallest phyla
  • haploid sporangiospores
  • diploid zygospores
• ascomycetes form asexual condiospores from specialised aerial structures, the spores then _ to form new fungi

  germinate
• sexual reproduction of ascomycetes:

  • mating
  • cell fusion
  • formation of specialised dikaryon structures
  • cell undergoes meiosis to produce four haploid sexual spores
  • each spore undergoes mitosis to form total of 8 spores
• ascomycetes characteristics:

  • haploid septate hyphae
  • haploid conidospores from conidiophores
  • haploid ascospores
  • most are saprophytes
• basidiomycetes undergo only sexual reproduction (asexual is very rare)
• basidiomycetes sexual reproduction:

  • spores released from fruiting body
  • 2 spores germinate followed by fusion of the hyphal cell
  • makes single organism with 2 nuclei
  • propagate and grow as dikaryon most of the time
  • spends most of time in soil
• deuteromycetes sexual reproduction cycle is absent
• true pathogens:


  affect healthy host, adapted to low oxygen tension and high temperature, restricted to geographical regions
• true pathogens are not obligate pathogens
• thermal dimorphism of fungi:

  • hyphal form in natural habitat less than 30 degrees celcius, reproduce through spores, saprophytic
  • yeast form in animal habitat 35-40 degrees celcius, reproduce through budding or endospores, parasitic
• histoplasmosis (true pathogen disease), pathogen is Histoplasma capsulatum
• infection cycle of Histoplasma capsulatum:

  • mainly from bird droppings
  • spores inhaled, infection local to the lungs
  • spores grow as yeast, forms endospores
  • if spreads, affects heart, lungs, liver tissues and becomes fatal
• coccidioidomycosis (true pathogen disease) caused by Coccidioides immitis
• coccidiodiomycosis can lead to fever, produces arthrospores (infectious agent)
• opportunistic pathogens:


  host defences are impaired
• opportunistic pathogens characteristics:

  • world-wide
  • many species and genera
  • no specific adaptations to host environment but are hardy to it
  • superficial to chronic systemic infections
• Candida albicans causes candidiasis (thrush):


  yeast causes localised infection in immunocompetent host, causes blood poisoning in immunocompromised host