M8 Barraclough- Introduction to fungi

what is the osmotrophic mode of nutrition?

• most fungi (excluding microsporidia and cyptomycota, no osmotrophism or hyphae, hence debatable) secrete enzymes into their environment to depolymerise + digest nutrients extracellularly

• eg. cellulose, proteins and lignin (partially)

• this relies on a high surface area to volume ratio- restricts hyphal diameter

what is the structure of the fungal cell wall?

• thick, rigid chitin inner layer

• glucan layer

• glycoprotein-rich outer layer

• the lipid bilayer normally contains ergosterol

what is the structure of fungal hyphae?

• tube-like eukaryotic cell structures that extend at the tips and branch

• can be compartmentalised with septa that allow for isolation, differentiation and mechanical strength

• non-septated hyphae are coenocytic and have multiple nuclei in one cell

• when branching, two adjacent hyphae can avoid eachother by negative autotropism or can rejoin together during anastomosis by positive autotropism

how is hyphal colony growth regulated?

• at the centre, the hyphae are more densely packed and fused together by anastomosis + positive autotrophy for transport and exchange

• moving further out, the hyphae become exploratory, unbranched and sparser, governed by negative autotrophy for space-filling

• these are called radial colonies

• as such, branching frequency is controlled by environmental conditions

  • under stress more exploratory hyphae are produced

  • under excess colonies become more dense

other growth forms exist- unicellular division by budding and binary fission in yeasts, though some species are dimorphic and change their growth form depending on the environment

what are the forms of asexual and sexual reproduction in fungi?

asexual:

• asexual production of spores

• anastomosis of genetically identical hyphae

• unicellular division in budding yeasts

sexual:

• spore production by fusion of gametes/2 haploid cells

• anastomosis, plasmogamy (cytoplasm fusion) and karyogamy (nuclear fusion) of genetically distinct but compatible hyphae

how do fungi survive in a haploid state?

• mutations are visible since genes are single copy

• but many hyphae are coenocytic, containing multiple nuclei- mutations can exist in the different nuclei, so multiple genotypes can exist at once and locally complement each other

• these heterokaryons are produced by anastomosis

  • the different nuclei increase genetic variation, so the phenotype depends on the interactions between the nuclei and can be spatially different/localised

  • however, hyphae may not be vegetatively compatible for anastomosis, dependent on the het (heterokaryon) loci and may die instead