ED. 13 Fungi

Kingdom overview

• Likely to be the first multicellular terrestrial organisms

• 900 million years ago - carboniferous radiation ca. 300 MYA 

• They are very good at recycling nutrients and causing disease to weaker organisms

• At least 6 phyla recognised so far

• Fungi use the environment as their gut, releasing enzymes extracellularly and break down complex molecules and take in simpler molecules 

• ‘True fungi’/ Fungi - a monophyletic group, other eukarya such as stramenpiles are not part of this group because they converge 

• they have chitin cell calls

How many fungi are there

• Estimates of species 1.5 mill - 3.5 mill - 9 mill species -> so far only 150k have been described by scientists, only 10k have been cultivated axenically (away from anything else) many of them remain unculturable typically because fungi often have complex nutritional requirements 

• Most fungi are microscopic for most of their life - a few form macroscopic structures e.g. mushrooms 

• Filamentous growth which are microscopic and hidden away 

• They might produce reproductive structures if they have enough nutrients 

• ‘Cryptic species’ - two or more species which are physically indistinguishable and have thought to be from one species

• Dark taxa - organisms we know only from DNA, we don’t know their phenotype

Roles of fungi

• Mutualists - lichens, mycorrhizas 

• Pathogens - rusts, blasts, blights, moulds, wilts, spots, smuts, mildews, cankers, diebacks, rots 

• Recyclers - degradation of wood, leaves and roots 

• Animal mutualist - rumen fungi, ant/termite/beetle farmers of recyclers 

• Animal pathogens - amphibian decline, bat decline, sea fan coral death, human mycoses

• Industry - bread, beer, wine, sake, cider, gin, rum, tequila, vodka, whiskey, marmite etc., ca. 44 trillion pounds 

• Eukaryotic models - yeasts, >6,600 genomes

Characteristics of most fungi

• Most fungi have sexual and asexual reproduction - meiotic and mitotic spores, or fragmentation

• Symbiosis - the living together or unlike organisms, 

• Biotrophy - connecting with another species to live 

• Indeterminate growth - able to take the shape or size of whatever they are growing in 

• Growth within solid matrices - e.g. soil, plant, animal, fungus 

• No motile cells - no cells that can swim

• Absorptive nutrition - enzymes only active outside, typically hydrolytic 

• Filamentous - form networks and connect with themselves, undergoing cell fusion 

• Cell walls of branched gluten, glycoprotein and chitin 

• Store glycogen and/or lipids - they have ergosterol in cell membranes, allowing the creation of antifungals 

• Hyperdiverse life cycles - diploid, haploid, dikaryon, multikaryon; homo-or heterokaryotic; parasexual 

• Parasexual - they go from diploid to haploid through losing chromosomes gradually 

Growth forms of fungi

• Unicells with flagella/or without flagella - fission yeasts and budding yeasts 

• Hypha - a filament of a muticellylar fungus, some are septate, others aseptate 

• Fungi evolved multicellularity at least 7 times (humans+plants only did this once) 

• Budding yeasts - go through mitosis and make a daughter cell which is smaller,  a scar is left where they give birth to the daughter cell 

• Yeasts have evolved from multicellular back to unicellular multiple times 

• Unicellular - ideal for when they are in liquid, 

• Filaments/multicellular - are ideal When they are in a solid

• Dimorphic fungi - both yeast and filamentous phases

Filamentous fungi - forms a mycelium, these can form a network, some filamentous fungi form tissues

• The mycelium tries to grow away from itself - negative autotropism 

• In the centre of the fungal network - there is positive auto tropism because they want connections with themselves for stability 

• Need alternating signalling to allow the fungi to find itself and grow towards itself 

• some filamentous fungi form tissues e.g. plectenchyma, rhizomorph, hymenium 

Hyphal structure

• Apical cell - this is the one that feeds, grows, steers - this is where the thinnest cell wall is 

• The older cells tend to have a bigger vacuole - under internal pressure and held by the cell wall 

• In the apical cell - exploration and colonisation is driven by internal pressure

Fusion

• Fusions - two hyphae fuse at the crossing point, or they form a bridge between two strands, or they just fuse side to side 

• Sometimes fusions are unsuccessful - it is tightly controlled by genetics

• Sexual fusion - mating loci, alleles must differ, so it is a successful fusion 

• Asexual fusion (making a network with themselves) - different set of loci called compatibility loci for self/non-self recognition, alleles must match

• There needs to be a balance between negative and positive autotropisms, this gives efficient resource translocation, mycelial stability, and genetic variation 

Fungi colonising plants

• Can be doing this to live somewhere, be parasitic etc. 

• Biotrophic - needing another organism to live 

• They will be breaking a hole in the cell wall of the plant, but not break the cell membrane allowing them to exchange resources 

• Or they could grow in between cells 

• Either create their entry point or enter through an existing entry point 

• Once inside, once they have enough resources they become necrotrophic

• They start to kill the cells using enzymes

Fungi colonising humans

• We are often inhaling a number of spores 

• Main fungi part of our flora is usually candida and malassezia - both are mainly going to live as yeasts, but they are both opportunistic pathogens, if we are in a weakened state they can switch and become filamentous and attack our cells 

Dikarya - ascomycota

• More than 50k species described 

• Licensed - they take on green algae 

• Commensal - grow within a living organism but don’t cause really any harm 

• Most spend their life as haploid septate filaments 

• Meiotic ascospores formed within a hypha called ascus - there are 8 ascospores within each ascus 

Development of sci with ascospores

• Development of sci with ascospores - two filaments one from male and other from female, at the tip there is repeated mitosis without cytokinesis so we end up with a lot of nuclei - these release pheromones which allows them to recognise that there is a female/male nearby 

• > the female grows a filament towards the male that fuses with it (called a trichogyne) the nuclei from the male moves through the trichogyne and ends up in the tip of the female filament, so there is a mixture of female and male nuclei

• The nuclei are lined up alternating female and male, they move into the tip of the filament - a hook shape is created by the tip moving in a positive autotrophic way -> mitosis occurs and the curved tip fuses with the same stem -> this forms a diploid cell 

• Then the tip goes through meiosis (two male and two female nuclei) -> another round of mitosis which results in 4 female and 4 male nuclei, the result is the formation of 8 nuclei 

• At the base of the tip there is a vacuole that forms internal pressure, this pushes the ascus pores together and essentially acts as a canon and disperses the ascus 

Development of conidia

• mitotic sports and asexual production

Ascomycota life cycle

Lichens

Symbiotic body - can only make that body using symbiosis 

• They are very early in the development of ecosystems

• They can grow on bare rock, bark, sand - they are tough but cannot cope with air pollution

• Mycobiont host, mainly ascomycetes, mostly obligate tropes

• Photobion, mostly Trebouxia (green algae), few with cyanobacteria, or both - Trebouxia so far unknown outside lichens: obligate biotrophs

• Together form a thallus unlike that of either alone: symbiosome, synergy

• dominant macro-organisms at highest latitudes and altitudes

Lichen anatomy

• Mycobiont of the lichen pokes through the cell wall of the plant cell but does not break open the cell membrane 

• ⁃ Without living with the lichen, the algae could not live on land - usually these alga are aquatic organisms 

Brewers/Bakers yeast

• Able to carry out aerobic fermentation in anaerobic conditions

• First eukaryotic genome that was sequenced 

• Diploid cell (bigger than haploid cells) can go through budding and produce daughter cells (4), then the mother cell bursts and the daughter cells are released

• -> the daughter cells can go through asexual budding and make more copies of themselves, when they detect each other they become sticky so male and female cells stick together 

• Mating type switching - the mother cell releases a daughter cell with the same mating type as the mother, the mother cell then switches mating type so that it can mate with the daughter cell just produced 

• Endonuclease cuts off the active locus and exonucelus destroys it and the mother is then able to switch mating types

• Mating types - are either a or alpha 

• Can only switch mating types after you bud off the daughter cell 

Basidiomycota

• Second most diverse phylum - so far 30k species

• Many are pathogens, some mycorrhizal 

• Most of their life cycle is spent as a dikaryotic septate filaments 

• Basidium is the tip of the hyphae, and outside of it, it will make 4 basidiospores - the basidiospores are meiotic

Development of basidia with basidiospore (meitotic spores)

• n+n = 2n (diploid) -> meiosis occurs so there is 4 nuclei, each nuclei moves into each basidiospore 

• Mitosis - a clamp connection is made where the tip grows into itself by positive auto tropism and nuclei goes through mitosis and the new copies  move through the connection and ends up towards the bottom of the tip so there are 4 nuclei 

Anatomy of Basidium

• At the top of the basidiospore carbohydrates are released and are hydroscopic (captures water, air has to be moist),

• condensation occurs and a lot of water forms in a droplet connected to the basidiospore which moves the centre of gravity

• the two droplets coalesce with each other and this propels the spore 

Basidiomycota life cycle 

Biomass recycling

• 95% of terrestrial biomass is wood

• Wood has polymers full of sugar which is ideal to feed on, however lignin creates obstacles in feeding on the sugars 

• A different kind of enzyme is evolved which is oxidative to be able to break down the lignin 

• The fungi break down the lignin all the way to c02 and water, this enables them to access the cellulose 

• The wood ends up being bleached and destroyed 

• Rhizomorphs contain a vasculature allowing mass flow of nutrients 

Main components of wood:

• lignin - complex heteropolymer, irregular cross-links, aromatic ring backbone, insoluble + impermeable, massively C-rich

• hemicellulose (pentose-hexose polymer) ← hemicellulases

• pectin (galacturonic acid polymer) ← pectinases

White rot fungi break down lignin, cellulose, hemicellulose and pectin

• some necrotrophic - this means it kills living host cells and feeds on the resulting dead organic matter 

• only some basidiomycetes and a few ascomycetes 

• the largest organisms 

• 1) Oxidative exoenzymes - peroxidases, phenoloxidase , laccases 

• 2) Hydrolytic exoenzymes 

• compartmentalisation and translocation