Fungi - intro

What type of cells are fungi

Eukaryotes

What kingdom are fungi closely related to, and what problem does that cause

Closely related to metazoa (animals). Both part of the Opisthokonts - cells that have a posterior flagellum, if they have a flagellum

Fungal diseases easily spread to humans and other animals

What are some of the groups that fungi are divided into (with examples)

Ascomycota - includes yeast, pathogens, lichen, common molds

Basidiomycota - includes mushrooms, toadstools, rusts, smuts, few lichens, yeasts

Other groups have a lot fewer species, Zoopagomycota includes animal pathogens & commensals, fungi, and amoeba

What organisms are often grouped with fungi despite not being closely related

Slime molds and water molds

Fungal diversity

Currently around 120,000 fungal species accepted, but that is estimated to only be 3-8% of total diversity

Examined ratio of fungi to plants in areas like Slapton Ley (13:1) and Esher common (8:1) - used these to scale up fungi estimates based on number of vascular plant species

Why are new fungi species continuously being found

Investigating less studied areas

Environmental sampling

Restudying known taxa and previous collections

Unique fungal characteristics

Cell wall made of chitin and glucan

Most grow by hyphae

Osmotrophic lifestyle

Dimorphic fungi - can switch between yeast and hyphal forms depending on the environment

Fungal features shared with plants

Usually haploid, vacuole, cell wall, asexual and sexual reproduction

Fungal features shared with animals

Heterotrophs, no chloroplasts, motile cells with single flagella

Variety of size of fungi

Some microscopic, but collective growth into mycelium can lead to huge structures

Largest fungus is Armillaria ostoyae which is a pathogen on trees - one estimated to be around 8,500 years old

Can also be large due to connections - rhizomorph is multiple hyphae parallelly oriented, able to transport nutrients long distances

Features of the fungi cell wall

Combining glucan and chitin

Can withstand high internal turgor pressure

Determines the shape, provides protection, and is the site of nutrient exchange

Fungi cell membrane

Conventional lipid bilayer structure

Most commonly contains ergosterol - therefore that is the basis of most anti-fungals that inhibit biosynthesis enzymes

Methods of fungal growth

Most grow through hyphae, extending at tips and branch points (filamentous growth)

Some are unicellular (yeasts) and reproduce by budding / fision

Some can switch between two lifestyles - dimorphic

Structure of hyphae

Formed of one or multiple cells surrounded by cell wall

Tube-like, extend at tips and branch points

Can be compartmentalised into septa which provide mechanical strength, allows for isolation of damaged/aging hyphae, allows for differentiation and sporulation

Septa differ across species

Hyphae without septa are ‘coenocytic’ and have multiple nuclei

Hyphae growth

Depends of the formation of branches → require a lot of cytoplasmic streaming, mixing and transporting of contents to enable membrane insertion and extension of the rigid cell wall

Branching can be apical or lateral

Can be part of negative auto-tropism (hyphae repelling one another via O₂/C₂O conc.)

Growth direction in most Dikarya (includes Ascomycota and Basidiomycota) enabled and defined by cell end markers - Spitzenkorper

How are mycelium formed

Fungal hyphae growth that forms a complex 3D network

The inner region has lots of branching and fused hyphae (anastomosis) as a result of positive auto-tropism → has good connectivity for transport

Outer region is more sparse, has more unbranched and exploratory hyphae - caused by negative auto-tropism for space-filling

What are fungi, due to their type of heterotrophic nutrition

Osmotrophs

What are osmotrophs

Absorb pre-digested food material through their body surface

Vital as decomposers and recyclers, associated with plant growth and terrestrial nutrient cycling

Fungi nutrition

Secrete enzymes (like cellulases, laccases, proteases) into the environment.

Digest nutrients externally

Absorb the nutrients - efficient due to their high SA:V ratio. Adsorption probably constrains hyphal diameter, not length

Accumulate large amounts of metal ions

What can fungi digest that makes them very important

Primary organisms that can digest wood - that makes up 95% of terrestrial biomass

Different types of fungal lifestyle

Saprophytes - feeding on dead/decaying matter

Symbionts - forming long term associations/interactions with other organisms

Pathogens - producing disease

Two types of fungi reproduction

Asexual and sexual reproduction

Fungi asexual production

Asexual production of spores

Growth of hyphae

Budding/fission

Fungi sexual reproduction

Fungi are mostly haploid so need fusion of hyphae and nuclei before sexual reproduction can begin

Anastomosis is the fusing of two hyphae, and it necessary before sexual reproduction

Hyphae can just remain fused or progress: plasmogamy (fusing of cytoplasm of two cells) → karyogamy (fusing of two nuclei) → meiosis

What mechanisms can control fusion of hyphae

Anastomosis itself has little control, but vegetative compatibility genes determine if fusion is maintained or aborted

Differences at the het loci can make cells incompatible, which leads to apoptosis of fused cells

Importance of fungi

Decomposers and recyclers - crucial for mineralising carbon

Mycorrhizal fungi essential for improving efficiency of root structures

Industrial purposes - bread, beer, cheese, antibiotics etc.

Examples of fungi symbiotic relationships

Mycorrhizae

Lichen

Features of mycorrhizae

Arbuscular mycorrhizae are associated with non-woody plants → the hyphae grow in the root cells and form ‘arbuscules’ which are the site of nutrient exchange. They are obligate biotrophs - completely dependent on plant for survival

Ectomycorrhiza are associated with woody trees - sheat forms around plant’s root structure and hyphae penetrates the outer layer to allow nutrient exchange

Potential benefit of arbuscular mycorrhizae (AM) colonisation

Can give increased resistance to pathogens, increased drought tolerance, salt tolerance etc.

Features of lichen

Fungus + photosynthetic alga / cyanobacterium

Fungi can partner with more than one phototroph

An essential part of ecosystems - especially in extreme climates. But sensitive to climate change and pollution

Why are fungi growing causes of concern

Major cause of plant disease

Growing concern for diseases affecting humans and animals - harder to treat than bacterial infection due to phylogenetic proximity

Examples of fungal diseases on animals

Cordyceps on ants → causes changes in behaviour, ‘zombie ants’

Chytrid fungus infects amphibians → only few can infect large numbers of amphibians

Effect of climate change on fungal disease

Exacerbates the problem
Increased spread of fungi and disease around the world

Fungi becoming pre-adapted to human body temperature, leading to increased resistance

Potential of fungi in medicine

Have secondary gene clusters that make a vast array of secondary metabolites

Includes stress-resistance, antimicrobial, psychoactive (parasitism/defence against insects) molecules

Includes dyes and litmus (from lichen)

Penicillin was the first developed antibiotic

Fungal genomics used for bioprospecting