Fungi

Question 1

Are fungi prokaryotes or eukaryotes?

Answer 1

Eukaryotes

Question 2

What are Opisthokonts?

Answer 2

Cells, which when flagellate, possess a single posterior flagellum.

Broad group of eukaryotes, including both the animal and fungus kingdoms.

Question 3

What is controversial about fungal phylogeny?

Answer 3

Fungi and Animals are closely-related but slime moulds and oomycetes aren’t – traditionally part of fungi and still very controversial.

Question 4

Describe fungi?

Answer 4

• eukaryotic
• typically haploid nuclei
• reproduce mainly with sexual/asexual spores
• rigid cell walls with chitin:
• mostly filamentous growth form
• heterotrophic
• microscopic hyphae: 2-10 µm in diameter
• macroscopic mycelium: can cover up to 10km area

Question 5

Detail about fungal rigid cell walls?

Answer 5

Rigid cell wall is due to the presence of chitin (also found in insect exoskeletons).

Determines the shape of the cell, provides protection, site of nutrient exchange and allows system to be pressurised via osmotic pressure.

Question 6

What are the filaments/hyphae?

Answer 6

Hollow tube containing cytoplasm with rigid wall

Lots of cytoplasmic streaming, mixing and transporting contents

Maybe compartmentalised with septa

Typical eukaryote with nucleus enclosed in a membrane, ER, mitochondria, golgi, etc

Growth only occurs at tip/branch point – vesicles containing lytic enzymes fuse with the membrane about 10 µm from the tip, the enzymes break down some of the wall polymers, allowing for local weakening.

High pressure in the hyphae cause the weakened wall to stretch. Vesicles containing new wall precursors fuse and new material is synthesised.

The hyphae re-hardens and the tip has moved forwards.

Fungal hyphae grow to form complex 3D networks by apical tip growth/branching, and by hyphal fusion.

Question 7

Describe the types of hyphae?

Answer 7

• Hyphae can be spetate or aseptate
• Septa provide mechanical strength, mechanism for isolating damaged/ageing hyphal lengths, allows differentiation
• Zygomycetes is normally aseptate to allow unrestricted cytoplasmic streaming but the septum allows isolation
• Ascomycetes has a Woronin body which can move to allow cytoplasmic streaming & allow nuclear/major organelle movement. Closed form provides isolation if a compartment is damaged
• Basidiomycetes has a dolipore septum, which allows cytoplasmic streaming & capable of regulation up to full isolation via degree of swelling

Question 8

How do fungi eat?

Answer 8

Digest externally, from enzymes released mostly from tip, then absorb via diffusion.

Imposes a surface:volume constraint.

Forced to be microscopic in one dimension.

Cellulose, lignin, protein are enzymatically depolymerised to simple sugars, amino acids, etc. which are then absorbed through cell wall & membrane

Question 9

How is growth of the whole colony regulated?

Answer 9

Branching frequency sensitive to environmental conditions.

More exploratory hyphae under nutrient stress, whereas more dense colonies under nutrient excess

Autotropism: sense neighbouring hyphae via O2 or CO2 concentrations followed by growth away/towards neighbours

Question 10

Fungal diversity?

Answer 10

70,000 named, but recent estimates are over 1.5 million species in biosphere

First fossils are from 400 mya – fossils indicate co-evolution with early vascular plants

Few marine and freshwater species (1% of species), but almost exclusively terrestrial occupying major decomposer and mutualistic niches

Question 11

When did a fungal bloom occur?

Answer 11

Fungal bloom occurred on dead organic matter after the meteor which struck the Earth 65 mya and caused a dust cloud to envelope the Earth and global deforestation due to lack of light.

Evidenced by layer of fungal spores at KT boundary.

Dinosaurs/Mammals exposed to high levels of airborne spores, and the resulting infections were more widespread for cold-blooded dinosaurs (high temps shown to inhibit fungal pathogens).

Dinosaur extinction over the next 300,000 years due to infection.

Mammals flourished.

Question 12

When did fungi cause a famine?

Answer 12

Great Famine in 1845

25% reduction in population, as 1/3 of population relied on potato as sole source of carbohydrates.

The Irish potato famine – Phytophthora infestans

Question 13

Outline dutch elm disease?

Answer 13

Ophiostoma ulmi

Current outbreak started in late 1960s from a beetle vector

More aggressive strain than it was in America

25 million elms out of 30 million trees were dead, a major source of biodiversity removed from British ecosystem

Question 14

Disease threat to humans and plants?

Answer 14

Humans – threat to immuno-compromised, but less of a threat than bacteria and viruses

Plants – greater threat than nematodes, bacteria and viruses. This impacts on crops which act as food to humans. However, saviours as mutualists.

Question 15

Are fungi haploid or diploid? Key features of this?

Answer 15

Most fungi are predominantly haploid, unlike most eukaryotes.

Therefore in fungi, most genes are single copy and mutations are visible like bacteria.

Many hyphae are coenocytic, many identical nuclei in the same piece of cytoplasm called homokaryon.

This means that two different mutations can compete in a local environment – bit like dominant & recessive in diploids

Question 16

Order of events in sexual reproduction in fungi?

Answer 16

Haploid initially so meiosis not the initial event in reproduction, it is hyphal fusion – plasmogamy.

Can occur tip-to-tip, tip-to-side or side-to-side.

The hyphae fuse and some exchange nuclei.

Local phenotypes occur when the nuclei don’t fuse, some phenotype may depend on the ratio of the nuclei

Heterokaryonic - can have multiple different nuclei.

Question 17

Heterokaryonic consequences?

Answer 17

• different nuclei provide genetic variation in mycelium
• mycelial phenotype depends on interactions between all nuclei
• phenotype of mycelium different in different parts of the mycelium

– spatially dependent phenotypes

• greater physiological flexibility to react to different nutritional environments
• increase genetic diversity without need for sexual reproduction
• allows acquisition of genetically different mitochondria, plasmids, etc. (cytoplasmic inheritance)

Question 18

Define heterokaryonic?

Answer 18

Multinucleate cell with genetically different nuclei.

Question 19

What happens if heterokaryonic hyphae generate asexual spores?

Answer 19

Homokaryonic spores are formed.

Spore chains formed by mitotic division from the nucleus.

Question 20

What determines whether two hyphae fuse into a stable heterokaryon?

Answer 20

Vegetative compatability genes.

If incompatible, death of fused cells occurs.

Question 21

Sexual reproduction sequence in fungi?

Answer 21

1. Stable heterokaryon
2. Karyogamy (nuclear fusion)
3. Meiosis (nuclear division)

Question 22

How are fungi identified and classified?

Answer 22

Morphological characters are still widely used for identification.

Sexual spores and structures are still widely used.

Question 23

Main features of penicillin?

Answer 23

Sir Joseph Lister (1871) observed mould growing on fruit and cheese had a antimicrobial effect

Sir Alexander Fleming (1928) noticed that numbers of staphylococci were lower when close to a mould growing on the same plate. Discovered the mould was effective against bacteria causing scarlet fever, meningitis, and diphtheria & published his results which were ignored.

In 1939, Florey and Chain showed the penicillin culture showed both antibacterial action and lack of mammalian toxicity when injected to mice.

Heatley devised methods for mass culture, assay and extraction using solvents.

Abraham purified the drug and worked out its structure.

Robinson and Cornforth put forward an alternative structure.

Hodgkin resolved the argument in Abraham’s favour using X-Ray crystallography.

Albert Alexander had sepsis, removed eye and infected lungs. Penicillin saw a 24 hour improvement but not enough to continue the treatment and he died.

During WW1, 15% of battle casualties died of infected wounds, after introduction of penicillin, WW2 death rates from infection were almost zero.

Penicillin is now produced in fed-batch submerged fermentation, where penicillin is always produced when the biomass increases

Unusually for eukaryotic genes, the penicillin genes are clustered together. Genes from different Penicillin species are usually in the same relative position.

Drug improvement based mainly on duplication of gene clusters, about a 15,000x increase in yield per g of culture in 70 years

Question 24

Why do antibiotics stop working?

Answer 24

1. Drug uptake/efflux – stop uptake or speed up removal
2. Drug modification – removes damaging part
3. Drug inactivation
4. Alteration of drug target

Question 25

Impact of Antimicrobial Resistance (AMR)?

Answer 25

WHO estimates 630,000 cases of multidrug-resistant TB worldwide in May 2013

1. Conjugation transfers plasmids at high frequency – many antibiotic resistance genes are plasmid encoded
2. To become a problem, needs an environment which selects for resistance, e.g. Hospitals
3. High levels of antibiotic use so resistance genes are selected for and get spread
4. Eventually acquired by pathogen and antibiotic is removed from armoury

Solutions:
new antibiotics (massive cost)
abstinence (withdraw certain antibiotics from use for periods – slimline genomes in bacteria so will remove plasmid if not helpful to survival, better standards of heath care in hospitals)

Question 26

Characteristics of a fungus suitable for industrial use?

Answer 26

1. Able to grow and form product in large-scale culture
2. Spores easy to inoculate and germinate in large fermenters
3. Rapid growth to produce the desired product in a relatively short period of time
4. Growth in a relatively inexpensive nutrient in bulk quantities
5. Not pathogenic
6. Amenable to genetic manipulation

Question 27

Production of citric acid?

Answer 27

Global production in 2007 – 1,700,000 tonnes

Uses in food industry – 70% - flavouring carbonated beverages, and fruit-flavoured drinks, jams, jellies, etc.

Uses in pharmaceutical industry – 20% - effervescent tablets, blood anticoagulant

Uses in chemical industry or cosmetics – 10% - cleaning oxides from boilers, nuclear reactors, pH control agent in textile industry and household detergents

Produced by Aspergillus niger
Metabolic pathway is central metabolism via glycolysis with citric acid produced in the Krebs cycle

Fungus tricked into producing excess citric acid by depriving it of iron – essential nutrient

Fungus produces citric acid as a siderophore to try and acquire iron from the environment

Question 28

How is it known whether a plant is susceptible to a pathogen?

Answer 28

For every resistance allele he found in the plant, a corresponding allele was found in the pathogen – if the pathogen didn’t have the allele then the plant was susceptible.

He termed the pathogen alleles avirulence genes.

Hence there was a gene‐for‐gene interaction between host and parasite – this gave rise to the gene-for-gene concept.

A lack of recognition of the pathogen (no elicitor or no receptor) allows infection by the biotrophic pathogen.

Question 29

How was wheat stem rust eradicated?

Answer 29

The problem was solved by the introduction of wheat resistance genes.

SR31 was the most successful derived from wheat x rye hybrid derivatives produced in Germany in the 1930s.

Wheat stem rust declined to almost insignificant levels nearly everywhere by mid 1990s.