Storage compounds – retaining nutrients
Littlefield and Heath 1979
Ultrastructure of Rust Fungi lipid glycogen

Nutrition of biotrophs
• Components are extracted through haustoria
• Nutrients are soluble and organic
• Extracellular degradation for cell penetration
• Extracellular factors establish/maintain a compatible infection
• Suppress senescence

Suppressing senescence www.mpiz-koeln.mpg.de/schlef/PSL_webpage.html

Livning substrates exploited by fungi
What is the nutrient flow direction?
http://www.ucmp.berkeley.edu/fungi/rhyniefungus.jpg

Arbuscular and ectomycorrhizal fungi

Amino acid biosynthesis

Secondary metabolites
• Glucose-derived – polysaccharides, peptidopolysaccharides, and sugar alcohols.
• Condensation products of acetate – derived from the acetate-malonate pathway of fatty acid synthesis, e.g. polyketides and phenolics.
• Condensation products of acetate derived from the mevalonic acid pathway, e.g. terpenes.
• Phenolics derived from the shikimic acid pathway of aromatic amino acid synthesis.
• Derivatives of other amino acid syntheses.

Secondary metabolites
Pigments
Hormones
Toxins
Co-regulated with sporulation

Secondary metabolites of Saccharomyces www.crc.dk/flab/ newpage13.htm

Genetics – study of heredity
• Transmission - the passage of traits from one generation to the next

Genetics – study of heredity
• Population - genetic diversity and change within natural populations

Genetics – study of heredity
• Molecular - details of gene structure and function

Our focus for genetics
• transmission and molecular genetics in experimental systems
• defining a population
– organisms in culture
– humungous fungus
– vegetative incompatibility

Transmission genetics
• Typical characteristics of fungal genomes
Small
– S. cerevisiae 6 MB – 6000 genes
– A. nidulans 13 MB – 12000 genes
– H. sapiens 1300 MB – 30000 genes

Typical characteristics of fungal genomes
• Little repetitive DNA – single copy genes
– 50-60% of nuclear genome is transcribed into mRNA in
S cerevisiae
– 33% in S. commune (basidiomycete)
– 1% in humans
• Introns
– few, often none
– small – 50-200bp vs ≥10 kb in mammals

Most higher fungi are vegetative haploids
• One genome copy per nucleus
• Alternatives?
– Plants?
– Algae?
– Animals?

Risks of haploidy
• No backup copy in case of genetic damage from UV or chemical mutagens
• Yeasts tend to be diploid ( S. cerevisiae except for lab strains) or have short G1 (S. pombe )
Chant and Pringle JCB 129:751

Advantages of haploidy
• A multinucleate cell can expose genome to mutagens
– most mutations are deleterious
– select for advantageous mutations in a heterokaryotic system
• Phenotypes of recessive mutations are obvious in the vegetative state, without generating homozygous recessives
• Lab strains of S. cerevisiae now generally include a mutation which stabilizes the haploid state

Transmission genetics – passage of inheritance
• Similar to more familiar mammalian systems, with bulk of life cycle haploid
• ‘Genders' are ‘mating types’
– cells are biochemically distinct but morphologically identical

Fungal mating systems

No mating factors  A. nidulans
• Inbreeding possible
– disadvantage – sex does not necessarily increase genetic diversity
– advantage – can form resistant spores even if no mating partner is available
– A. nidulans ascospores from 1995 still viable after 4°C storage, whereas conidia viability is severely reduced after several months at 4°C

One factor (zygo, asco, some basids)
• Bipolar mating system
• meiosis will give two types of segregants
– N. crassa a and 
–
Rhizopus + and
–

One factor (zygo, asco, some basids)
•
Advantage
– outbreeding
•
Disadvantage
– cannot produce resistant sexual spores unless a partner is available
• ‘Coping’ with one-factor mating systems
–
Some fungi have multiple alleles at the mating locus
–
Mating type switching in Saccharomyces

One factor (zygo, asco, some basids)
• In
S. cerevisiae "a" cells produce a-factor, a peptide sexual hormone, and

-receptor; converse for
 cells
• hormones/receptors interaction promotes schmooing, wall changes promote adhesion

Two factors, A/B (often in basids)
• Tetrapolar mating system  meiosis give four types of segregants
• A
1
B
1
:: A
2
B
2

A
1
B
1
, A
1
B
2
, A
2
B
1
, A
2
B
2

A and B functions are distinct
• in homobasids (.....?)
– A controls pairing and synchronous division of nuclei, hook cell formation;
– B controls septal dissolution and hook cell fusion ( b -glucanase activity) and nuclear migration

A and B functions are distinct
• in heterobasids (....?)
– A controls pathogenicity;
– B controls filamentous growth

Systems restricting outcrossing in one-factor mating type systems
• self-fertility  S. cerevisiae has "mating type switching"
• molecular basis  both mating genes have a storage site and an expression site.
• if the appropriate partner cell is not available when mating conditions are presented (how would this be detected?)will induce swi expression

Systems restricting outcrossing in one-factor mating type systems
• vegetative (somatic) incompatibility
• het genes are important for mating, but prevent vegetative fusion

Systems restricting outcrossing in one-factor mating type systems
• vegetative (somatic) incompatibility
• in Fusarium – vegetative incompatibility is important for maintaining distinct populations with different host specificities
• Fusarium oxysporum f. sp. groups

Mutants in experimental fungal systems
• spontaneous mutations or mutagenesis (uv, chemicals)
• each gene is named for 1st described mutation
• Example: gene for pigmentation is called
“white” because the mutant lacked colouration

Different species, different naming system
• Saccharomyces cerevisiae
• Schizosaccharomyces pombe
• Aspergillus nidulans
• Neurospora crassa
• Generally, three-letters plus a letter or number – hypA, CDC2, cdc28