Littlefield and Heath 1979 Ultrastructure of Rust Fungi

glycogen

lipid

Storage compounds – retaining nutrients

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

PigmentsHormonesToxins

Co-regulated withsporulation

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

• A1B1 :: A2B2 A1B1, A1B2, A2B1, A2B2

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 (-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