Transcript Fungi are organisms with a common lifestyle

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