Two factors, AB mostly basids

1
Two factors, A/B (mostly basids)

• Tetrapolar mating system ? meiosis gives four
  types of segregants
• Need different alleles at each mating locus
• A1B1 A2B2 ?A1B1, A1B2, A2B1, A2B2
• Schizophyllum has 28,000 mating type
  combinations

2
Basidiomycete mating

• Mating ? dikaryon formation ? fruiting
• Mating uses pheromones receptors that signal
  through a kinase cascade

fruit body
for heterobasidiomycetes
3
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 (precise b-glucanase activity) and nuclear
  migration

4
Basidiomycete mating, part 1
Precise branch positioning and determinate
growth
Septum modification and nuclear
migration Nuclear proliferation and hyphal
invasion
5
Clamp connections in basidiomycete dikaryons
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A and B functions are distinct

• in heterobasids (....?)
• A controls pathogenicity
• B controls filamentous growth

karyogamy, meiosis, sporulation
7
Using genetics to explore biologymutants 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

8
Gene naming systems

• Genes generally, three-letters plus a letter or
  number cdc2, CDC28, nimX (species specific)
• Alleles generally a numeral, e.g. nimX3
• Allele descriptors nimX3 Y305H
• Other descriptors wildtype, D dominant, ts
  temperature sensitive, D deletion
• Gene product p34cdc2, NIMX
• Gene name product identifier nimXcdc2

9
Mating and progeny analysis in Aspergillus
nidulans
10
The genetics of spore colour in A. nidulans

• The ability to make pigment is controlled by the
  gene called white
• The first mutant had white spores, wA
• Wildtype green is WA (typically wildtype alleles
  are not written in the genotype)
• WA and wA are allelic

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Colour can be modified

• If a strain is WA, then it has pigmented spores
• Colour can be modified by other genes
• Green pigment formation takes two steps, only the
  first of which is controlled by white

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Mating white and green strains

• Cross WAwA (two alleles of the same gene locus)
• Pattern of colour inheritance?
• WAwA ? WA wA 11

13
Two steps for making green pigment

• The yellow gene is required for making green
  pigment
• Wildtype allele is YA mutant is yA
• If a strain has yellow spores, which allele is at
  the w/W locus?
• YAyA ? progeny genotype, phenotype, proportions?

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Mating a white and a yellow strain

• wAyA
• Implications regarding w/W?
• Since the yellow strain is yellow, it must be WA
  yA
• The white strain is wA but unknown for yA vs YA
• Epistasis

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How do we find out?

• Mate wA strain to yA
• If wA, YA WA, yA ?? white yellow
• wA, YA WA, yA wA, yA WA, YA
• white yellow white green
• If wA, yA WA, yA ?? white yellow
• wA, yA WA, yA wA, yA WA, yA
• white, yellow, white, yellow