Lecture 5: Basidiomycota

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Lecture 5 Basidiomycota Sexual reproduction and
somatic incompatibility

• Anastomosis - fusion of branches of the same or
• different hyphae

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• 7) Mating event
• Fusion of monokaryons
• formation of dikaryon
• sexual compatibility
• recognition and fusion
• of monokaryons
• controlled by mating
• type genes (MAT)
• 3 mating systems
• homothallic
• heterothallic
• secondary homothallic

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• Homothallic, heterothallic, secondary
  homothallic
• homothallic
• dikaryon formation occurs in single,
  self-compatible thallus
• 25 of all species of basidiomycetes
• heterothallic
• self-sterile (self-incompatible) individuals
  
• requires the union of two, unique compatible
  thalli
• 75 of all species of basidiomycetes

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• Mating type genes (MAT genes)
• a MAT locus consists of few to several tightly
  linked genes
• Unifactorial (bipolar)
• - one locus or factor (A)
• - requires 2 unlike alleles for mating type
  compatibility
• - 25 of heterothallic species
• Bifactorial (tetrapolar)
• - two unlinked loci or factors on different
  chromosomes
• (A and B)
• - requires 4 unlike alleles for mating type
  compatibility
• - 75 of heterothallic species

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• Unifactorial (bipolar)
• one locus (unifactorial) controls mating
• two different alleles (bipolar) required for
• compatibility of nuclei
• Hymenomycetes can have from
• tens to hundreds of alleles/locus
• (A is locus, numbers, e.g., 1,2,3... are alleles)
• A1 X A2 (karyogamy) ----gt A1A2 (meiosis)
• ----gt A1 A2 (spores)

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• Bifactorial (tetrapolar)
• two loci (factors) control mating
• four different alleles (tetrapolar) - two at
  both factors -
• required for compatibility of nuclei
• numerous alleles/locus as in unifactorial
  systems
• A1B1 X A2B2 (karyogamy) ----gt A1B1A2B2 (meiosis
  
• crossingover) ----gt A1B1, A2B2, A1B2, A2B1

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Mating type systems are usually determined in
sibling spore mating. For example, 10 spores
from a single basidiocarp are germinated to
establish 10 monokaryons. These 10 monokaryons
are then paired against each other in all
possible combinations (10 X 10 100). Those
monokaryons that possess compatible mating type
alleles will fuse to form dikaryons. Those
monokaryons that do not possess compatible mating
type alleles will fail to form dikaryons. The
major limiting factor with this approach is that
the spores of many species of basidiomycetes do
not germinate in culture.
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Sibling spore mating
Bipolar/ Unifactorial 5050 (CI)
Tetrapolar/bifactorial 2575 (CI)
C compatible, I incompatible
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• Ultimate function of A B alleles discerned via
  partial
• compatibility crosses
• fusion between A1B1 X A1B2 (identical A alleles)
• no clamp connections formed
• fusion between A1B1 X A2B1 (identical B alleles)
  
• clamp connections formed but no migration of
  nuclei

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• Secondary homothallic
• fungi that produce binucleate basidiospores
• contain two haploid nuclei with compatible mating
  type alleles
• basidiospore germinates directly into a dikaryon
• no true monokaryotic stage to life cycle
• superficially resembles a homothallic species

A1
A2
A1
A1
A1
A2
A2
A2
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• Somatic Incompatibility
• fusion of dikaryons
• self vs. nonself recognition
• controlled by het genes
• multigenic and multiallelic
• function in the dikaryotic state

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• A.H.R. Buller - unit mycelium
• mycelia are cooperative genetic mosaics
• Rayner - individualistic mycelium
• mycelium is more than simple, additive
  assemblage of hyphae
• Self vs. nonself reactions - individual
  identities
• Mycelium capable of indeterminate growth
• One of the major challenges facing a mycelium is
  encountering
• other mycelia of the same or similar species

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Things a dikaryotic mycelium must do
migrate between discontinuous nutrient deposits
capture resources compete with
neighbors respond to changes in
microenvironment form symbioses
sporulate
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• Self-fusions
• fusion between genetically identical hyphae
• combination of hyphae into aggregates or somatic
  tissues
• e.g. rhizomorphs of Armillaria, extend at a rate
  faster
• than the mycelium
• specialized function (division of labor)
• absorptive components and sinks
• translocation of nutrients along tugor
  gradients controlled
• by nutrient supply and water potential

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• Mechanisms that prevent or restrict nonself
  anastomosis
• somatic incompatibility (SI) non-self rejection
• multiple mechanisms and outcomes
• SI distinguish between pre- and post- fusion
• rejection results in the death of fusion
  compartments and
• associated hyphae
• septal maintenance plugging of septa thus
  not allowing
• physiological access

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• SI in basids
• occur between dikaryons that are heterokaryotic
• genetic system is polygenic and multiallelic and
  may generate
• innumerable individual specificities analogous
  to human
• HLA histocompatibility system
• in culture results are demarcation zones or
  barrage lines between
• colonies compared against control self-pairings
  
• often observed in nature e.g., spalted wood,
  inhibition lines
• among wood decay fungi

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2 spores germinate to produce monokaryons Primary
mycelia fuse to produce dikaryon (mating)
Dikaryons encounters other dikaryons and behave
as an unique individual (somatic
incompatibility) Unique somatic compatible
units, i.e., unique secondary mycelia, called
genets or clones Two somatically incompatible
secondary mycelia may go on to produce
basidiospores and primary mycelia that are mating
compatible.
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How would you determine how many individuals of
the same species exist in a given area?