Summary of previous lesson

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Summary of previous lesson

• Janzen-Connol hypothesis explanation of why
  diseases lead to spatial heterogeneity
• Diseases also lead to heterogeneity or changes
  through time
• Driving succession
• The Red Queen Hypothesis selection pressure will
  increase number of resistant plant genotypes
• Co-evolution pathogen increase virulence in
  short term, but in long term balance between host
  and pathogen
• Density dependance

2
Disease and competition

• Competition normally is conducive to increased
  rates of disease limited resources weaken hosts,
  contagion is easier
• Pathogens can actually cryptically drive
  competition, by disproportionally affecting one
  species and favoring another

3
Janzen-Connol

• Regeneration near parents more at risk of
  becoming infected by disease because of proximity
  to mother (Botryosphaeria, Phytophthora spp.).
  Maintains spatial heterogeneity in tropical
  forests
• Effects are difficult to measure if there is
  little host diversity, not enough
  host-specificity on the pathogen side, and if
  periodic disturbances play an important role in
  the life of the ecosystem

4
Diseases and succession

• Soil feedbacks normally its negative. Plants
  growing in their own soil repeatedly have higher
  mortality rate. This is the main reason for
  agricultural rotations and in natural systems
  ensures a trajectory towards maintaining
  diversity
• Phellinus weirii takes out Douglas fir and
  hemlock leaving room for alder

5
The red queen hypothesis

• Coevolutionary arm race
• Dependent on
• Generation time has a direct effect on rates of
  evolutionary change
• Genetic variability available
• Rates of outcrossing (Hardy-weinberg equilibrium)
• Metapopulation structure

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Diseases as strong forces in plant evolution

• Selection pressure
• Co-evolutionary processes
• Conceptual processes potentially leading to a
  balance between different ecosystem components
• How to measure it parallel evolution of host and
  pathogen

7

• Rapid generation time of pathogens. Reticulated
  evolution very likely. Pathogens will be selected
  for INCREASED virulence
• In the short/medium term with long lived trees a
  pathogen is likely to increase its virulence
• In long term, selection pressure should result in
  widespread resistance among the host

8
More details on

• How to differentiate linear from reticulate
  evolution comparative studies on topology of
  phylogenetic trees will show potential for
  horizontal transfers. Phylogenetic analysis
  neeeded to confirm horizontal transmission

9
Phylogenetic relationships within the
Heterobasidion complex
Fir-Spruce
Pine Europe
Pine N.Am.
10
Geneaology of S DNA insertion into P ISG
confirms horizontal transfer.Time of
cross-over uncertain
NA S
NA P
EU S
890 bp CIgt0.9
EU F
11
Complexity of forest diseases

• At the individual tree level 3 dimensional
• At the landscape level host diversity,
  microclimates, etc.
• At the temporal level

12
Complexity of forest diseases

• Primary vs. secondary
• Introduced vs. native
• Air-dispersed vs. splash-dispersed, vs. animal
  vectored
• Root disease vs. stem. vs. wilt, foliar
• Systemic or localized

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Stem canker on coast live oak
14
Progression of cankers
Hypoxylon, a secondary sapwood decayer will
appear
Older canker with dry seep
15
Root disease center in true fir caused by H.
annosum
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HOST-SPECIFICITY

• Biological species
• Reproductively isolated
• Measurable differential size of structures
• Gene-for-gene defense model
• Sympatric speciation Heterobasidion, Armillaria,
  Sphaeropsis, Phellinus, Fusarium forma speciales

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Phylogenetic relationships within the
Heterobasidion complex
Fir-Spruce
Pine Europe
Pine N.Am.
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Recognition of self vs. non self

• Intersterility genes maintain species gene pool.
  Homogenic system
• Mating genes recognition of other to allow for
  recombination. Heterogenic system
• Somatic compatibility protection of the
  individual.

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Recognition of self vs. non self

• What are the chances two different individuals
  will have the same set of VC alleles?
• Probability calculation (multiply frequency of
  each allele)
• More powerful the larger the number of loci
• and the larger the number of alleles per locus

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INTERSTERILITY

• If a species has arisen, it must have some
  adaptive advantages that should not be watered
  down by mixing with other species
• Will allow mating to happen only if individuals
  recognized as belonging to the same species
• Plus alleles at one of 5 loci (S P V1 V2 V3)

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MATING

• Two haploids need to fuse to form nn
• Sex needs to increase diversity need different
  alleles for mating to occur
• Selection for equal representation of many
  different mating alleles