Summary of previous lesson

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Summary of previous lesson

• ASCOMYCETES, BASIDIOMYCETES, OOMYCETES
• DISEASE TRIANGLE humans
• Locus/ allele/ polymorphisms
• Invasive organism
• Genetic traits of invasive populations reduced
  genetic diversity and differentiation among new
  populations because of founder effect and lack of
  equilibrium

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Definitions

• Alternatively fixed alleles
• Dominant vs. co-dominant markers
• Genotype

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Alternatively fixed alleles

• Two flower species (species 1 and species 2) can
  have one of two features
• Long (L) or short (s) leaves
• Red ( R) or white (w) flowers
• Ten individuals from species 1 have the following
  traits
• LR LR LR LR LR LR LR sR sR sR
• Ten individuals from species 2 have the following
  traits
• sw sw sw sw sw sw sw Lw Lw Lw

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Which one is the alternatively fixed allele?

• Both alleles will differentiate the groups
  (frequencies are significantly different)
• Only one will be diagnostic because alternatively
  fixed
• It is the color of the flower all flowers in
  species 1 are R, all flowers in species 2 are w
  (all implies your sampling size is adequate!!)

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Dominant vs. co-dominant markers

• Flowers are red or white or yellow, DNA sequence
  is agg, agt, agc DNA fragment is 10, 12 0r 14 bp
  long (CO-DOMINANT, we know what alternative
  alleles are)
• Flowers are red or non-red, DNA is agg or not,
  size is 10bp or not. We only see the dominant
  allele and we express it in binary code
  1(present), 0(absent)

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Limitations of co-dominant markers

• Not all non-red flowers are the same, but we
  assume they are (non red flowers can be orange or
  yellow)
• If at one locus we have a dominant A allele and a
  recessive a allele, using a codominant marker we
  would say AAAa but not aa. We know in reality
  AA and Aa are quite different.

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Genotype

• A unique individual as defined by an array of
  genetic markers. (the more markers you have the
  less mistaken identity you will have.
• blonde

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• Blonde
• Blue-eyed

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• Blonde
• Blue-eyed
• Hairy

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• Blonde
• Blue-eyed
• Hairy
• 6 feet tall

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• Blonde
• Blue-eyed
• Hairy
• 6 feet tall
• Missing two molars

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In the case of microbes it will probably be
something like

• Genotype A 01010101
• Genotype B 00110101
• Genotype C 00010101

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Summary of third lesson

• DNA polymorphisms can be diagnostic
• Mutations/Sex/Barriers to mating
• Plant Diseases can be biotic (interaction between
  host and causal agent ), or abiotic
• Many organisms can cause plant diseases, but
  fungi are the No.1 cause
• Diversity of fungi, but all have ideal structure
  for plant infection
• hypha/cord/rhizomorph/infection peg/appressorium
• Sexual vs. asexual reproduction can do both

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Fungi again!

• ASCOMYCETES
• BASIDIOMYCETES
• OOMYCETES (fungus-like, water molds)

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ASCOMYCETES

• Yeasts (fermentation, human mycoses)
• Truffles, morels
• Penicillia (penicillin), Fusaria (potent toxins,
  damping off of seedlings), molds

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Ascus is the sack in which the spores are
contained
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Asci can be placed on a disk (apothecium), many
apothecia can be together in a fruitbody
Morel fruitbody
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Asci can be carried inside a flask (perithecium)
Nectria
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Ploidy is mostly n
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BASIDIOMYCETES

• Mushrooms. mycorrhizal
• Wood decay organisms
• Rusts, Smuts
• Yeasts and damping off

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Toadstools and huitacochle are both basidiomycetes
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Basidium means club, it carries the
basidiospores (dispersion propagules) naked
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Most of their life, they are nn (dikaryons),
some rare ones are diploid
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Oomycetes

• Belong to the kingdom Stramenopila, used to be
  called Chromista
• Phytophthora, Pythium, Saprolegnia

H20
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Hyphae, sporangia, and zoospores of P. ramorum
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Most of their lifecycle they are 2n Have
cellulose in cell wall Not fungi!!, but
look like them because of convergent evolution
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Fungi do not photosynthesize

• Biotrophic mycorrhyzae, rusts
• Endophites clavicipetaceae,
• Necrotrophic most pathogens
• Saprobes primary (involved in litter
  decomposition)

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DISEASE!!

• Symptoms vs. signs e.g. chlorosis vs. fruit-body
• The disease triangle

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Disease triangle

• Effect of humans

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Human activities affecting disease incidence in
forests

• Introduction of exotic pathogens
• Planting trees in inappropriate sites
• Changing stand density, age structure,
  composition, fire frequency
• Wound creation
• Pollution, etc.

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Effects of fire exclusion
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DISEASE plant microbe interaction

• 1-Basic compatibility need to be present
• 2- Chemotaxis, thighmotropy
• 3- Avirulence in pathogen matched by resistance
  in host according to the gene for gene model
• 4-Pathogenicity factors such as toxins and
  enzymes important in the infection process

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1- Basic compatibility

• Size of infectious propagules
• Timing of susceptibility in host and production
  of infectious structures

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2- Finding the host

• Chemotaxis pathogen has receptor that detects
  food base in oomycetes zoospores will all swim
  towards host
• Thigmotropy recognizing morphological structures
  that indicate presence of host prelude to
  production of infective structures such as
  infection pegs and appressoria

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3- Infecting the host

• Pathogen will produce array of enzymes to infect
  host cells
• Upon identification of infection, host will
  produce array of antimicrobial compounds , or
  will kill some of its cells to halt infection
  process (hypersensitive response)

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3- Infecting the host

• Plant that are resistant, must be able to react
  (dominant R resistant allele)
• Plants that cannot react (r allele) are always
  sensitive
• Pathogens that are not noticed by plant can
  infect (recessive avirulence allele)
• Pathogens that are noticed may be stopped
  (dominant A avurulence allele)

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3- Infecting the host

• RA no disease
• Radisease
• radisease
• rAdisease
• There will be a strong selection in favor of R
  alleles but R comes at a cost

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4- Causing disease

• Correlated to ability of pathogen to invade plant
  cell, pathogenicity is usually a dominant trait

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Categories of wild plant diseases

• Seed decay
• Seedling diseases
• Foliage diseases
• Systemic infections
• Parasitic plants
• Cankers, wilts , and diebacks
• Root and butt rots
• Floral diseases

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Seed diseases

• Up to 88 mortality in tropical Uganda
• More significant when seed production is episodic

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Stress cone crop
BS on DF
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Seedling diseases

• Specific diseases, but also diseases of adult
  trees can affect seedlings
• Pythium, Phytophthora, Rhizoctonia, Fusarium are
  the three most important ones
• Pre- vs. post-emergence
• Impact up to 65 mortality in black cherry.
  These diseases build up in litter
• Shady and moist environment is very conducive to
  these diseases

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Foliar diseases

• In general they reduce photosynthetic ability by
  reducing leaf area. At times this reduction is
  actually beneficial
• Problem is accentuated in the case of small
  plants and in the case other health issues are
  superimposed
• Often, e.g. with anthracnose,needle cast and
  rust diseases leaves are point of entry for twig
  and branch infection with permanent damage
  inflicted

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Systemic infections

• Viral?
• Phytoplasmas
• Peronospora and smuts can lead to over 50
  mortality
• Endophytism usually considered beneficial

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Grass endophytes

• Clavicipetaceae and grasses, e.g. tall fescue
• Mutualism antiherbivory, protection from
  drought, increased productivity
• Classic example of coevolutionary development
  Epichloe infects flowers of sexually
  reproducing fescue, Neotyphodium is vertically
  transmitted in species whose sexual reproductive
  ability has been aborted

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Parasitic plants

• True (Phoradendron) and dwarf mistletoe
  (Arceuthobium)
• Effects
• Up to 65 reduction in growth (Douglas-fir)
• 3-4 fold mortality rate increase
• Reduced seed and cone production
• Problem accentuated in multistoried uneven aged
  forests

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Cankers, wilts, and die-backs

• Includes extremely aggressive, often easy to
  import tree diseases pine pitch canker, Dutch
  elm disease, Chestnut blight, White pine blister
  rust
• Lethal in most cases, generally narrow host range
  with the exception of Sudden Oak Death

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Root diseases

• Extremely common, probably represent the most
  economically damaging type of diseases
• Effects tree mortality (direct and indirect),
  cull, effect on forest structure, effect on
  composition, stand density, growth rate
• Heterobasidion, Armillaria, Phellinus weirii,
  Phytophthora cinnamomi

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Removing food base causes infection of roots of
other trees
Hyphae in plant tissue or soil (short-lived)
Melanin-covered rhizomorphs will allow for fungus
to move to new food Sources (Armillaria mellea)
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Effects of fire exclusion
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Floral diseases

• Pollinator vectored smut on silene offers an
  example of well known dynamic interaction in
  which pathogen drives genetic variability of
  hosts and is affected by environmental condition
• Puccinia monoica produces pseudoflowers that
  mimic real flowers. Effects reduction in seed
  production, reduction in pollinators visits

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Density-dependence

• Most diseases show positive density dependence
• Negative dependence likely to be linked to
  limited inoculum e.g. vectors limited
• If pathogen is host-specific overall density may
  not be best parameter, but density of susceptible
  host/race
• In some cases opposite may be true especially if
  alternate hosts are taken into account

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Counterweights to numerical effects

• Compensatory response of survival can exceed
  negative effect of pathogen
• carry over effects?
• NEGATIVE progeny of infected individuals less
  fit
• POSITIVE progeny more resistant (shown with
  herbivory)

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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

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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