Transcript Lecture #4

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
Definitions
• Alternatively fixed alleles
• Dominant vs. co-dominant markers
• Genotype
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
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!!)
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)
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 AA=Aa
but not aa. We know in reality AA and Aa are quite
different.
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
• Blonde
• Blue-eyed
• 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
In the case of microbes it will
probably be something like
• Genotype A= 01010101
• Genotype B= 00110101
• Genotype C= 00010101
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
Fungi… again!
• ASCOMYCETES
• BASIDIOMYCETES
• OOMYCETES (fungus-like, water molds)
ASCOMYCETES
• Yeasts (fermentation, human mycoses)
• Truffles, morels
• Penicillia (penicillin), Fusaria (potent
toxins, damping off of seedlings), molds
Ascus is the sack in which the
spores are contained
Asci can be placed on a disk
(apothecium), many apothecia
can be together in a fruitbody
Morel fruitbody
Asci can be carried inside a flask
(perithecium)
Nectria
Ploidy is mostly
n
BASIDIOMYCETES
• Mushrooms. mycorrhizal
• Wood decay organisms
• Rusts, Smuts
• Yeasts and damping off
Toadstools and huitacochle are
both basidiomycetes
Basidium means “club”, it carries
the basidiospores (dispersion
propagules) naked
Most of their life, they are
n+n (dikaryons), some rare
ones are diploid
Oomycetes
• Belong to the kingdom Stramenopila, used
to be called Chromista
• Phytophthora, Pythium, Saprolegnia
H20
Hyphae, sporangia, and zoospores of P. ramorum
Most of their lifecycle
they are 2n
Have cellulose in cell
wall
Not fungi!!, but look
like them because of
convergent evolution
Fungi do not photosynthesize
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Biotrophic: mycorrhyzae, rusts
Endophites: clavicipetaceae,
Necrotrophic; most pathogens
Saprobes: primary (involved in litter
decomposition)
DISEASE!!
• Symptoms vs. signs; e.g. chlorosis vs. fruitbody
• The disease triangle
Disease triangle
Effect of humans
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.
Effects of fire exclusion
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
1- Basic compatibility
• Size of infectious propagules
• Timing of susceptibility in host and
production of infectious structures
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
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)
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)
3- Infecting the host
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RA= no disease
Ra=disease
ra=disease
rA=disease
There will be a strong selection in favor of R
alleles but R comes at a cost
4- Causing disease
• Correlated to ability of pathogen to invade
plant cell, pathogenicity is usually a
dominant trait
Categories of wild plant diseases
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Seed decay
Seedling diseases
Foliage diseases
Systemic infections
Parasitic plants
Cankers, wilts , and diebacks
Root and butt rots
Floral diseases
Seed diseases
• Up to 88% mortality in tropical Uganda
• More significant when seed production is
episodic
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
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
Systemic infections
• Viral?
• Phytoplasmas
• Peronospora and smuts can lead to over
50% mortality
• Endophytism: usually considered beneficial
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
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
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
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
Removing food base causes
infection of roots of other trees
Hyphae in plant
tissue or soil (shortlived)
Melanin-covered rhizomorphs will
allow for fungus to move to new food
Sources (Armillaria mellea)
Effects of fire exclusion
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
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
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)
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
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