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Parasitism
Photo of Cordyceps fungus growing out of the insect it parasitized from Wikimedia Commons
Exploitation (+/- or antagonistic interaction)
Parasites consume tissues or fluids of their host organisms;
typically infecting just 1 individual, generally without killing it
(at least not immediately)
Ectoparasite (external) vs. Endoparasite (internal)
Macroparasite vs. Microparasite
Free-living vs. Symbiont
Complex Life Cycle (generally involving multiple host species) vs. Simple Life Cycle
Horizontally Transmitted vs. Vertically Transmitted
Photo of human head louse from Wikimedia Commons
Exploitation (+/- or antagonistic interaction)
Pathogens
Parasites that cause disease
(which manifests as pain, dysfunction or death)
Photomicrograph of an Ebola virion (a complete virus particle) from Wikimedia Commons
Brood Parasites
Rely on other organisms to raise their young
Photo of Reed Warbler & Cuckoo chick from Wikimedia Commons
Kleptoparasites
Thieves (many are therefore competitors)
Photo from http://antediluviansalad.blogspot.com/2012_09_02_archive.html
Symbionts
Live in close association with their hosts;
amensal, commensal, parasitic, or mutualistic
Some mutualistic defensive symbionts protect their
hosts against parasitic symbionts
Photomicrograph of endophytes in a plant from http://www.entm.purdue.edu/turfgrass/research.html
Co-cladogenesis & co-speciation
Two associated lineages (especially symbionts)
diverge together, and potentially co-speciate
Dipodomys
merriami
Photos from Wikimedia Commons; figure from Light & Hafner (2008) Systematic Biology
Fahrenholzia
pinnata
Horizontal vs. Vertical Transmission
Horizontal – between individuals that are not
linked by a parent-child relationship
Vertical – from mother to offspring
Mother
Horizontal
Other non-offspring
member of
the population
Vertical
Daughter
Son
or
Horizontal
Other non-parent
member of
the population
Cospeciation
Host switch
Duplication
Host
Parasite
Failure to speciate
From J. Weckstein (2003)
Missing the boat
Extinction
Coexistence
Which are most likely under strictly vertical transmission?
*When is it Coevolution?
Reciprocal adaptive evolution in each of 2 interacting species
in response to adaptations in the other species
Tribolium castaneum is infected by microsporidian Nosema whitei;
parasite virulence and host recombination frequencies co-evolve
Photo of T. castaneum flour beetle from Wikimedia Commons; *original idea from Janzen (1980) Evolution
Macroparasite vs. Microparasite
Catagories based on function rather than taxonomy or phylogenetics
Macroparasites – parasites grow, but have no direct reproduction within
the host (they produce infective stages that must colonize new hosts);
typically much larger and have longer generation times than
microparasites; immune response in host is typically absent or very shortlived; infections are often chronic as hosts are continually reinfected;
e.g., helminthes, arthropods, etc.
Microparasites – parasites that reproduce within the host, often within the
host’s cells, and are generally small and have short lifespans relative to
their hosts; hosts that recover often have an immune period after infection
(sometimes for life); infections are often transient; e.g., bacterial, viral,
fungal infectious agents, as well as many protozoans, etc.
Parasites can influence individuals, populations,
interactions between species, communities &
ecosystems
Tribolium castaneum outcompetes T. confusum when both are healthy;
T. confusum outcompetes T. castaneum in the presence of
protist parasite Adelina tribolii (which is especially virulent towards T. castaneum)
Cain, Bowman & Hacker (2014), Fig. 14.17; after Park (1948)
Parasites can influence individuals, populations,
interactions between species, communities &
ecosystems
Red Grouse are naturally
infected by a nematode
parasite; antihelminthics cure
the infected hosts
“Proportion of grouse treated…
No treatment, dashed line; 5%, dotted line;
10%, thick solid Line; and 20%, thin solid line.”
Photo of from http://www.pbase.com/wildbirdimages/image/129932709; fig. from Hudson et al. (1998) Science
Modeling Microparasite Disease Dynamics
The SIR Model – a compartmental model
(population is sub-divided into compartments) for epidemiology
Birth
a
Susceptible
hosts (S)
a
β
transmission
Infected
hosts (I)
α+b
b
Death

After Anderson & May (1979ab) & May (1983)
a
v
recovery
Recovered &
Immune
hosts (R)
b
Modeling Microparasite Disease Dynamics
Coupled differential equations; one for each host compartment:
dS/dt = fxn; dI/dt = fxn; dR/dt = fxn
S, I & R are numbers of individuals; the other variables are rates
Birth
a
Susceptible
hosts (S)
a
β
transmission
Infected
hosts (I)
α+b
b
Death

After Anderson & May (1979ab) & May (1983)
a
v
recovery
Recovered &
Immune
hosts (R)
b
Modeling Microparasite Disease Dynamics
dI/dt = βSI – (α + b + v)I
dI/dt = βSI – mI
βSI =
Disease transmission rate
m=
combined death & recovery rate
Birth
a
Susceptible
hosts (S)
a
β
transmission
Infected
hosts (I)
α+b
b
Death

After Anderson & May (1979ab) & May (1983)
a
v
recovery
Recovered &
Immune
hosts (R)
b
Modeling Microparasite Disease Dynamics
If dI/dt > 0, disease will establish & spread
βSI – mI > 0
ST > m / β
We refer to this as the threshold density for disease progression
What are the public policy or management implications?
Transmission & Virulence
Transmission – passing a parasite or pathogen
from an infected host to another individual
Virulence – the host’s parasite-induced loss of fitness
Host Fitness
(Inversely related to
Pathogen Virulence)
Production & spread of disease organisms from a host
(Transmission)
Darwinian (Evolutionary) Agriculture &
Darwinian (Evolutionary) Medicine
The application of modern evolutionary theory to
understand crop & livestock production (Darwinian Agriculture)
and human health & disease (Darwinian Medicine)
E.g., Why do new diseases continue to appear
in human, crop & livestock populations?
Mind-Controlling Parasites
“Cocoon web” of spider
parasitized by specialist wasp
Normal web
Cain, Bowman & Hacker (2014), Fig. 14.22,
from Eberhard (2000) Nature & (2001) Journal
of Arachnology; photo of Eberhard from
http://www.stri.si.edu/english/about_stri/
headline_news/news/article.php?id=1015
Bill
Eberhard