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Lecture 12

BIOB50H3 Lecture Notes - Lecture 12: Sarcoptes Scabiei, Angiostrongylus, Protist

Biological Sciences
Course Code
Marc Cadotte

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BIOB50Winter2012 Lecture 12: Parasitism (Chapter 13)
Case Study: Enslaver Parasites
o Some parasites can alter the behaviour of their host in order to complete their life cycles
o The hairworm may induce its host, the cricket to commit suicide by altering its perception of thirst NT taurine
in insects regulates the brain’s ability to sense a lack of water when this is changed, the perception of thirst is
altered to the point where the cricket jumps in water because of the sensation of thirst and drowns allowing the
hairworm to continue its life cycle
o Not all enslaver parasites appear to act by manipulating the host’s chemistry, but the mechanisms are still
o Many other parasites enslave their hosts
o Some fungi alter the perching behaviour of their fly hosts so that their spores can be dispersed more easily
o Figure 13.2 Enslaved by a Fungus flies infected by the fungus perch in an atypical (HIGH) position from which
fungal spores can easily spread to healthy flies which typically perch on the upper surfaces of the LOW lying
vegetation where they are exposed to fungal spores
o Rats infected with the protozoan parasite Toxoplasma gondii do not avoid cats, and in some cases are actually
attracted to cats
o This increases the chance that the parasite will be transmitted to the next host in its complex life cycle which is
the cat
Case Study Revisited: Tropical Parasitoid Wasp Example
o The parasitoid wasp Hymenoepimecis argyraphaga manipulates its host, the orb-weaving spider Plesiometa
argyra to spin a special cocoon web
o The larval stage of the wasp attaches to the exterior of a spider’s abdomen and sucks the spider’s body fluids
o When fully grown it induces the spider to make a special “cocoon web”
o Once the spider has built the web, the larva kills and eats the spider
o The larva then spins a cocoon and attaches it to the cocoon web
o As the larva completes its development in the cocoon, the cocoon web serves as a strong support that protects
the larva from being swept away from torrential rains
o A parasitized spider builds normal webs right up to the night when the wasp induces it to make a cocoon web
suggest was may inject chemical that alters spider behaviour
o Eberhard (2001) removed larvae from their hosts several hours before time when a cocoon web would be
usually made wasp removal resulted in the construction of a web which was similar to the cocoon web but
more often resulted in the construction of a web that differed substantially from both normal and cocoon webs
o In the days that followed the removal, some spiders partially recovered the ability to make normal webs
o Conclusion: parasite induces construction of a cocoon web by injecting a fast acting chemical into the spider
o The achemical appears to act in dose dependent manner otherwise all spider would build cocoon webs
instead of webs of an intermediate form
o The chemical appears to interrupt the spiders’ usual sequence of web building behaviours
o Symbionts are organisms that live in or on other organisms
o More than half of the millions of species that live on Earth are symbionts
o Our own bodies can be home to many other species

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o Some symbionts are mutualists but the majority are parasites
o A parasite consumes the tissues or body fluids of the organism on which it lives, called the host
o Pathogens are parasites that cause diseases
o Figure 13.3 The Human Body as a Habitat different parts of our body provide suitable habitat for a wide range
of symbionta, many of which are parasites some are pathogens (cause disease)
Parasite Natural History
o Parasites, which constitute roughly 50% of the species on Earth, typically feed on only one or few host species
o Macroparasites are large such as arthropods and worms
o Microparasites are microscopic such as bacteria
o Most parasites feed on only one or a few individual host organisms
o Defined broadly, parasites include herbivore such as aphids or nematodes, that feed on one or a few host plants
o Parasitoids whose larvae feed on a single host, almost always kill it
o Many species are host to more than one parasite species
o Ectoparasites live on the outer body surface of the host
o They include plant parasites such as dodder dodder obtains water and food from the host plant via
specialized roots called haustoria
o Mistletoes are hemiparasitic they get water and nutrients from the host but can also photosynthesize
o Many fungal and animal parasites are ectoparasites
o More than 5000 species of fungi attack important crop and horticultural plants
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