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Symbiotic Relationships
Symbiosis are the intimate relationships that exist
between species.
Symbiotic relationships give at least one of the
participants a nutritional or survival advantage.
Parasitism
Commensalism
Mutualism
Parasitism
An interaction
between two
organisms where one
species benefits
(parasite) and the
other species (host)
is ‘harmed’ in some
way.
Parasites
Obligate
Facultative
Parasites dependant on
the host for survival
(viruses)
Parasites that do not
need a host to survive.
They can live with or
without them.
Outside or Inside ?
Ectoparasites
Endoparasites
Live on surface of the host
Ticks
Fleas
leeches
Live within their host
Tapeworm
Malaria (plasmodium)
Sleeping sickness
(trypanosome)
Parasite and Host relationship
 Balance between receiving nutrients and damage
done.
 Most parasites do not kill their host.
 Host could have some means of defence.
 Death of the host means loss for parasite.
 Relationship usually stable between them.
Transmission of parasite to new host
Direct Contact
Parasite moves between hosts e.g rhinovirus,
influenza, head lice.
Resistant stages
Offspring released by parasite are in dormant or
resistant form. When consumed by a host they
become active e.g. liver fluke (snail and sheep)
Secondary host species or vectors
Organisms that transmit parasites e.g. fleas and ticks.
The infectious stage develops inside them and is
passed on e.g. toxoplasmosis, malaria
Host – parasite specificity
 Parasites have often developed with host
 Parasites have often shared an evolutionary path.
 As hosts defence system affects parasites the
most well adapted survive.
Defence against parasites
Animals
Lymphocytes – antibodies
Macrophages
Preening/grooming
Migration
Plants
Chemicals
Localised death of cells to isolate
Galls
Plant parasites
 Viruses
tobacco mosaic virus
potato leaf roll virus
 Bacteria
fireblight in apples and pears
 Fungi
wheat rust
grey mould
Koch’s postulate for diseases
 Pathogen should be present in all diseased
organisms but not healthy ones
 Pathogen should be able to be isolated and grown
in pure culture
 A healthy animal inoculated with the pure culture
should get the same disease as original animals
 The pathogen should be able to be isolated from
the newly diseased organisms
Commensalism
An interaction between two organisms where
one species (commensal) benefits and the other
species (host) is unaffected.
Commensalism
 Are there really any pure commensal relationships?
Mutualism
An interaction between
two organisms where
both species benefit
from the relationship.
Mutualism
Organisms involved in mutualistic
relationships often benefit from increased
levels of metabolites, protection, removal of
parasitic organisms.
Organisms also show complementary
structures or behaviours.
Mutualism
Lichens - fungi and algae
Sea anemone and hermit crab
Mutualism
Plants often have symbiosis with fungi
especially in root systems.
Ectomycorrhizae – fungus on outside of cells
forming covering sheath of mycelium.
Endomycorrhizae – fungus found inside cortex
cells.