Symbiosis

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Symbiosis
Learning outcomes To know the following
interspecies relationships and examples of
each 1. parasitism 2. mutualism.
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Detailed learning outcomes

• Symbiotic relationships are co-evolved and
  specific intimate relationships between members
  of two different species.
• 1. Parasitism
• A parasite benefits in terms of energy or
  nutrients, whereas a host is harmed by their
  loss. Parasites often have a limited metabolism
  so cannot survive out of contact with a host.
• 2. Mutualism
• Both mutualistic partner species benefit from
  the relationship. One example is that of
  cellulose-digesting protozoa/bacteria living in
  the guts of many herbivores.

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Symbiotic relationships
Symbiosis
Mutualism
Parasitism
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Symbiosis
Symbiosis is the close relationship between two
different species, eg the relationship between
nitrogen-fixing soil bacteria (Rhizobia) and
leguminous plant roots (peas, beans and clover).

• There are two types of symbiosis
• parasitism
• mutualism.

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Parasitism

• A parasite lives on another organism (the host)
  from which it obtains food. This
• benefits the parasite
• harms (or kills) the host.

Harmed
Benefits
Some parasites (ectoparasites) live on the outer
skin of the host
leech
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Parasitism
Some parasites (endoparasites) live inside the
host (usually in vertebrates)
tapeworm
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Living with parasites

• Only poor parasites kill their host as this is
  not to their advantage, especially for obligate
  parasites.
• There is a balance between parasitic damage and
  host defence, so the relationship is relatively
  stable.
• Diseases often occur in cycles as parasite and
  host co-evolve.

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Secondary host species or vectors

• Vectors are organisms that transmit parasites,
  usually insects and ticks. These vectors are
  essential for the development of the infectious
  stage of the lifecycle of some parasites before
  they are transmitted to the next host (eg
  mosquitoes transmit the malarial parasite).
• Some of these vectors therefore have a double
  aspect of parasitism.

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Hostparasite co-evolution

• As a host evolves defence mechanisms against the
  parasite, so the parasite evolves mechanisms to
  overcome the defence.
• This has led to many hostparasite interactions
  being species specific, in other words most
  parasites only infect a particular host.

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Types of parasite

• Parasites may be animals or plants.
• Viruses are parasites.
• Unicellular organisms, such as some bacteria, are
  parasites.
• Multicellular organisms, such as worms and
  insects, may be parasites.

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Mutualism
Mutualism is a specialist relationship between
two organisms that is beneficial to both.
Benefits
Benefits
Lichens are composed of fungus and algae in a
beneficial relationship. http//www.snh.org.uk/ima
ges/publications/lichens03.jpg
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Leguminous plants and Rhizobium bacteria

• These bacteria live freely in soil.
• They are able to infect the roots of leguminous
  plants and form nodules.
• Inside the nodules bacteria obtain energy from
  their host and supply nitrogen compounds in
  return.
• Picture www.apsnet.org

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Fungi and algae (lichens)

• Lichens are able to colonise areas where other
  plants are unable to grow.
• The fungal part of the lichen produces tough
  cellulose walls and secretes mineral-digesting
  chemicals while receiving carbohydrate.
• The algal part of the lichen photosynthesises and
  receives protection and essential minerals.

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Herbivores and gut bacteria

• Herbivores cannot produce cellulose-digesting
  enzymes.
• Bacteria which can do this are found in the
  alimentary tract of herbivores.
• These bacteria receive food and shelter, and aid
  plant digestion.

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Ants and aphids

• Some species of ants farm aphids. They move
  them from plant to plant and protect them from
  predators.
• In return the aphids secrete a sugary liquid that
  the ants drink.

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Terrestrial plants and pollinating animals

• Many plants have evolved special flower shapes
  which allow only one kind of animal near them.
• The animal depends on the nectar it receives as a
  reward for its visit.
• The plant depends on the animal to transport its
  pollen between flowers.

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Summary
Parasitism Mutualism




Species A
-

Species B
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Acknowledgments

• Many thanks to R. Purdie from Sanquhar Academy
  for the use of some materials.