SYMBIOSIS
Definition of Symbiosis
• Symbiosis is a set of species interactions involving
close physical association of participants, and
frequently, evidence of co-evolution.
• Symbiotic interactions maybe exploitative or
cooperative. Cooperative interactions are thought to
have evolved from exploitative interactions when
benefits outweighed costs for both partners.
• There are 3 types of symbiotic interaction:
parasitism, commensalism, and mutualism.
Significance of symbiosis
• Of the three types of symbiotic interactions, two are
cooperative, rather than exploitative.
• Many interactions are cryptic and may not produce
immediate survival or reproduction, so that how they
function in population regulation is poorly understood.
• Some historians argue that the “Neo-Malthusian
Perspective” has blinded us to the importance of
symbiosis.
• Examples like hermatypic corals show that entire
global ecosystems depend on symbiosis for their
existence.
Facultative Versus Obligative Symbiosis
• FACULTATIVE symbiosis occurs
between otherwise free-living
partners. Their survival does not
depend on the interactions, but
fitness changes as a result.
• May exist with little co-evolution
• No specialized structures
assuring the interaction may be
present.
• In OBLIGATIVE symbiosis, one
or both partners must engage in
interaction to survive or
reproduce.
• Elaborate co-evolution often
apparent.
• Specialized structures or
behaviors exist and require
energy to develop or maintain.
Definitions of Types
• Parasitism: A parasite lives in close physical association with a host
from which it receives nourishment, lowering the fitness of the host.
Exploitation is usually sub lethal. (+/-)
• Mutualism. Both “interactors” increase their fitness as a result of the
exchange. Mutualism thought to have evolved from parasitism or
commensalism. (+/+)
• Commensalism: Commensal and host live in close association, with
the former receiving nourishment or shelter, without reducing the
fitness of the host. Many commensalisms may be cryptic parasitisms
or mutualisms. (+/0)
Parasitic castration: Sacculina sp.
• A highly evolved parasitc barnacle,
Sacculina grows within decapod
hosts as a branching system of
tubules that extract nutrients,
diverting most of the hosts energy
to its own bulbous reproductive
structure on the outside of the
host.
• Loss of nutrients from host and
hormonal releases from the
parasite prevent crabs from
developing functional gonads.
• Crabs become infected when the
Sacculina cyprid settles at the base
of a bristle and injects a mass of
undifferentiated cells into the host
blood stream. The cell mass
develops into the adult parasite.
Commensalisms: Pearl Fish and the
sea cucumber
Pearl fish seek shelter
in the rectum
of their sea
cucumber hosts.
Facultative or
Obligative?
Micorrhizea: a mutualistic association
between plant and root fungus
Mutualism Confirmed
• Isotopes injected in the vascular system (phloem) of
host trees have been traced first to the roots, then to
mycorrhizae infecting the roots. The hosts, thus, give
nourishment to the fungi.
• The following lab experiment is part of the growing
evidence that the mycorrhizae confer benefit to their
host plants.
An Experiment to test
(1) whether mycorrhizea
help plants acquire nutrients,
and if so, (2) whether the
fertility of soil determines
whether the helpful
mycorrhizea are present.
Inocula of fungi (treatments)
and no inoculum (control)
were initiated in nutrient poor
soil
As a second treatment factor,
replicates of inocula received
varying amounts of nutrient.
(Nutrient additions not shown in
figure. See text for details)
Results: When nutrients available, mycorrhizea
help plants acquire them, grow larger.
Results: Increased nutrient uptake by
mycorrhizea increases host plant reproduction.
Environmental Context
• Mycorrhizea are widespread in terrestrial
plant communities.
• The most common rainforest trees often
share the same species of mycorrhizea,
suggesting that the association is essential to
their success.
Coral-Algae Mutualism: a crucial
interaction for reef-building corals.
Close up of colony showing external
anatomy of polyps
Anatomy of hermatypic corals.
Polyp
Algal inclusions
In the gastrodermis
Theca
The algal symbiont
• ALgal symbionts are goldenbrown algae, zooxanthelea.
• Nearly all symbionts of the
large phylum Cnidaria
belong to the same genus.
Different strains occur in
different species or clones.
The symbionts are
transmitted by adult
cnidarians to their
embryonic young. Shown is
a free living symbiont from
culture.
Electron micrograph of Symbiodinium
in host tissues, highly transformed.
Experiment demonstrating that:
Symbionts promote rapid skeletal growth of
hermatypic corals. Goreau et al.
Experiments demonstrating that:
Photosynthetic products translocated.
Growing tips covered with opaque or clear caps, or uncovered.
Growth rates remain same for all three.
Autoradiography showed
phosphate and
Carbon isotopes incorporated
into protein and carbohydrates
at growing tip. Interpretation?
Separate HPLC analyses showed
algae leak glucose glycerol
and amino acids.
Experiments demonstrating that:
Symbionts “recognize” hosts.
Summary Coral/Algae Mutualism
Additional info
• About 1/3 protein of host orginates from algal amino
acids.
• Uptake of CO2 increases PH, allowing faster CaCO3
deposition.
• Symbionts release about 50% of what they produce
(by weight).
• Hosts extracts are interchangable, just as symbiont is
not specific to host.
A ‘soft coral’:
no CaCO3 skeleton, no symbionts.
This type of coral does not build reefs.
Conclusion
• Mutualistic interactions are ancient and widespread.
• Matrix forming species (e.g. trees, corals), which are
the foundation of important communities, depend for
their success on obligative mutualisms.
• Therefore, the “struggle for existence” involves both
cooperative and exploitative interactions in complex
networks that we are only beginning to understand.