Symbiosis (from Greek συμβίωσις "living
together", from σύν "together" and βίωσις
"living") is any type of a close and long-term
biological interaction between two different
biological organisms, be it mutualistic, commensalistic,
or parasitic.
The organisms, each termed a symbiont, may
be of the same or of different species.
In 1879, Heinrich Anton de Bary defined it
as "the living together of unlike organisms".
The term was subject to a century-long debate
about whether it should specifically denote
mutualism, as in lichens; biologists have
now abandoned that restriction.
Symbiosis can be obligatory, which means that
one or both of the symbionts entirely depend
on each other for survival, or facultative
(optional) when they can generally live independently.
Symbiosis is also classified by physical attachment;
symbiosis in which the organisms have bodily
union is called conjunctive symbiosis, and
symbiosis in which they are not in union is
called disjunctive symbiosis.
When one organism lives on the surface of
another, such as head lice on humans, it is
called ectosymbiosis; when one partner lives
inside the tissues of another, such as Symbiodinium
within coral, it is termed endosymbiosis.
== Definition ==
The definition of symbiosis was a matter of
debate for 130 years.
In 1877, Albert Bernhard Frank used the term
symbiosis to describe the mutualistic relationship
in lichens.
In 1879, the German mycologist Heinrich Anton
de Bary defined it as "the living together
of unlike organisms".
The definition has varied among scientists,
with some advocating that it should only refer
to persistent mutualisms, while others thought
it should apply to all persistent biological
interactions, in other words mutualisms, commensalism,
or parasitism, but excluding brief interactions
such as predation.
Current biology and ecology textbooks use
the latter "de Bary" definition, or an even
broader one where symbiosis means all interspecific
interactions; the restrictive definition where
symbiosis means only mutualism is no longer
used.In 1949, Edward Haskell proposed an integrative
approach, proposing a classification of "co-actions",
later adopted by biologists as "interactions".Biological
interactions can involve individuals of the
same species (intraspecific interactions)
or individuals of different species (interspecific
interactions).
These can be further classified by either
the mechanism of the interaction or the strength,
duration and direction of their effects.
=== Obligate versus facultative ===
Relationships can be obligate, meaning that
one or both of the symbionts entirely depend
on each other for survival.
For example, in lichens, which consist of
fungal and photosynthetic symbionts, the fungal
partners cannot live on their own.
The algal or cyanobacterial symbionts in lichens,
such as Trentepohlia, can generally live independently,
and their symbiosis is, therefore, facultative
(optional).
== Physical interaction ==
Endosymbiosis is any symbiotic relationship
in which one symbiont lives within the tissues
of the other, either within the cells or extracellularly.
Examples include diverse microbiomes, rhizobia,
nitrogen-fixing bacteria that live in root
nodules on legume roots; actinomycete, nitrogen-fixing
bacteria such as Frankia, which live in alder
root nodules; single-celled algae inside reef-building
corals; and bacterial endosymbionts that provide
essential nutrients to about 10%–15% of
insects.Ectosymbiosis is any symbiotic relationship
in which the symbiont lives on the body surface
of the host, including the inner surface of
the digestive tract or the ducts of exocrine
glands.
Examples of this include ectoparasites such
as lice; commensal ectosymbionts such as the
barnacles, which attach themselves to the
jaw of baleen whales; and mutualist ectosymbionts
such as cleaner fish.
=== Competition ===
Competition can be defined as an interaction
between organisms or species, in which the
fitness of one is lowered by the presence
of another.
Limited supply of at least one resource (such
as food, water, and territory) used by both
usually facilitates this type of interaction,
although the competition may also exist over
other 'amenities', such as females for reproduction
(in case of male organisms of the same species).
== Mutualism ==
Mutualism or interspecies reciprocal altruism
is a long-term relationship between individuals
of different species where both individuals
benefit.
Mutualistic relationships may be either obligate
for both species, obligate for one but facultative
for the other, or facultative for both.
A large percentage of herbivores have mutualistic
gut flora to help them digest plant matter,
which is more difficult to digest than animal
prey.
This gut flora is made up of cellulose-digesting
protozoans or bacteria living in the herbivores'
intestines.
Coral reefs are the result of mutualisms between
coral organisms and various types of algae
which live inside them.
Most land plants and land ecosystems rely
on mutualisms between the plants, which fix
carbon from the air, and mycorrhyzal fungi,
which help in extracting water and minerals
from the ground.An example of mutualism is
the relationship between the ocellaris clownfish
that dwell among the tentacles of Ritteri
sea anemones.
The territorial fish protects the anemone
from anemone-eating fish, and in turn the
stinging tentacles of the anemone protect
the clownfish from its predators.
A special mucus on the clownfish protects
it from the stinging tentacles.A further example
is the goby, a fish which sometimes lives
together with a shrimp.
The shrimp digs and cleans up a burrow in
the sand in which both the shrimp and the
goby fish live.
The shrimp is almost blind, leaving it vulnerable
to predators when outside its burrow.
In case of danger, the goby touches the shrimp
with its tail to warn it.
When that happens both the shrimp and goby
quickly retreat into the burrow.
Different species of gobies (Elacatinus spp.)
also clean up ectoparasites in other fish,
possibly another kind of mutualism.A non-obligate
symbiosis is seen in encrusting bryozoans
and hermit crabs.
The bryozoan colony (Acanthodesia commensale)
develops a cirumrotatory growth and offers
the crab (Pseudopagurus granulimanus) a helicospiral-tubular
extension of its living chamber that initially
was situated within a gastropod shell.Many
types of tropical and sub-tropical ants have
evolved very complex relationships with certain
tree species.
=== Endosymbiosis ===
In endosymbiosis, the host cell lacks some
of the nutrients which the endosymbiont provides.
As a result, the host favors endosymbiont's
growth processes within itself by producing
some specialized cells.
These cells affect the genetic composition
of the host in order to regulate the increasing
population of the endosymbionts and ensure
that these genetic changes are passed onto
the offspring via vertical transmission (heredity).A
spectacular example of obligate mutualism
is the relationship between the siboglinid
tube worms and symbiotic bacteria that live
at hydrothermal vents and cold seeps.
The worm has no digestive tract and is wholly
reliant on its internal symbionts for nutrition.
The bacteria oxidize either hydrogen sulfide
or methane, which the host supplies to them.
These worms were discovered in the late 1980s
at the hydrothermal vents near the Galapagos
Islands and have since been found at deep-sea
hydrothermal vents and cold seeps in all of
the world's oceans.As the endosymbiont adapts
to the host's lifestyle, the endosymbiont
changes dramatically.
There is a drastic reduction in its genome
size, as many genes are lost during the process
of metabolism, and DNA repair and recombination,
while important genes participating in the
DNA-to-RNA transcription, protein translation
and DNA/RNA replication are retained.
The decrease in genome size is due to loss
of protein coding genes and not due to lessening
of inter-genic regions or open reading frame
(ORF) size.
Species that are naturally evolving and contain
reduced sizes of genes can be accounted for
an increased number of noticeable differences
between them, thereby leading to changes in
their evolutionary rates.
When endosymbiotic bacteria related with insects
are passed on to the offspring strictly via
vertical genetic transmission, intracellular
bacteria go across many hurdles during the
process, resulting in the decrease in effective
population sizes, as compared to the free-living
bacteria.
The incapability of the endosymbiotic bacteria
to reinstate their wild type phenotype via
a recombination process is called Muller's
ratchet phenomenon.
Muller's ratchet phenomenon, together with
less effective population sizes, leads to
an accretion of deleterious mutations in the
non-essential genes of the intracellular bacteria.
This can be due to lack of selection mechanisms
prevailing in the relatively "rich" host environment.
== Commensalism ==
Commensalism describes a relationship between
two living organisms where one benefits and
the other is not significantly harmed or helped.
It is derived from the English word commensal,
used of human social interaction.
It derives from a medieval Latin word meaning
sharing food, formed from com- (with) and
mensa (table).Commensal relationships may
involve one organism using another for transportation
(phoresy) or for housing (inquilinism), or
it may also involve one organism using something
another created, after its death (metabiosis).
Examples of metabiosis are hermit crabs using
gastropod shells to protect their bodies,
and spiders building their webs on plants.
== Parasitism ==
In a parasitic relationship, the parasite
benefits while the host is harmed.
Parasitism takes many forms, from endoparasites
that live within the host's body to ectoparasites
and parasitic castrators that live on its
surface and micropredators like mosquitoes
that visit intermittently.
Parasitism is an extremely successful mode
of life; as many as half of all animals have
at least one parasitic phase in their life
cycles, and it is also frequent in plants
and fungi.
Moreover, almost all free-living animal species
are hosts to parasites, often of more than
one species.
== Mimicry ==
Mimicry is a form of symbiosis in which a
species adopts distinct characteristics of
another species to alter its relationship
dynamic with the species being mimicked, to
its own advantage.
Among the many types of mimicry are Batesian
and Müllerian, the first involving one-sided
exploitation, the second providing mutual
benefit.
Batesian mimicry is an exploitative three-party
interaction where one species, the mimic,
has evolved to mimic another, the model, to
deceive a third, the dupe.
In terms of signalling theory, the mimic and
model have evolved to send a signal; the dupe
has evolved to receive it from the model.
This is to the advantage of the mimic but
to the detriment of both the model, whose
protective signals are effectively weakened,
and of the dupe, which is deprived of an edible
prey.
For example, a wasp is a strongly-defended
model, which signals with its conspicuous
black and yellow coloration that it is an
unprofitable prey to predators such as birds
which hunt by sight; many hoverflies are Batesian
mimics of wasps, and any bird that avoids
these hoverflies is a dupe.
In contrast, Müllerian mimicry is mutually
beneficial as all participants are both models
and mimics.
For example, different species of bumblebee
mimic each other, with similar warning coloration
in combinations of black, white, red, and
yellow, and all of them benefit from the relationship.
== Amensalism ==
Amensalism is an asymmetric interaction where
one species is harmed or killed by the other,
and one is unaffected by the other.
There are two types of amensalism, competition
and antagonism (or antibiosis).
Competition is where a larger or stronger
organism deprives a smaller or weaker one
from a resource.
Antagonism occurs when one organism is damaged
or killed by another through a chemical secretion.
An example of competition is a sapling growing
under the shadow of a mature tree.
The mature tree can rob the sapling of necessary
sunlight and, if the mature tree is very large,
it can take up rainwater and deplete soil
nutrients.
Throughout the process, the mature tree is
unaffected by the sapling.
Indeed, if the sapling dies, the mature tree
gains nutrients from the decaying sapling.
An example of antagonism is Juglans nigra
(black walnut), secreting juglone, a substance
which destroys many herbaceous plants within
its root zone.A clear case of amensalism is
where sheep or cattle trample grass.
Whilst the presence of the grass causes negligible
detrimental effects to the animal's hoof,
the grass suffers from being crushed.
Amensalism is often used to describe strongly
asymmetrical competitive interactions, such
as has been observed between the Spanish ibex
and weevils of the genus Timarcha which feed
upon the same type of shrub.
Whilst the presence of the weevil has almost
no influence on food availability, the presence
of ibex has an enormous detrimental effect
on weevil numbers, as they consume significant
quantities of plant matter and incidentally
ingest the weevils upon it.
== Cleaning symbiosis ==
Cleaning symbiosis is an association between
individuals of two species, where one (the
cleaner) removes and eats parasites and other
materials from the surface of the other (the
client).
It is putatively mutually beneficial, but
biologists have long debated whether it is
mutual selfishness, or simply exploitative.
Cleaning symbiosis is well-known among marine
fish, where some small species of cleaner
fish, notably wrasses but also species in
other genera, are specialised to feed almost
exclusively by cleaning larger fish and other
marine animals.
== Co-evolution ==
Symbiosis is increasingly recognized as an
important selective force behind evolution;
many species have a long history of interdependent
co-evolution.
=== Symbiogenesis ===
Eukaryotes (plants, animals, fungi, and protists)
developed by symbiogenesis from a symbiosis
between bacteria and archaea.
Evidence for this includes the fact that mitochondria
and chloroplasts divide independently of the
cell, and the observation that some organelles
seem to have their own genome.The biologist
Lynn Margulis, famous for her work on endosymbiosis,
contended that symbiosis is a major driving
force behind evolution.
She considered Darwin's notion of evolution,
driven by competition, to be incomplete and
claimed that evolution is strongly based on
co-operation, interaction, and mutual dependence
among organisms.
According to Margulis and her son Dorion Sagan,
"Life did not take over the globe by combat,
but by networking."
=== Co-evolutionary relationships ===
==== 
Mycorrhizas ====
About 80% of vascular plants worldwide form
symbiotic relationships with fungi, in particular
in arbuscular mycorrhizas.
==== Pollination ====
Flowering plants and the animals that pollinate
them have co-evolved.
Many plants that are pollinated by insects
(in entomophily), bats, or birds (in ornithophily)
have highly specialized flowers modified to
promote pollination by a specific pollinator
that is correspondingly adapted.
The first flowering plants in the fossil record
had relatively simple flowers.
Adaptive speciation quickly gave rise to many
diverse groups of plants, and, at the same
time, corresponding speciation occurred in
certain insect groups.
Some groups of plants developed nectar and
large sticky pollen, while insects evolved
more specialized morphologies to access and
collect these rich food sources.
In some taxa of plants and insects, the relationship
has become dependent, where the plant species
can only be pollinated by one species of insect.
==== Acacia ants and acacias ====
The acacia ant (Pseudomyrmex ferruginea) is
an obligate plant ant that protects at least
five species of "Acacia" (Vachellia) from
preying insects and from other plants competing
for sunlight, and the tree provides nourishment
and shelter for the ant and its larvae.
== See also ==
Aposymbiotic
Cheating (biology)
Human Microbiome Project
Microbial consortium
Socio-ecological system
== Notes
