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Evolving together: the biology of symbiosis, part 2
GREGORY G. DIMIJIAN, MD
Symbiotic trade-offs dominate the world of biology and medicine in
colonist-host relationships and between separate, mutually dependent
organisms of different species. Infectious and parasitic diseases can be
better understood by exploring the dynamic continuum between pathogenicity and mutualism, between antagonism and cooperation—the
sliding scale along which microorganisms can move in a moment’s notice with a single nucleotide substitution. Organisms practicing piracy
or pastoralism may be close genetic relatives. Mergers occur not only
between cells but also between genomes; viruses co-opt host genes and
in turn insert themselves into host genomes. Separate organisms, from
ants to fungi to plants, establish symbiotic ties with each other that bind
over deep time, generating much of the diversity we see in nature.
B
iological and medical research has shown that organisms
are not isolated but, at deep levels of biological organization, weave intimate and complex tapestries with other
life forms. These associations—some of which are symbiotic—
may last for the lifetime of 1 or all partners and may be cooperative, antagonistic, or somewhere in between, with a dynamic and
changing relationship. Part 1 of this article reviewed a number
of functions or “services” provided by a symbiotic partner: photosynthesis, chemosynthesis, nitrogen fixation, luminescence,
and nutrition (1). This article reviews several more cooperative
and noncooperative symbioses, beginning with cellular organelles and moving to insects and plants.
BACTERIAL ANCESTRY OF CELLULAR ORGANELLES
Among the most intimate of symbioses are those between
eukaryotic cells and their mitochondria and chloroplasts. Mitochondria are the powerhouses of eukaryotic cells, the little organelles that manufacture adenosine triphosphate (ATP), the
universal energy currency in eukaryotes. Chloroplasts are organelles of plant cells that carry out photosynthesis, converting
light energy from the sun to chemical energy and “fixing” the
carbon of carbon dioxide in organic compounds.
Both organelles carry abundant evidence that their ancestors
were free-living bacteria. They are the size of bacteria and have
a remnant genome of their own, consisting of a loop of DNA.
They replicate on their own in the cytoplasm, independently of
the host cell. The cell has only 2 copies of most nuclear genes,
but in most cells there are hundreds to thousands of mitochondria. Mitochondrial DNA (mtDNA) is so much more abundant
than nuclear DNA that it is often the only DNA that can be
recovered from fossils. A significant part of our genome thus lies
BUMC PROCEEDINGS 2000;13:381–390
mitochondrion
foldings of
inner membrane
cell nucleus
chloroplast
loops of mtDNA,
which code for proteins of the respiratory chain
mitochondria
Figure 1. Eukaryotic cell drawn with only nucleus, mitochondria, and chloroplasts.
The hundreds of mitochondria in every cell produce 90% or more of the energy
needed by the cell. The many infoldings of the inner mitochondrial membrane
serve to increase the surface area on which membrane-bound reactions occur.
Mitochondrial DNA (mtDNA) is highly exposed to the reactive free radicals of this
membrane system. Like bacteria, mitochondria contain their own ribosomes, used
in protein synthesis.
outside of the 46 chromosomes, a fact often forgotten in descriptions of human genome sequencing (Figure 1).
Mitochondria and chloroplasts, relict bacteria that moved
into precursors of eukaryotic cells, became subcellular organelles
bound inextricably to their host. The symbiotic union became
so strong that many genes migrated to the cell’s nucleus, where
they code for proteins bearing signals ensuring that they are targeted back to the organelles.
The endosymbiont hypothesis that posits this origin of mitochondria and chloroplasts was originally championed by the
biologist Lynn Margulis in the middle decades of the 20th century, long before it became widely accepted. She now writes: “The
branches on the tree of life do not always diverge. One branch
can merge with another, and from these unions new limbs can
grow, unlike anything seen before” (2).
From the Department of Psychiatry, The University of Texas Southwestern Medical Center at Dallas.
Corresponding author: Gregory G. Dimijian, MD, 3277 Brincrest Drive, Dallas,
Texas 75234 (e-mail: [email protected]).
Editor’s note: See interview of the author on p. 391.
381
Mitochondrial genome sequences point to a single ancestral
genome closely related to the rickettsial subdivision of the αProteobacteria, a group of obligate intracellular bacteria that
include Rickettsia and Ehrlichia, the closest known living relatives
of mitochondria. About 2 billion years have elapsed since freeliving α-Proteobacteria became permanent passengers inside
their host cells. As cells diversified, their mitochondria also became more diverse, with different mitochondrial lineages retaining different traces of their free-living past (3).
Chloroplast genomes, in contrast, are most like those of cyanobacteria, photosynthetic bacteria that were once called “bluegreen algae.” Chloroplasts are the photosynthetic specialists of a
family of membrane-bound plant cell organelles called plastids,
all of which are believed to have evolved from cyanobacteria.
Lynn Margulis elaborates on the evidence of ancestry:
Bacteria, merging in symbiosis, leave us hints of their former independence. Both mitochondria and plastids are bacterial in size and
shape. Most importantly, these organelles reproduce so that many
are present at one time in the cytoplasm but never inside the nucleus.
Both types of organelles . . . not only proliferate inside cells but reproduce differently and at different times from the rest of the cell in
which they reside. Both types, probably 1000 million years after their
initial merger, retain their own depleted stores of DNA. The ribosomal DNA genes of mitochondria still strikingly resemble those of
oxygen-respiring bacteria living on their own today. The ribosomal
genes of plastids are very much like those of cyanobacteria (4).
Margulis concludes with a jolting insight: we have Lamarckianism after all—the inheritance of acquired genomes. This is not
exactly the scenario proposed by Lamarck, the inheritance of
environmentally acquired traits, but is even more remarkable—
the acquisition of another organism and its genes.
Lewis Thomas, who died in 1993, wrote eloquently on mitochondria and chloroplasts:
There they are, moving about in my cytoplasm, breathing for my
own flesh, but strangers. They are much less closely related to me
than to each other and to the free-living bacteria out under the hill.
. . . The usual way of looking at them is as enslaved creatures, captured to supply ATP for cells unable to respire on their own, or to
provide carbohydrate and oxygen for cells unequipped for photosynthesis. This master-slave arrangement is the common view of
full-grown biologists, eukaryotes all. But there is the other side.
From their own standpoint, the organelles might be viewed as having learned early how to have the best of possible worlds, with least
effort and risk to themselves and their progeny. Instead of evolving as we have done . . . they elected to stay small and stick to one
line of work. To accomplish this, and to assure themselves the longest possible run, they got themselves inside all the rest of us.
It is a good thing for the entire enterprise that mitochondria and
chloroplasts have remained small, conservative and stable, since
these two organelles are, in a fundamental sense, the most important living things on earth. Between them they produce the oxygen and arrange for its use. In effect, they run the place (5).
Mammalian mtDNA has a mutation rate 10 times that of
nuclear DNA. What can explain the difference? Mitochondria
are highly exposed to free oxygen radicals generated by their
intense metabolic activity, and they have no effective DNA repair system. These handicaps may have favored transfer of most
mitochondrial genes to the nucleus. A high mutation rate may
also have resulted from the rarity of introns (noncoding sequences) in mtDNA; a mutation is therefore more likely to strike
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a coding sequence. There is evidence that mitochondrial mutations accumulate over a person’s lifetime and contribute to a
decline in energy production in many tissues (6).
In keeping with these findings, we are harshly reminded of
mitochondrial diseases, highly variable syndromes characterized
by degenerative changes in muscle, eye, and central nervous system. Mitochondrial diseases are maternally inherited because
most human mitochondria are transmitted only in the cytoplasm
of the egg. There is preliminary evidence of limited paternal
transmission of mtDNA, and nuclear mitochondrial genes are
transmitted in both egg and sperm chromosomes, but there is no
clinical evidence of paternal transmission of mitochondrial diseases (7).
Tracing mtDNA into the past is “cleaner” than tracing
nuclear genes, because little or no recombination of mitochondrial genes occurs between mitochondria of males and females.
“Mitochondrial Eve” was thus proposed, the hypothetical most
recent female ancestor of contemporary humans. If we ever succeed in finding a reliable path to a Mitochondrial Eve through
the jungle of mtDNA branchings, she would represent the point
in the past at which all contemporary human mtDNA originates.
But there are flaws in the argument that she is our true ancestor.
She need not have contributed any other genes to contemporary
humans. Although every gene must have a common ancestor,
the ancestors of different genes could be different individuals
living at different times. Herein lies the danger in relying on a
single genetic locus to reconstruct human history or the history
of any species. With these limitations in mind, mitochondrial
gene trees can nevertheless be useful in phylogenetic and population studies, because they provide higher resolution than
nuclear DNA and offer unique information on where humans
originated and how their populations spread.
In summary, mitochondria and chloroplasts are remnants of
bacteria that have become semi-independent organelles of eukaryotic cells. The endosymbiotic theory is a milestone of 20thcentury biology, showing that symbiosis can be a creator of new
worlds of organisms, indeed, the very world we know.
Other cellular organelles
Is it possible that still other microorganisms have become incorporated into cells as symbiotic organelles? Margulis thinks so
and argues that flagella and cilia were once free-living bacteria.
The absence of genes in these organelles makes this hypothesis
difficult to test. But other researchers are finding tantalizing evidence of such unions. An unusual self-replicating organelle, the
apicoplast, has been identified inside the cells of 3 important
human parasites: Toxoplasma gondii, Plasmodium falciparum, and
Cryptosporidium (8). The apicoplast has its own maternally inherited DNA and resembles chloroplasts but has no photosynthetic function. Apicoplasts appear to have been acquired by
endosymbiosis of a green alga, and they replicate by fission within
the cytoplasm of the host cell. Their genomes are abnormally
small; have they lost genes to the nucleus? Apicoplast DNA
encodes bacteria-like proteins, suggesting that we may be able
to derive antiparasitic drugs from agents that act on the apicoplasts. This may explain the susceptibility of the malaria parasite to antibiotics such as doxycycline and azithromycin, which
interfere with bacterial protein synthesis. Ciprofloxacin inhib-
BAYLOR UNIVERSITY MEDICAL CENTER PROCEEDINGS
VOLUME 13, NUMBER 4
its replication of the apicoplast in T. gondii, and this inhibition
blocks parasite replication (9).
The engulfing of cells and co-opting of cell parts may be more
common than we think. Marine ciliates can be seen today phagocytizing other single-celled photosynthetic organisms and harvesting their chloroplasts, maintaining the chloroplasts as
functioning sugar-producing organelles in their own cells. The
smoking gun in this endosymbiotic interpretation is the finding
of 4 membranes around the organelle—2 from the green alga and
2 from the alga’s chloroplast. The alga and its chloroplast seem
to have been engulfed intact. A majority of ciliates and flagellates in the anoxic bottom waters of the Santa Barbara Basin have
been found to harbor bacterial symbionts, and the ecosystem has
been called a “symbiosis oasis” (10). A vast menagerie of protists, including amoebae, ciliates, flagellates, and other singlecelled organisms, awaits an understanding of their complex
anatomy, including bizarre organelles that may have originated
by endosymbiosis (11).
Changing views of the “tree of life”
What do these new findings suggest about the tree of life?
Until recently we imagined a solid trunk representing the earliest life forms, spreading out into an ever-branching crown. The
newly revised view is that early organisms constituted a soup of
changing genetic entities with promiscuous horizontal (lateral)
gene exchange (12). With such beginnings it would be difficult
to trace family or gene lineages into the distant past. Carl Woese,
who pioneered our understanding of the 3 domains of life
(Archaea, Bacteria, Eucarya), puts it this way:
Initially . . . lateral gene transfer . . . was pandemic and pervasive to
the extent that it, not vertical inheritance, defined the evolutionary dynamic. As increasingly complex and precise biological structures and processes evolved, . . . the evolutionary dynamic gradually
became that characteristic of modern cells. The various subsystems
of the cell “crystallized,” i.e., became refractory to lateral gene transfer. . . . Organismal lineages, and so organisms as we know them, did
not exist at these early stages. The universal phylogenetic tree, therefore, is not an organismal tree at its base but gradually becomes one
as its peripheral branchings emerge. The universal ancestor is not a
discrete entity. It is, rather, a diverse community of cells that survives and evolves as a biological unit. This communal ancestor has
a physical history but not a genealogical one (13).
If for now we can only postulate that lateral gene transfer
characterized the early soup of organisms, we have no doubt that
it occurs today between bacteria, enabling genes for antibiotic
resistance and virulence factors to spread quickly and easily.
Bacteria need not delay the spread of such genes until they reproduce; they can merely spread the instructions around to contemporary cells. Viruses achieve similar genetic exchange during
replication, when they trade gene segments among strains; this
“antigenic shift” accounts in part for new influenza A strains
every year (with pig and avian viral segments reassorting). Viruses also serve as vectors for bacterial genes and for genes we
artificially insert into their small genomes. There is a trace of the
soup still with us in microbial gene exchange.
VIRAL REMNANTS IN THE GENOME
Vertebrate genomes are cluttered with ghosts of past infections in the form of endogenous retrovirus sequences. These
OCTOBER 2000
proviruses, as they are called, may be cause for concern when
animal organs are transplanted to humans, enabling proviruses
to cross the “species barrier.” Matt Ridley writes:
There are several thousand nearly complete viral genomes integrated into the human genome; these HERVs, or human endogenous retroviruses, account for 1.3% of the entire genome. That
may not sound like much, but “proper” genes account for only 3%.
If you think being descended from apes is bad for your self-esteem,
then get used to the idea that you are also descended from viruses
(14).
Evolution is opportunistic. One of these genomic “fossils”
appears to have been appropriated by mammalian hosts for a
useful purpose. One feature of the pathology of retroviruses is
their ability to fuse host cells together into a syncytium. Provirus genes are known to be expressed in the placenta, where the
fetal-maternal interface, or syncytiotrophoblast, is a thin layer
of fused fetal trophoblast cells (15).
Andean mummies 1500 years old have been found to harbor proviral DNA from the human T-cell lymphotropic virus type
I (16). An ancient infection thus left its traces in fossil human
genomes, evidence that human T-cell lymphoma virus-1 was
carried with ancient Mongoloids to the Andes before the Colonial era. In contemporary human genomes there are endogenous
retroviruses dating back at least 30 million years which encode
a factor analogous to an HIV-1 protein, suggesting that an immunodeficiency virus genome infected primate germ cells long
before hominids evolved (17).
Even more common than endogenous retroviruses are retrotransposons, believed to be relict fragments of viral genomes that
have acquired the ability to multiply and reinsert new copies of
themselves anywhere in the genome. Retrotransposons represent
an enormous fraction, possibly 15%, of the human genome. They
can act as insertional mutagens by inserting copies of themselves
in the middle of genes, causing mutations.
Most eukaryotic genomes are littered with these seamlessly
integrated parasitic elements. The term “symbiosis” is not usually used to describe these mergers, but they are clearly examples
of ancestral genomes that fused with those of their original hosts.
SYMBIOTIC HUMAN PATHOGENS
Surprising numbers of pathogenic viruses and bacteria have
become symbiotic with host organisms for most or all of the host’s
lifetime, only occasionally causing serious illness (Table). A challenging question is: Why do these symbionts ever produce illness? If pathogen and host have achieved a kind of truce through
coevolution, why is the relationship not 100% friendly?
Herpesviruses
Consider the extraordinary achievement of latency, characteristic of herpesviruses. Herpes simplex virus type 1 enters the
sensory processes of neurons on the lips and moves into neuronal
cell bodies in the trigeminal ganglion, where it may reside for
the lifetime of the host. The virus has achieved the ideal state
of harmlessly infecting a healthy, active host, which can broadcast it far and wide. At times the virus is truly latent and is not
being replicated by host cells; at other times it is replicated and
moves down axon terminals onto mucosal surfaces where it is
spread to other hosts. Usually it does not cause illness, but occa-
EVOLVING TOGETHER: THE BIOLOGY OF SYMBIOSIS, PART 2
383
Table. Some symbiotic pathogens of humans
Herpesviruses:
• Herpes simplex viruses types 1 and 2
• Varicella-zoster virus
• Human herpesvirus 4 (Epstein-Barr virus)
• Human cytomegalovirus
• Human herpesvirus 6
• Human herpesvirus 7
Neisseria meningitidis
Haemophilus influenzae
Helicobacter pylori
Candida spp.
sionally, even in immunocompetent hosts, it causes life-threatening illness, from encephalitis to neonatal herpes. Latency may
be terminated if the host is stressed, when viral replication resumes and lesions are produced on the face and in the mouth.
During latency a single viral gene—appropriately named
LAT—is abundantly transcribed and appears to block cellular
apoptosis. Infected ganglion cells thus cannot initiate their own
self-destruction by using the major histocompatibility complex
to display viral proteins on the cell surface and are preserved,
serving as periodic virus factories (18).
Humans are not unique hosts to symbiotic herpes simplex
viruses. Monkeys stressed by capture and transport break out with
gingivostomatitis from their own latent herpesvirus strains.
Other herpesviruses display the trait of latency. Varicellazoster virus causes chickenpox early in life and then enters latency in the dorsal root ganglia. It can later reactivate as herpes
zoster (shingles). Studies have demonstrated the identity of the
viruses causing both diseases. Human cytomegalovirus, another
herpesvirus, causes a latent infection with periodic reactivation
for the life of its human host; incidence of host infection reaches
70% in cities of the USA and nearly 100% in parts of Africa.
Human herpesvirus 4 (or Epstein-Barr virus) produces a latent
B-cell infection for the life of the host, hiding in the very cells
that make antibodies. Human herpesviruses 6 and 7 are believed
to initiate a lifelong infection in nearly all humans in early childhood (19).
Bacteria
Not to be outdone by viruses, pathogenic bacteria boast many
species that have become symbiotic. Neisseria meningitidis is a
major cause of bacterial meningitis and septicemia, often spreading on college campuses, with a case fatality rate of 10% to 12%.
In spite of its reputation as an aggressive pathogen, it is more
often found as a harmless commensal in the human nose and
pharynx. It has been estimated that at any given time between
5% and 10% of humans carry the organism. It causes sporadic
cases of disease at the much lower incidence of 1 to 5 per 100,000
a year. The meningococcus lives only in humans, passing from 1
person to another in aerosols; there are no other animal or environmental niches that it is known to colonize. It is adapted to
living peacefully in humans and has evolved a variety of mecha384
nisms to evade our defenses, including the ability to produce
variable cell-surface proteins that provide an ever-changing target for the immune system (20).
Haemophilus influenzae, another symbiotic bacterial pathogen
of humans, has no other known natural host. It is among the normal bacterial flora of the pharynx and, to a lesser extent, the
conjunctivae and genital tract. Up to 80% of healthy persons are
carriers of the bacterium, which only occasionally causes serious
illness such as meningitis.
Helicobacter pylori is a spiral bacterium uniquely adapted to
the low pH of the stomach. Its ecology is revealed in its unusually small genome, which lacks genes found in other bacteria
adapted to more variable environments. H. pylori is spread by
vomitus and feces and possibly by flies and is especially common
in developing countries. By age 10, >50% of children worldwide
carry the symbiotic bacterium. An untreated infection is believed
to last the lifetime of the host.
Fungi
Fungi of the genus Candida are normal commensals of humans, found on the skin, in sputum, in the female genital tract,
and throughout the entire gastrointestinal tract. They are usually found as single cells without the branching mycelia seen in
bread mold fungi. Serious infections, uncommon in healthy persons, include vaginitis, oral thrush, esophagitis, pneumonia, endocarditis, and meningitis.
Pathogens and disease
Many, if not most, microbial pathogens are not symbiotic
with hosts. Influenza viruses, for example, cause a time-limited
infection and are eliminated by the immune system if the host
survives. Yet the closer we look, the more often we discover longterm carrier states and low-grade, chronic, long-lasting infections. Mycobacterium tuberculosis, Plasmodium vivax, and HIV-1
often colonize a host for the better part of its lifetime. Should
we consider these pathogens symbiotic? Whatever our preference, we should address the extraordinary fact that many pathogens are adapted to colonize our bodies for the better part of our
lifetime. We are normal animals in this respect.
Is disease sometimes the sorry expression of a residual degree
of virulence maintained by natural selection because of competition among strains or species of pathogens? If virulence often
means a higher rate of replication, a strain of pathogen with a
higher virulence might outcompete another strain and be the one
we see today. Coevolution with hosts might provide an upper
limit to virulence in pathogens, as premature host death would
limit spread. Disease would then occur in a minority of hosts but
be inevitable because of competition among pathogens.
There is also another cause of disease, of increasing importance in today’s world. Crossing the species barrier exposes a
pathogen to a new host with which it has had no chance to coevolve, and either the pathogen or the host may be no match
for the other. In today’s globally disrupted ecosystems there are
rampant opportunities for species barriers to be crossed for the
first time. The situation is analogous to a perforated colon, in
which previously harmless bacteria end up where they don’t belong.
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VOLUME 13, NUMBER 4
SYMBIOTIC COLONISTS IN INVERTEBRATES AND PLANTS:
WOLBACHIA AND AGROBACTERIUM
The tiny bacterium Wolbachia spends its entire life within the
cells of the ovaries and testes of many arthropod species, not causing illness but nevertheless playing havoc with the sex ratio of
its host’s progeny. It is passed on to the host’s offspring only in
eggs, not in sperm. Thus the bacterium’s strategy for passing on
its genes: skew the sex ratio markedly in favor of females, or spare
no guns and do away with males altogether. In either case the
strategy works.
Wolbachia achieves this devious end by altering host chromosome behavior, reducing male numbers or completely eliminating males. In the latter case the host becomes asexual; the
whole infected population becomes parthenogenetic for as many
generations as the bacterium lives in the host’s gonads. Many
cases of parthenogenesis, once thought to be genetic and permanent, are being found to be infectious in origin. Asexuality in
some wasps, for example, can be reversed by treating the wasps
with antibiotics or subjecting them to heat in the laboratory,
which kills the bacteria. Two genders magically reappear among
the offspring. Antibiotics cure asexuality!
How could natural selection bring about this behavior in
Wolbachia? One possibility is through competitive exclusion.
Once a microbe became capable of invading the ovary and living there, a competing strain that discovered a way to skew the
host’s sex ratio would multiply more rapidly than the other strain
and would be the one we see today.
Wolbachia infection is amazingly common, occurring in members of >90% of known arthropod species, including 5 orders of
insects. In most cases the result is a skewed sex ratio, not asexual
reproduction.
New strategies of disease prevention sometimes come from
the most unlikely places. Studies are under way to use Wolbachia
to disrupt transmission of pathogens carried by arthropod vectors. Genetically modified Wolbachia could spread a second maternally inherited gene into the vector population very rapidly,
just as it spreads its own genes; the inserted gene could disrupt
the pathogen’s life cycle (21).
Another symbiotic bacterial species has been harnessed as a
gene vector. Agrobacterium tumefaciens, itself a natural genetic
engineer, infects a wounded plant and inserts genes from its plasmid into the genome of the host plant. Some of the transferred
genes code for the synthesis of hormones that transform plant
cells to tumor cells, forming a large “crown gall” tumor that
houses the bacteria. Others direct the synthesis of chemicals
called opines that cannot be metabolized by the host and that
provide nitrogen-rich and carbon-rich compounds to the bacteria. The transferred genes thus arrange for room and board for
their original owner, which provides no known service in return.
Human genetic engineers use Agrobacterium as a vector for
“artificial” genes to be inserted into plant cells. In the laboratory these genes are integrated into the large bacterial plasmid,
converting Agrobacterium into a shuttle for inserting the genes
into plants. Some of these genes code for antimicrobial compounds, including the enzyme replicase, which disrupts viruses.
Disease-resistant varieties of potatoes have been created in this
way. Another gene, removed from the bacterium Bacillus
thuringiensis (“Bt”) and inserted into the Agrobacterium plasmid,
OCTOBER 2000
Figure 2. Swollen thorns of whistling thorn acacia, with resident ants, in Serengeti
Plains of Tanzania, East Africa. Photo: Gregory G. Dimijian, MD.
directs the synthesis of an insecticidal toxin. Inserted by Agrobacterium into the corn genome, it continues to code for toxin
in its new host, “Bt corn,” which is resistant to lepidopteran pests.
REMAINING APART: ANT-PLANT MUTUALISMS
Some of the most remarkable symbiotic associations occur
between organisms that remain physically separate, with neither
colonizing the body of the other. Both remain separate, but their
lives and activities are so interdependent that they evolve almost
as 1 organism.
Almost half of all insect species are herbivores, which defoliate plants. If evolution is opportunistic, plants should not have
failed to notice the power of ants to defend against herbivores,
from mammals to invertebrates, including other ants. Because
ants live in colonies, they can collectively defend against herbivores with the striking force of a legion of workers, all willing to
sacrifice themselves at a moment’s notice.
Plants have indeed seduced ants into a dazzling variety of
cooperative exchanges. In return for shelter, nectar, or food bodies, a colony of ants can defend every square inch of the plant.
Nest cavities are provided in the form of swollen thorns (Figure
2), hollow stems, leaf pouches, and even hollow roots. Nectar
or food bodies are provided strategically on growing shoot tips
or young leaves, where plant tissues are most vulnerable to browsing. Ants defend against herbivorous insects and vertebrates and
even prevent vines from climbing up a trunk and taking over a
tree.
In the Peruvian Amazon a light tap on the trunk of a Triplaris
tree will bring a rain of stinging ants down onto one’s head and
shoulders from the foliage overhead. In Costa Rica an Inga plant
provides extrafloral nectaries for ants to drink from and receives
protection in return. In East Africa hollow thorns develop on
whistling thorn acacias even if the plants are grown from seed
in a nursery; the thorns provide nest cavities for ants. Two of the
ant species that colonize whistling thorns are so militant that
within seconds of touching a tree an intruder is attacked by a
horde of biting ants.
In 1872 an extraordinary naturalist, Thomas Belt, first proposed that ants protect trees. Belt spent 4 years in the Nicaraguan
EVOLVING TOGETHER: THE BIOLOGY OF SYMBIOSIS, PART 2
385
Figure 3. Two ants drink from the nectar organs of a caterpillar, which in turn
feeds at a flower of an acacia. Both plant and caterpillar are believed to be protected in this apparent 3-way mutualism. Research of Diane Wagner at the Southwestern Research Station of the American Museum of Natural History. Photo:
Gregory G. Dimijian, MD.
rain forest, observing plants and animals and keeping careful
notes. In his classic account, The Naturalist in Nicaragua, he writes:
These ants form a most efficient standing army for the plant, which
prevents not only the mammalia from browsing on the leaves, but
delivers it from the attacks of a much more dangerous enemy—the
leaf-cutting ants. For these services the ants are not only securely
housed by the plant, but are provided with a bountiful supply of
food, and to secure their attendance at the right time and place,
the food is so arranged and distributed as to effect that object with
wonderful perfection. . . . At the end of each of the small divisions
of the compound leaflet there is, when the leaf first unfolds, a little
yellow fruit-like body united by a point at its base to the end of the
pinnule. Examined through a microscope, this little appendage
looks like a golden pear. When the leaf first unfolds, the little pears
are not quite ripe, and the ants are continually employed going from
one to another, examining them. When an ant finds one sufficiently
advanced . . . it breaks it off and bears it away in triumph to the
nest. All the fruit-like bodies do not ripen at once, but successively.
. . . Thus the young leaf is always guarded by the ants; and no caterpillar or larger animal could attempt to injure them without being attacked by the little warriors (22).
can feed with impunity on plants that otherwise would not tolerate them (poisoning them with toxins synthesized in the
leaves). Instead the plants allow them to browse on the leaves,
in exchange for the protection provided by the ants they attract
(Figure 3). Some caterpillars can even summon ants from nests
at the base of the plant by releasing chemicals as signals or by
producing vibrations that the ants can feel.
Ant societies have invited other species into their midst repeatedly through evolutionary history. Many ants adopt aphids,
mealybugs, or other arthropods that provide a steady diet of honeydew and provide protection in return. Some of these complex
associations appear to be 3-way mutualisms, with plant, caterpillar, and ant benefiting in complex and varying ways.
There are pitfalls, however, in making these interpretations.
The ants we see on 1 plant may not be the same species that
coevolved with the other 2 members of the mutualism, but may
instead be interlopers, even cheaters. In addition, the mutualisms
may not always be specific but may be more general, with plants
and caterpillars sharing ants of different species. The ants may
also protect other insects and plants at the same site. Pitfalls
include the assumption that ants and plants coevolve; it is more
difficult to measure the reliance of ants on plants than vice versa.
Under the stress of herbivory, plants have evolved services used
by ants, but have the ants also evolved in concert or are they just
taking the offerings and behaving like ants?
CHEATERS: PARASITES OF MUTUALISMS
Acacias in Central America are protected by obligate acaciaants but after a prolonged drought or other disturbance may be
temporarily deprived of the resident ant colony. During this vulnerable period a cheater ant species may occupy the plant, feeding on the food bodies and enjoying the shelter of the nest
cavities but not protecting the plant. The temporary cheater is
a parasite of the ant-acacia mutualism, benefiting from the services of the plant but providing no service in return (24).
A cheater species has evolved in the savanna of East Africa,
where whistling thorn acacias offer services to ants. This parasitic ant species (Crematogaster nigriceps) occupies an individual
acacia and enjoys shelter and food without providing protection.
Its parasitic behavior does not stop there; the ants proceed to
prune the tree, removing the growing shoots, and sterilize the
tree by chewing off its flower buds. This odd behavior was an
enigma to researchers until it became apparent that the ants were
preventing growth toward other trees that were colonized by
competitively superior ant species, which would stream over to
their tree if the trees were in contact (25).
Today we call the small food bodies on the acacia leaves
Beltian bodies. Belt’s hypothesis was dismissed by leading biologists of the time, who retorted that considering ants beneficial
to plants was like considering fleas beneficial to dogs. It took
experiments by Janzen in the 1960s to resolve this issue and show
that when the ants were removed, herbivores decimated the
acacias (23). Since Janzen’s experiments,
ants have been shown repeatedly to inpathogenicity;
crease plant fitness by foraging actively
cheating in mutualisms
commensalism
mutualism
in large numbers over plant surfaces, removing arthropods that consume plant
parts.
antagonism
cooperation
Caterpillars, like plants, are opportunists in exploiting ants. By providing Figure 4. An unbroken and dynamic continuum spans the extremes of organismal interactions from adversarial
ants with nectar from glands on their to cooperative. A symbiont may move along the continuum in response to changing environmental conditions,
such as nutrition of the host, or over evolutionary time. A symbiont may instead acquire mutations or new
bodies, many caterpillars have gained genes that assign it a very different place on the continuum from its closely related recent ancestor. Changes
protection by ants in the same way that toward antagonism may involve acquiring greater pathogenicity or, in the case of mutualists, cheating on the
plants have. Some of these caterpillars mutualism.
386
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A beetle in Costa Rican rain forests can stimulate Piper antplants to produce food bodies, as if ants were present. The beetles
take over the plant, exploiting nest sites usually occupied by ants
and consuming the food produced for the ants but providing no
protection in return. The beetles even prey on any ant brood
present on the plant (26).
Parasites of mutualisms constitute a cost to both parties.
There is often little defense, and cheaters thrive and diversify.
The biologist Judith L. Bronstein has divided cheaters into
several useful categories. Some species cheat exclusively, other
species consist of some individuals that cheat and others that
cooperate, and still other species consist of individuals that cheat
some of the time and not at other times. Mutualist and cheater
species are often close relatives from sister taxonomic groups,
consistent with the hypothesis that a lineage has escaped the cost
of mutualistic investment. The cheater species may even be derived from the mutualist species (JL Bronstein, personal communication).
Cheating may thus be analogous to pathogenicity, both occurring along an unbroken continuum in which a pathogen or
cheater evolved from a cooperative species or vice versa. This
insight may be revolutionary in exploring disease, as the evolutionary dynamics of cheating vs cooperating in mutualisms may
be virtually identical to the dynamics of host-pathogen relations,
as both involve selfish evolutionary trade-offs (Figure 4).
LEAFCUTTER ANTS
Among the few organisms that cultivate their own food are
leafcutter ants, which maintain fungal gardens in their underground nests. Leafcutters are the dominant plant-eaters of the
New World tropics; some 15% of leaves of tropical forests in
Central and South America disappear down their nests (Figure
5). There the smaller workers chop up and clean the fragments
and mix them with the fungi. The fungi break down the cellulose walls of the plant cells, make the protein available, use some
of it themselves, and detoxify some of the plant alkaloids. The
ants then feed on the fungi (special nutrient-filled hyphae are
provided by fungi cultivated by more recently evolved leaf-
Figure 5. Column of leafcutter ants carrying fragments of leaves back to the nest,
during a downpour in the lowland tropical rain forest of the La Selva Field Station of the Organization for Tropical Studies in Costa Rica. Thomas Belt wrote
that the endless trains of leafcutters were “a mimic representation of a moving
Birnam Wood.” Photo: Gregory G. Dimijian, MD.
OCTOBER 2000
Figure 6. Leafcutter ant queen tending her new fungal culture and the eggs she
has laid in its midst. The fungus is carried in her buccal pouch when she leaves the
colony on her mating flight. After mating she digs a narrow shaft in the soil and
starts her garden with the fungus. She feeds the garden with her own eggs and
excrement, and only after the garden is doing well will she attempt to raise workers. If she should lose the garden at this stage she will die, as she is unable to obtain more of the fungus. Photo courtesy of David Roberts, Nature’s Images, Inc.
cutters). In return, the fungus is assured a regular food supply of
leaves and a sheltered nest and is guaranteed transportation to
new nests by the founding queen (Figure 6).
In their evolutionary past the ants captured the fungus, or—
as E. O. Wilson reminds us—maybe it was the other way around.
In either case they now own each other.
No one suspected that a third mutualist would be found in
leafcutter colonies, but a remarkable study has just revealed one.
It turns out to be a bacterium that protects the fungal garden from
a fungal pathogen (27). Found on the bodies of the ants, the filamentous bacterium produces antibiotics specifically targeted to
suppress the growth of the pathogenic fungus. The 3-party mutualism—ant, fungus, and bacterium (from 3 kingdoms—animals,
fungi, and prokaryotes)—appears to be an ancient association, the
partners having coevolved over the past 10 million years or more.
A new queen carries some of the bacteria with her, along with
the fungal inoculum, when she leaves the nest to mate and found
a new colony.
It may not be a coincidence that the bacteria belong to the
genus Streptomyces, from which over half of our antibiotics have
been derived. Unlike our antibiotics, however, this one (with its
evolving variations) has remained effective for 10 million years.
SLAVE-MAKING ANTS (SOCIAL PARASITISM)
Each day at about 4 PM in the pine-oak-juniper woodlands
of the Chiricahua Mountains of southeastern Arizona, raiding
columns of Polyergus ants scurry across and under the leaves on
the forest floor, following their leader scouts to the underground
nest of another ant species of the genus Formica. There the
Polyergus queen fights to the death with the Formica queen, eventually killing her (Figure 7). She licks her victim and acquires
the scent of the dying queen, after which the Formica workers
treat her as their own, allowing her raiding workers to pick up
the pupae and carry them off. Back over the forest floor they go,
carrying the pupae back to their own nest (Figure 8). When the
pupae emerge from their cocoons they will form social bonds with
EVOLVING TOGETHER: THE BIOLOGY OF SYMBIOSIS, PART 2
387
Figure 7. Polyergus ant queen killing the Formica queen, photographed in the
lab at the American Museum of Natural History’s Southwestern Research Station.
Both queens were captured separately and placed together at the time of day
when they normally encounter each other, during a raid. When this murder occurs in the Formica nest, the victorious queen licks her victim’s body, acquiring
the scent that terminates the retaliation of the nest workers. The pupae can then
be captured and abducted without worker resistance. Research of Howard Topoff
and Christine Johnson (27). Photo: Gregory G. Dimijian, MD.
their slave-making nestmates through olfactory imprinting. A
visitor to the Southwestern Research Station of the American
Museum of Natural History may be privileged to witness this
daily event, where it has been studied for over a decade.
The Polyergus slave makers have lost the ability to maintain
their nest or care for their young. The queen cannot rear her
brood without slave labor. The workers, which conduct the daily
slave raids, do little else. They do not forage for food, feed their
brood or queen, or even clean their own nest. All of these chores
are performed with no complaints by the slaves, which forage for
food and regurgitate food to colony members of both species. The
slaves even send out scouts to locate a new nesting site when the
population of the nest grows too large. When they all move, the
slaves carry all eggs, larvae, pupae, and every host adult, even the
host queen, to the new nest site (28).
The 2 ant species are symbiotic but their bodies remain separate, like ants and plants, ants and caterpillars, and ants and their
fungal gardens. The relationship is both symbiotic and parasitic;
the term “social parasitism” is appropriate, with 1 party duped
into serving the other. The slave workers help the genes of another species achieve future representation.
This extraordinary scenario is not an aberration. Of the
~8800 known species of ants, some 200 species have evolved
some degree of symbiotic relationship with other ants (29). At
1 end of the continuum are temporary parasites and at the other
end, the “inquiline” ants. Inquilines are parasitic species that
spend their entire lives in the nest of the host species; in the most
extreme example the entire worker caste has been eliminated,
as have the slave raids. The queen is absurdly tiny and rides on
the host queen’s back. Males and new queens produced by the
diminutive queen copulate inside the host nest; the newly mated
queens then locate other host colonies to parasitize, and the cycle
is repeated, with complete dependence on the host. The parasitic ants bear the marks of extensive morphological degeneration correlated with loss of function; even the brain is diminished
388
Figure 8. Returning Polyergus workers carrying captured Formica
pupae destined to become slave workers for the colony. Photo:
Gregory G. Dimijian, MD.
in size. Yet for the survival of their genes they must retain the
skills of locating and parasitizing new hosts. Their condition is
an evolutionary sink; a return to a nonparasitic mode of life is
unlikely. What do they gain? The services of their host. What
does the host gain? Nothing, as far as we know, but there is always a chance that we are wrong.
A bizarre feature of these ant symbioses has evaded explanation: parasitic ants generally originate from the closely related
ants that are their hosts. This seems paradoxical; how can a species generate its own parasite? One hypothesis proposes that a
population splits and becomes geographically isolated; if the 2
resulting populations reunite and cannot interbreed, one may
specialize as a parasite of the other (30).
FLOWERS AND POLLINATION
The highly specialized and coevolved relationships between
flowers and their pollinators—morphological, physiological, and
behavioral—have fascinated biologists ever since they were first
described. Pollination is one of the most widespread mutualisms
in nature. Early radiations of flowering plants occurred concomitantly with radiations of insects that pollinated them; Cretaceous
and Tertiary flower-visiting insects included an impressive variety of beetles, flies, moths, wasps, and bees. Butterflies joined
them in the mid-Tertiary. Many flower parts and insect body
structures are morphologically convergent, having apparently
evolved together (31).
Two extraordinary pollination mutualisms—those of yucca
plants with yucca moths, and fig trees with fig wasps—and 1
nonsymbiotic association merit description here.
Yucca plants
The female yucca moth is the sole pollinator of many species of yucca plants. She inserts her ovipositor into the ovary of
the flowers and deposits her eggs, then walks up to the stigma of
the flower and actively deposits a small amount of pollen. In
doing so she makes it possible for seeds to develop; seeds will
become the exclusive food of her progeny. The mutualism is so
specific that other pollinators are excluded; this has created an
absolute mutual dependence of the plant on the moth and vice
versa. Neither can reproduce without the other.
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VOLUME 13, NUMBER 4
This has been called the “perfect” mutualism, with the idea
that both parties need and benefit each other. Closer scrutiny
reveals conflicts of interest and self-defense; the yucca has developed subtle strategies to prevent exploitation. Fruits often fail
to mature if they contain heavy egg loads or low pollen counts;
this may be a way to select against moths laying many eggs or
providing low-quality pollination. This is the less cooperative
face of mutualism (32).
Another fly has been found in the idealistic ointment. When
2 kinds of moths depend on the same yucca, the door is opened
for one to become a parasite of both the yucca and the other
moth. This very scenario is taking place in Florida, where 2 moth
species lay eggs in the same yucca. The cheater moth has split
off from the other species, no longer wasting its energy on growing special mouth parts for gathering pollen. Both species inject
their eggs under the surface of the fruit, but the cheater species
does so later, after the fruit has begun maturing—thus avoiding
the yucca’s defense. These cheater moths are parasites of the
mutualism, their success depending on the mutualism itself (33).
Some biologists would classify this mutualism as nonsymbiotic, because plant and pollinator live apart. It is included
in this review because of the tightly interwoven life histories and
obligatory association of both partners.
Fig trees
The second pollination mutualism, this one stranger than fiction, has evolved between fig trees and their only pollinators, tiny
fig wasps. Each species of fig tree is pollinated only by its own
species of wasp, which can breed only inside the fig, at a time
when the fig is in flower. The female wasp, bearing pollen, enters a fig “fruit,” pollinates the flowers (which line the inside surface of the fig), and lays eggs, usually dying there. Each larva
develops at the expense of a single tiny fig flower. When the new
generation of wasps reaches adulthood (still inside the fig), they
mate, and females leave, loaded with pollen. The figs then ripen
and are eaten by frugivores, which disperse the seeds. As in the
case of the yucca and yucca moth, this is a very tight mutualism,
and neither fig tree nor wasp can reproduce without the other
(23).
Orchids
Orchids are renowned for their intimate and intricate coevolutionary associations with insect pollinators. They provide a
lesson, however, in when not to call an association mutualistic.
The flowers of Ophrys orchids have evolved elaborate ways of
mimicking the appearance, hairiness, and sex pheromones of
female bees (34). This visual, tactile, and chemical mimicry
entices male bees to attempt to copulate with the flower, and the
bee may spend several hours in this activity, even ejaculating,
but taking no food for himself. Instead, he flies away with pollen on his body, which he delivers to the next flower. The relationship is thus not mutualistic but parasitic, as only 1 partner
is known to benefit. It is also not symbiotic. An evolutionary
arms race is likely to be in progress, with male bees being selected
to discriminate between real and floral females and the orchid
selected for maintaining the ruse.
OCTOBER 2000
SHIFTING IDENTITIES
There may be a fine line between pathogenicity and mutualism—between 1 end of the continuum and the other.
There is a growing belief that many mutualistic symbioses
originate as parasitic symbioses. A striking example of a transition from pathogen to mutualist has been documented in a fungus that infects plant cells; mutation at a single locus was found
in the nonpathogenic strain, which otherwise was genetically
identical to the pathogenic strain and retained the same host
specificity (35).
Even Wolbachia bacteria may shift along the continuum. The
filarial worm Onchocerca volvulus, a leading cause of blindness
in Africa, appears to depend on endosymbiotic Wolbachia for its
normal fertility. Treating the worms with tetracycline leads to
elimination of the bacterial endosymbionts and sterility of adult
worms. Worms not originally infected with Wolbachia are not
harmed by the antibiotic (36).
For intracellular bacteria, the term “infection” applies to both
pathogens and mutualists; in both cases cells are “infected.” In
order to understand infection by a pathogen, it may help to
understand infection by a mutualist.
Brucella abortus, a mammalian pathogen that causes brucellosis, and Rhizobium meliloti, a phylogenetically related plant
mutualist, establish chronic infections in their respective hosts.
The same gene product is needed to establish and maintain
chronic intracellular infection in both bacterial species. Both
bacteria are endocytosed by host cells and live for prolonged
periods in intracellular compartments. A pathogen and a mutualist may thus be closely related and utilize similar molecular
pathways for infecting the host (37).
CONCLUSIONS
Symbiotic associations between organisms appear to constitute the biological and social milieu of animals and plants in ways
that we never expected even a half-century ago. Larger organisms are colonized by smaller ones, and organisms of similar size
associate intimately without merging. Genomes have merged and
coalesced since the early soup of lateral gene transfer.
Movement along the interactional continuum from antagonistic to cooperative is dynamic and changing, with any 1 symbiotic relationship having both antagonistic and cooperative
elements active at the same time. Many mutualistic relationships
may have begun as antagonistic ones. Microbes and hosts share
overlapping life histories in the context of a dynamic standoff,
each looking after its own genetic interests. The consequences
range from fatal infections to mutualisms and everything in between. The complex ways in which cheaters exploit mutualisms
may provide revolutionary insights into infectious and parasitic
diseases, as many cheaters and pathogens are close relatives of
species at the other end of the continuum.
Host disease occurs in at least 2 evolutionary settings: 1) a
highly coevolved pathogen has titrated its pathogenicity to a
level high enough to be competitive with other strains (and other
species) and low enough to maximize host survival and spread
to other hosts; and 2) a pathogen recently having crossed the
“species barrier” may be more virulent than its best interests dictate, but it has not had time to coevolve with the host.
EVOLVING TOGETHER: THE BIOLOGY OF SYMBIOSIS, PART 2
389
Models predict that long-term interactions are evolutionarily
stable only when both interacting species possess mechanisms to
prevent excessive exploitation. Evolutionary time is needed, as
E. O. Wilson reminds us, for symbiotic bargains to be struck.
The understanding of symbiotic relationships promises new
insights into biology and medicine, from the evolution of species to the dynamic relations between microorganisms and hosts.
Acknowledgments
Miriam Muallem, librarian at Medical City Dallas Hospital,
provided invaluable expertise in acquiring reference material.
Mary Beth Dimijian provided helpful editing advice, and Keith
Johansen, MD, made the important recommendation that the
symbiosis material be presented in 2 parts.
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