hand, 466 distinct homologs of the light-driven
proton pump bacteriorhodopsin are found in the
surface waters of the Sargasso Sea, whereas none
are found in the deep-sea whale falls or in soil.
The analysis of operons likewise reveals
similarities and differences in functional systems
(Fig. 4, upper right) that suggest features of the
environments. The most discriminating operons
tend to be systems for the transport of ions and
inorganic components, highlighting their impor-
tance for survival and adaptation. With respect
to ionic and osmotic homeostasis, for example,
the two maritime environments are similar—
both show a strong enrichment in operons that
contain transporters for organic osmolites and
sodium ion exporters coupled to oxidative phos-
phorylation. The soil sample, on the other hand,
has a strong enrichment in operons responsible
for active potassium channeling. These biases
nicely reflect the relative abundance of these
ions in the respective environments: Whereas
typical ocean water contains considerably more
sodium ions than potassium, the soil sample
examined here containedhighpotassiumand
low sodium concentrations (13).
Examination of higher order processes
reveals known differences in energy production
(e.g., photosynthesis in the oligotrophic waters
of the Sargasso Sea and starch and sucrose
metabolism in soil) (7) or population density
and interspecies communication Eoverrepresen-
tation of conjugation systems, plasmids, and
antibiotic biosynthesis in soil (Fig. 4, lower
left)^ (22). The broad functional COG catego-
ries, on the other hand, primarily suggest
differences in genome size and phylogenetic
Notably, many uncharacterized genes and
processes are among the most overrepresented
categories in each sample. This hints at an
abundance of previously unknown functional
systems, specific to each environment, whose
occurrence patterns may offer useful guidance
for further, more directed experimental and com-
putational investigations. More extensive
sampling in both time and space will reveal
which features are broadly distributed within a
given environment and which are unique to the
places and times sampled here. Nonetheless, this
analysis of genes and functional modules in
environments reveals expected contrasts, hints at
certain nutrition conditions, and points to novel
genes and systems contributing to a particular
Blife-style[ or environmental interaction.
The predicted metaproteome, based on
fragmented sequence data, is sufficient to iden-
tify functional fingerprints that can provide
insight into the environments from which
microbial communities originate. Information
derived from extension of the comparative meta-
genomic analyses performed here could be used
to predict features of the sampled environments
such as energy sources or even pollution levels.
At the same time, the environment-specific dis-
tribution of unknown orthologous groups and
operons offers exciting avenues for further inves-
tigation. Just as the incomplete but information-
dense data represented by expressed sequence
tags have provided useful insights into vari-
ous organisms and cell types, EGT-based eco-
genomic surveys represent a practical and
uniquely informative means for understanding
microbial communities and their environments.
References and Notes
1. E. F. DeLong, N. R. Pace, Syst. Biol. 50, 470 (2001).
2. P. Hugenholtz, Genome Biol 3, REVIEWS0003 (2002).
3. M. R. Liles, B. F. Manske, S. B. Bintrim, J. Handelsman,
R. M. Goodman, Appl. Environ . Mic robiol. 69, 2684 (2003).
4. O. Beja et al., Science 289, 1902 (2000).
5. A. H. Treusch et al., Appl. Environ. Microbiol. 6, 970
6. G. W. Tyson et al., Nature 428, 37 (2004).
7. J. C. Venter et al., Science 304, 66 (2004).
8. V. Torsvik, L. Ovreas, T. F. Thingstad, Science 296,
9. Y. A. Goo et al., BMC Genomics 5, 3 (2004).
10. C. R. Smith, A. R. Baco, in Oceanography and Marine
Biology: An Annual Review, R. N. Gibson, R. J. A.
Atkinson, Eds. (Taylor & Francis, London, 2003), vol.
41, pp. 311–354.
11. J. B. Hughes, J. J. Hellmann, T. H. Ricketts, B. J.
Bohannan, Appl. Environ. Microbiol. 67, 4399 (2001).
12. R. K. Colwell, personal communications (1994–2004).
13. Supplementary online material.
14. R. Overbeek, M. Fonstein, M. D’Souza, G. D. Pusch, N.
Maltsev, Proc. Natl. Acad. Sci. U.S.A. 96, 2896 (1999).
15. R. L. Tatusov et al., BMC Bioinformatics 4, 41 (2003).
16. C. von Mering et al., Nucleic Acids Res. 33, D433 (2005).
17. A. Bateman et al., Nucleic Acids Res. 32 (Database
special issue), D138 (2004).
18. S. K. Rhee et al., Appl . Environ. Microbiol. 70, 4303 (2004).
19. D. E. Robertson et al., Appl. Environ. Microbiol. 70,
20. C. von Mering et al., Proc. Natl. Acad. Sci. U.S.A.
100, 15428 (2003).
21. M. Kanehisa, S. Goto, S. Kawashima, Y. Okuno, M.
Hattori, Nucleic Acids Res. 32 (Database special
issue), D277 (2004).
22. R. Daniel, Curr. Opin. Biotechnol. 15, 199 (2004).
24. This work was performed under the auspices of the
DOE’s Office of Science, Biological and Environmental
Research Program; the University of California, Law-
rence Livermore National Laboratory, under contract
no. W-7405-Eng-48; Lawrence Berkeley National
Laboratory under contract no. DE-AC03-76SF00098;
and Los Alamos National Laboratory under contract
no. W-7405-ENG-36. S.G.T. was supported by grant
no. THL007279F, an NIH National Research Service
Award (NRSA) Training and Fellowship grant to E.R.
K.C. was supported by NSF grant no. EF 03-31494.
Sequencing of the environmental libraries was per-
formed under a license agreement with Diversa (J. R.
Short, U.S. patent no. 6455254). We gratefully ac-
knowledge the efforts of C. Baptista, L. Christoffersen,
J. Garcia, K. Li, J. Ritter, P. Sammon, S. Wells, D.
Whitney, J. Eads, T. Richardson, M. Noordewier, and L.
Bibbs. We thank C. Smith for providing the whale fall
samples; K. Remington for providing Sargasso Sea
sample information; N. Ivanova, N. Kyrpides, and
members of the Rubin laboratory for helpful com-
ments on the manuscript; and J. Chapman, I. Grigoriev,
E. Szeto, J. Korbel, T. Doerks, K. Foerstner, E.
Harrington, and M. Krupp for assistance with data
processing and analysis. These Whole Genome
Shotgun projects have been deposited with the DNA
Data Bank of Japan, the European Molecular Biology
Laboratory (EMBL) Nucleotide Sequence Database, and
the GenBank in collaboration (DDBJ/EMBL/GenBank)
under the project accessions AAFX00000000 (soil),
AAFY00000000 (whale fall 1), AAFZ00000000 (whale
fall 2), and AAGA00000000 (whale fall 3). For each
project, the version described in this paper is the
first version, AAFX01000000, AAFY01000000,
AAFZ01000000, and AAGA01000000. The 16S rRNA
sequences from the soil and three whale fall samples
have been deposited under GenBank accession nos.
AY921654 to AY922252. The metagenomic data will
also be incorporated into the U.S. Department of
Energy Joint Genome Institute Integrated Microbial
Genomes system (www.jgi.doe.gov/) to facilitate
detailed comparative analysis of the data in the context
of all publicly available complete microbial genomes.
Supporting Online Material
Materials and Methods
Figs. S1 to S7
References and Notes
23 November 2004; accepted 4 February 2005
A Cellular MicroRNA Mediates
Antiviral Defense in Human Cells
In eukaryotes, 21- to 24-nucleotide-long RNAs engage in sequence-specific
interactions that inhibit gene expression by RNA silencing. This process has
regulatory roles involving microRNAs and, in plants and insects, it also forms
the basis of a defense mechanism directed by small interfering RNAs that
derive from replicative or integrated viral genomes. We show that a cellular
microRNA effectively restricts the accumulation of the retrovirus primate
foamy virus type 1 (PFV-1) in human cells. PFV-1 also encodes a protein, Tas,
that suppresses microRNA-directed functions in mammalian cells and displays
cross-kingdom antisilencing activities. Therefore, through fortuitous recogni-
tion of foreign nucleic acids, cellular microRNAs have direct antiviral effects in
addition to their regulatory functions.
In plants and insects, viral double-stranded
RNA is processed into small interfering RNAs
(siRNAs) by the ribonuclease (RNase) III en-
zyme Dicer. These siRNAs are incorporated
into the RNA-induced silencing complex to
target the pathogen_s genome for destruc-
tion (1, 2). Plant and insect viruses can
counter this defense with silencing suppres-
www.sciencemag.org SCIENCE VOL 308 22 APRIL 2005
sor proteins, which often have adverse side
effects on microRNA (miRNA) functions
(3, 4). Although undisputed in plants and
insects, a defensive role for RNA silencing
in vertebrates has not been demonstrated.
Virus-derived small RNAs have not been
detected in infected vertebrate cells, with
the exception of miRNAs produced by the
Epstein-Barr virus, but the role of those
molecules remains unclear (5). Moreover,
some mammalian virus-encoded proteins that
suppress RNA silencing have only been
investigated in heterologous systems (6).
Because RNA silencing suppresses mobili-
zation of endogenous retroviruses in plants,
yeast, worms, and flies (7), we reasoned that
retrotransposition of mammalian exogenous
viruses might also be subject to this process.
Therefore, we studied the primate foamy
virus type 1 (PFV-1), a complex retrovirus
(akin to human immunodeficiency virus) that,
in addition to the Gag, Pol, and Env proteins,
produces two auxiliary factors, Bet and
Tas, from the internal promoter (IP) (Fig.
PFV-1 accumulation was strongly en-
hanced in 293T cells expressing the P19
silencing suppressor (Fig. 1B). This sug-
gested that a siRNA and/or miRNA pathway
limits PFV-1 replication in human cells, be-
cause P19 specifically binds to and inacti-
vates both types of small RNAs (4, 9, 10).
Viral sequences spanning the 12-kb-long
PFV-1 genome (Fig. 1A) were fused to the
3¶ untranslated region (UTR) of a green flu-
orescent protein (GFP)–tagged reporter gene,
Propre de Recherche (UPR) 2357, Insti-
tut de Biologie Mole
culaire des Plantes, 12 rue du
ral Zimmer, 67084 Strasbourg Cedex, France.
Proligo, Paris, France.
CNRS UPR9051, Ho
Louis, Paris, France.
INSERM U462, Ho
*To whom correspondence should be addressed.
E-mail: email@example.com (C.-H.L.); olivier.
Fig. 1. RNA silencing
limits PFV-1 accumu-
lation in mammalian
cells. (A) Schematic of
the PFV-1 genome. Bent
arrows indicate the start
of transcription between
the 5¶-proximal long-
terminal repeat (LTR)
and the IP. Viral se-
quences (F1 to F10) used
for GFP transcriptional
fusions are indicated.
(B) mRNA accumula-
tion from PFV-1 in 293T
cells that do (þ)ordo
not (–) stably express
the P19 protein. Cells
were harvested 48 hours
after transfection. North-
ern analysis confirms
P19 expression. rRNA,
ethidium bromide stain-
ing of ribosomal RNA;
NI, noninfected. (C) The
GFP sensors F1 to F11 were transfected together with (þ) or in the absence of (–) PFV-1. Their
expression was assayed 48 hours later by Northern (first upper panel) and Western (fourth panel)
analysis. (Second upper panel) PFV-1 RNA accumulation. (Bottom) Staining of total protein for
loading control. Relative RNA or protein accumulation is shown at the bottom of each panel, with
control levels arbitrarily set to 1.
Fig. 2. miR-32 effectively
limits PFV-1 replication. (A)
Position of the computa-
tionally predicted miR-32
target relative to PFV-1
transcripts. (B) The miR-
32 target sequence or a
mutated form thereof (–)
was fused to the 3¶UTR
of a GFP reporter gene (þ).
Constructs were trans-
fected in HeLa cells and
harvested 48 hours later.
GFP and GFP mRNA ac-
cumulation were assess-
ed by Western (top) and
Northern (bottom) analy-
sis. (C) HeLa cells were
transfected with PFV-1 to-
gether with LNAs (10 nM)
directed against miR-32 or
miR-23. Total RNA was
extracted 48 hours after
transfection and subjected
to Northern analysis. (D)
PFV-1 was transfected in
HeLa cells (transfection 1).
Separate cells were trans-
fected with a luciferase-
based reporter (Luc) driven
by the PFV-1 IP, which is
activated by the transacti-
vator Tas (transfection 2).
Transfections 1 and 2 were mixed 24 hours later and further cocultured for 48
hours. Luciferase expression in cells from transfection 2, resulting from their
infection by virions released from transfection 1, was then quantified. hpt,
hours post-transfection. (E) The miR-32 target sequence within PFV-1D32
contains two synonymous mutations (arrows). Northern analysis of mutant
and wild-type virus mRNAs was carried out 48 hours after transfection.
22 APRIL 2005 VOL 308 SCIENCE www.sciencemag.org
and the resulting constructs (F1 to F11) were
cotransfected with PFV-1 into baby hamster
kidney (BHK) 21 cells. Any viral-derived
siRNA would induce RNA silencing of the
corresponding reporter fusions, diagnosed as
reduced GFP mRNA accumulation. However,
the mRNA levels from those constructs were
similar in noninfected and infected cells (Fig.
1C). Use of a highly sensitive RNase pro-
tection assay likewise failed to provide evi-
dence for viral-derived siRNAs (fig. S1).
The GFP levels from fusion F11 were dis-
proportionably reduced compared to the accu-
mulation of the F11 mRNA (Fig. 1C). They
were also reduced compared to the GFP levels
from constructs F2 and F10. Although a pos-
sible result of intrinsic protein instability, the
effect was reminiscent of the translational in-
hibition directed by animal miRNAs (11).
However, it was independent of the presence
or absence of PFV-1 (Fig. 1C), suggesting that
any miRNA involvement was likely cellular
rather than viral. Using the DIANA-microT
algorithm (12), we found a high probability hit
(free energy of –21.0 kcal/mol) between the
PFV-1 F11 sequence and the human miR-32
(Fig. 2A) (13). The predicted miR-32 target
sequence was sufficient to promote transla-
tion inhibition of the GFP mRNA (Fig. 2B),
unlike a derivative thereof that carried four
mutations disrupting annealing of the small
RNA. Moreover, translation inhibition by
miR-32 was suppressed in P19-expressing
cells (fig. S2).
The miR-32 target is in open reading frame
(ORF) 2, shared by the Bet and EnvBet
proteins, and is also within the 3¶UTR of all
remaining PFV-1 mRNAs (Fig. 2A). To ad-
dress the antiviral effect of miR-32, we used
antisense locked nucleic acid (LNA) oligo-
nucleotides (fig. S3), which yield highly stable
hybrids (14). In HeLa and BHK-21 cells, the
transfected anti-miR-32 LNA prevented
translation inhibition by miR-32, whereas a
control LNA with antisense sequence of the
unrelated miR-23 did not (fig. S3). At LNA
concentrations of 10 nM, accumulation of
PFV-1 mRNAs was higher in the anti-miR-
32–treated cells than in the anti-miR-23–
treated cells (Fig. 2C). Use of a luciferase-
based assay also indicated that the anti-
miR-32, unlike the anti-miR-23, almost
doubled progeny virus production (Fig. 2D).
Although these results are consistent with
an antiviral effect of miR-32, we could not
discard the possibility of an indirect action of
anti-miR-32 LNA causing, for instance,
ectopic expression of cellular miR-32 targets,
which could in turn increase viral fitness. The
miR-32 target sequence in PFV-1 was thus
modified to contain two synonymous muta-
tions that abolished the miR-32 pairing
but preserved the Bet amino acid content
(Fig. 2E). The mRNA levels from the miR-
32–resistant virus (PFV-1D32) were three
times as high as those from the unmodified
virus, consistent with the anti-miR-32 re-
sults (Fig. 2, E and C). Therefore, miR-32 ex-
erts a direct, sequence-specific effect against
Does PFV-1 encode a silencing suppres-
sor to counter the antiviral effect of miR-32?
The constitutive presence of miR-32 required
that the putative suppressor be synthesized
precociously, which is the case of the Tas and
Bet proteins (Fig. 2A). As Bet is dispensable
for productive replication, Tas appears the
most likely candidate (15). miR-32–mediated
translational inhibition was indeed suppressed
in Tas-expressing BHK21 cells (Fig. 3A). This
was not specific for the sequence or activity
of miR-32, because Tas, like P19, also sup-
pressed endonucleolytic cleavage of GFP sen-
sors carrying a perfect miR-23 target (Fig. 3B
and fig. S2). Probably as a consequence of its
suppressor function, Tas promoted the nonspe-
cific overaccumulation of all cellular miRNAs
inspected, which we also observed 5 days after
miRNA overaccumulation is also seen with
several plant viral suppressors that interfere
with the miRNA pathway (3, 4).
To validate the silencing suppression
activity of Tas in a heterologous system, we
used an Arabidopsis line expressing an RNA
interference (RNAi) construct targeted against
chalcone synthase (CHS), which is responsible
for the brown seed-coat pigmentation (4). This
line accumulates CHS siRNAs and, conse-
Fig. 4. (A) Transgenic Tas suppresses CHS RNAi in Arabidopsis.(B) Northern analysis of CHS
siRNAs in two independent Tas-expressing lines. Col0, nontransformed plants; CHS, the reference
RNAi line. (C) Developmental defects and (D) miRNA accumulation in Tas-expressing Arabidopsis.
miR156 and miR172 are evolutionarily conserved miRNAs that promote cleavage and translation
inhibition, respectively. miR163 is a cleavage-promoting, Arabidopsis-specific miRNA.
Fig. 3. Tas suppresses miRNA-directed silencing in mammalian cells. (A) The reporter constructs
used in Fig. 2B were transected in control BHK21 cells (mock) or in cells stably expressing Tas. GFP
expression was assayed by Western analysis (top) 48 hours after transfection. Tas expression was
confirmed by Northern analysis (bottom). (B) A sequence with 100% complementarity to miR-23
(þ) or a mutated derivative thereof (–) was inserted into the 3¶UTR of the GFP reporter gene.
Constructs were transfected in BHK21 cells (mock) or in cells stably expressing Tas (Tas), and the
GFP mRNA was assayed by Northern analysis 48 hours later. (C) Northern analysis of cellular
miRNAs from BHK21 cells expressing (þ) or not expressing (–) Tas (left) and from noninfected (–)
or PFV-1–infected (þ) BHK21 cells (right). Total RNA was extracted 5 days after infection.
www.sciencemag.org SCIENCE VOL 308 22 APRIL 2005
quently, produces pale yellow seeds (Fig.
4A, left). Transgenic Tas expression restored
anthocyanin synthesis (Fig. 4A, right) because
of a strong decrease in CHS siRNA levels
(Fig. 4B). Tas-expressing plants also ex-
hibited developmental anomalies, including
leaf elongation and serration (Fig. 4C),
reminiscent of those elicited in Arabidopsis
by viral suppressors interfering with miRNA
functions (3, 4). As in mammalian cells, Tas
enhanced miRNA accumulation (Fig. 4D),
independently of their nature or mode of
action, suggesting that it suppresses a fun-
damental step shared between the miRNA and
siRNA pathways that is conserved from plants
These results indicate that RNA silencing
limits the replication of a mammalian virus,
PFV-1, and that a cellular miRNA contrib-
utes substantially to this response. As a coun-
terdefense, PFV-1 produces Tas, a broadly
effective silencing suppressor. Because all
our experiments were conducted with Tas-
expressing viruses, because of the essential role
of the protein for replication (15), the strong
effect of Tas on siRNA accumulation observed
in Arabidopsis could account for our failure to
detect siRNAs in mammalian cells (fig. S1).
Therefore, we do not yet rule out their impli-
cation in the antiviral response reported here.
Our findings with miR-32 and PFV-1 were
in fact anticipated in plants by Llave, who
pointed out several near-perfect homologies
between Arabidopsis small RNAs and viral
genomes (16). The chances of a match be-
tween cellular miRNAs and foreign (i.e.,
viral) RNAs increase proportionally with the
size of sampled sequences. The extent to
which cellular miRNAs will be selected to
target pathogen genomes upon their initial
interaction with viruses may vary. En-
dogenous viruses might effectively coevolve
with miRNAs for defensive or developmen-
tal purposes (17, 18), such that viral control
might eventually constitute the sole function
of some cellular miRNAs. Exogenous viruses
with high mutation rates could, on the other
hand, rapidly escape this miRNA interference
through modification of the small RNA com-
plementary regions (19).
Our results support the emerging notion that
miRNAs might be broadly implicated in viral
infection of mammalian cells, with either posi-
tive or negative effects on replication (5, 20).
They also indicate that virtually any miRNA
has fortuitous antiviral potential, independent-
ly of its cellular function. Moreover, because
the repertoire of expressed miRNAs likely
varies from one cell type to another (11), this
phenomenon could well explain some of the
differences in viral permissivity observed be-
tween specific tissues.
Note added in proof: Recent findings
indicate that a single 8-oligonucleotide seed
(small RNA positions 1 to 8 from the 5¶ end)
is sufficient to confer strong regulation by
animals miRNAs. Thus, fortuitous targeting
of foreign RNAs by cellular miRNAs could
be widespread (21, 22).
References and Notes
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O. Voinnet, Plant Cell 16, 1235 (2004).
5. S. Pfeffer et al., Science 304, 734 (2004).
6. W. X. Li et al., Proc. Natl. Acad. Sci. U.S.A. 101, 1350
7. V. Schramke, R. Allshire, Curr. Opin. Genet. Dev. 14,
8. M. Heinkelein et al., EMBO J. 19, 3436 (2000).
9. K. Ye, L. Malinina, D. J. Patel, Nature 426, 874
10. J. M. Vargason, G. Szittya, J. Burgyan, T. M. Tanaka
Hall, Cell 115, 799 (2003).
11. D. P. Bartel, Cell 116, 281 (2004).
12. M. Kiriakidou et al., Genes Dev. 18, 1165 (2004).
13. M. Lagos-Quintana, R. Rauhut, W. Lendeckel, T. Tuschl,
Science 294, 853 (2001).
14. J. S. Jepsen, M. D. Sorensen, J. Wengel, Oligonucleo-
tides 14, 130 (2004).
15. G. Baunach, B. Maurer, H. Hahn, M. Kranz, A. Rethwilm,
J. Virol. 67, 5411 (1993).
16. C. Llave, Mol. Plant Pathol. 5, 361 (2004).
17. H. Seitz et al., Nat. Genet. 34, 261 (2003).
18. S. Mi et al., Nature 403, 785 (2000).
19. A. T. Das et al., J. Virol. 78, 2601 (2004).
20. S. Lu, B. R. Cullen, J. Virol. 78, 12868 (2004).
21. J. Brennecke et al., PLoS Biol. 15, e85 (2005).
22. B. Lewis, C. Burge, D. Bartel, Cell 120, 15 (2005).
23. We thank S. W. Ding, B. Cullen, and P. Zamore for
critical reading of the manuscript and access to
data; members of the Voinnet lab for discussions;
and R. Wagner’s team for excellent plant care. Sup-
ported by an Action The
matique Incitative sur
Programme from the CNRS, the Fondation pour
la Recherche Me
dicale, and the Universite
Supporting Online Material
Materials and Methods
Figs. S1 to S4
References and Notes
16 December 2004; accepted 8 February 2005
Postsecretory Hydrolysis of
Nectar Sucrose and Specialization
in Ant/Plant Mutualism
J. Rattke, W. Boland
Obligate Acacia ant plants house mutualistic ants as a defense mechanism
and provide them with extrafloral nectar (EFN). Ant/plant mutualisms are
widespread, but little is known about the biochemical basis of their species
specificity. Despite its importance in these and other plant/animal inter-
actions, little attention has been paid to the control of the chemical com-
position of nectar. We found high invertase (sucrose-cleaving) activity in
Acacia EFN, which thus contained no sucrose. Sucrose, a disaccharide common
in other EFNs, usually attracts nonsymbiotic ants. The EFN of the ant acacias
was therefore unattractive to such ants. The Pseudomyrmex ants that are
specialized to live on Acacia had almost no invertase activity in their digestive
tracts and preferred sucrose-free EFN. Our results demonstrate postsecretory
regulation of the carbohydrate composition of nectar.
Many plants produce nectar in their flowers
(floral nectar) and on vegetative parts Eextrafloral
nectar (EFN)^ to mediate their interactions
with animals. The chemical composition of
nectar strongly affects the identity and behav-
ior of the attracted insects and thus the out-
come of the interaction (1–3). Particularly
important chemical factors include amino
acid content (4–6) and the ratio and amount
of the main sugars: glucose, fructose, and
sucrose (3). However, previous studies have
focused on nectar as a Bstanding crop,[ leav-
ing open the question of how its chemical
composition is controlled.
Floral nectar is produced to attract polli-
nators, whereas EFN acts to defend plants
indirectly Esee (7) for a description of EFN in
more than 80 plant families^.Mostinter-
actions among animals and both floral and
extrafloral nectars are thus believed to be
mutualistic. Highly specialized mutualisms
are surprisingly rare in nature, because they
are associated with specific coevolutionary
problems (8). In mutualisms in general, one
partner provides a service for the other and
receives some kind of reward (9). In defen-
sive ant/plant mutualisms, the presence of
ants serves as an indirect defense mechanism
and, in return, they receive food rewards and/
or nesting space (10).
Ant/plant mutualisms differ widely in their
specificity and thus are particularly suitable for
Department of Bioorganic Chemistry, Max-Planck-
Institute for Chemical Ecology, Hans-Kno
D-07745 Jena, Germany.
*To whom correspondence should be addressed at FB 9
ty of Duisburg-Essen, Universita
tsstraße 5, D-45117
Essen, Germany. E-mail: Heil_Martin@web.de
22 APRIL 2005 VOL 308 SCIENCE www.sciencemag.org