Wasp Gene Expression Supports an Evolutionary Link Between Maternal Behavior and Eusociality

Abstract

The presence of workers that forgo reproduction and care for their siblings is a defining feature of eusociality and a major challenge for evolutionary theory. It has been proposed that worker behavior evolved from maternal care behavior. We explored this idea by studying gene expression in the primitively eusocial wasp Polistes metricus. Because little genomic information existed for this species, we used 454 sequencing to generate 391,157 brain complementary DNA reads, resulting in robust hits to 3017 genes from the honey bee genome, from which we identified and assayed orthologs of 32 honey bee behaviorally related genes. Wasp brain gene expression in workers was more similar to that in foundresses, which show maternal care, than to that in queens and gynes, which do not. Insulin-related genes were among the differentially regulated genes, suggesting that the evolution of eusociality involved major nutritional and reproductive pathways.

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DOI: 10.1126/science.1146647
, 441 (2007); 318Science
et al.Amy L. Toth,
Link Between Maternal Behavior and Eusociality
Wasp Gene Expression Supports an Evolutionary
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Atlantic are available, except a recent record
from the Irish margin by Peck et al.(23). The
authors recorded identical d
18
O N. pachyderma (s)
and G. bulloides values during the LGM that were
attributed to a continuous discharge of meltwater
from the British Ice Sheet and year round mixing
that homogenized the upper waters.
Several processes can create variability in the
d
18
O of foraminifera. Seasonal variability was
interpreted by Ganssen and Kroon (19) to explain
why G. bulloides d
18
O was more positive than
G. inflata d
18
O in the modern North Atlantic at
57°N, which was attributed to a later seasonal
period of G. bulloides production further south.
The uniform d
18
O of the foraminifera during HEs
would require improbable ecological changes in
preferred depth-habitat zones or in seasonal
behavior if these values were not the result of
uniform upper–water-mass conditions. Upwell-
ing of
18
O-depleted water produced by brine-
rejection at higher latitudes might affect the d
18
O
of deeper-dwelling foraminifera; however , it is
difficult to imagine it influencing d
18
Ovaluesin
all three taxa. W ithout a decrease in temperature
by ~2.5°C, it is impossible to lower the salinity
by 0.8 per mil (‰)(24)(andbyextensionto
lower d
18
Oby0.5‰) while remaining along the
same isopycnal surface, which by itself would
correspond to a 0.5‰ increase in the d
18
Oof
calcite. Because these effects cancel each other
out, such a mechanism is inadequate to explain
the anomalously low d
18
O values in all three
planktonic foraminifera (Fig. 3B).
Intensified vertical mixing and deepening of
the mixed layer during HEs is the mechanism
remaining to explain the data. Atmospheric con-
ditions directly influence the mixed layer through
turbulence, and wind driven Langmuir circula-
tion could be the prime driver of the turbulence
(25, 26). As a result, the upper ocean often be-
comes well mixed to depths as great as 600 m
(27). Our d
18
O data from planktonic foramini-
fera that live at different depth ranges illustrate
the extent of this process, suggesting that during
the times of HEs the near-surface waters were
homogenized by stronger mixing.
During the last glacial cycle, large ice sheets
in the Northern Hemisphere and steeper merid-
ional temperature gradients in the atmosphere
must have reorganized atmospheric circulation.
As a result, winter sea-ice cover extended fur-
ther south, and glacial winds were stronger and
more zonal (28, 29). These winds would have
intensified the vertical mixing and turbulence in
the upper water masses. It is counterintuitive to
visualize such a mechanism during HEs when
the glacial North Atlantic was flooded with melt-
water resulting in stronger stratification. However,
our d
18
O data demonstrate that homogeniza-
tion of upper water masses did occur, suggesting
that this mechanism functioned at the core site.
Additional evidence of this mechanism comes
from the measurements of Ca
+2
and Na
+
ions
derived from sea salt and continental dust in the
Greenland ice core (30). Both Ca
+2
and Na
+
ions
in the ice core show rapid increases from their am-
bient concentration during stadials. Abrupt, many-
fold increases of these chemical species suggest
that storminess during the glacial period caused
stronger vertical mixing at the atmosphere/ocean
boundary at the subpolar and subtropical fronts.
The question of why weaker homogenization
of near-surface waters occurred during other D/O
cycles not associated with HEs could be raised,
because Greenland ice core data show similar
patterns of glaciochemical species. One possibil-
ity is that unfavorable composition or insufficient
volumes of meltwater were available to perturb
the near-surface waters during these D/O ice-
rafting cycles as the icebergs originated from
the smaller ice sheets. Hence, even though the
glacial climate was windier and stormier, the
near-surface waters continued to be stratified.
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31. We thank E. Goddard for helping to acquire part of the
isotope data and D. J. W. Piper and B. P. Flower for
discussions to improve an initial version of the manu-
script. H.R. thanks Fonds pour la Formation de
Chercheurs et l'Aide à la Recherche, Québec, for its
support through a postdoctoral fellowship. E.A.B. was
supported by grants from NSF and the Cambridge–
Massachusetts Institute of Technology.
Supporting Online Material
www.sciencemag.org/cgi/content/full/1146138/DC1
SOM Text
Fig. S1
Table S1
References
6 June 2007; accepted 11 September 2007
Published online 20 September 2007;
10.1126/science.1146138
Include this information when citing this paper.
Wasp Gene Expression Supports an
Evolutionary Link Between Maternal
Behavior and Eusociality
Amy L. Toth,
1
*
Kranthi Varala,
2
Thomas C. Newman,
1
Fernando E. Miguez,
2
Stephen K. Hutchison,
3
David A. Willoughby,
3
Jan Fredrik Simons,
3
Michael Egholm,
3
James H. Hunt,
4
Matthew E. Hudson,
2
Gene E. Robinson
1,5
The presence of workers that forgo reproduction and care for their siblings is a defining feature of
eusociality and a major challenge for evolutionary theory. It has been proposed that worker behavior
evolved from maternal care behav ior. We explor ed this idea by studying gene expression in the
primitively eusocial wasp Polistes metricus . Because little genomic information existed for this species,
we used 454 sequencing to generate 391,157 brain complementary DNA reads, resulting in robust hits
to 3017 genes from the honey bee genome, from which we identified and assayed orthologs of 32
honey bee behaviorally related genes. Wasp brain gene expression in workers was more similar to
that in foundresses, which show maternal care, than to that in queens and gynes, which do not.
Insulin-related genes were among the differentially regulated genes, suggesting that the evolution of
eusociality involved major nutritional and reproductive pathways.
A
major challenge in biology is to under-
stand the evolution of animal society in
molecular terms. Eusociality is the most
extreme form of cooperation, typified by indi-
viduals that care for siblings rather than repro-
duce themselves, i.e., “workers. ” The evolution
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of eusociality has been ascribed to kin or colony-
level selection (1, 2), but these explanations do
not specify mechanistic routes.
It has long been suggested (3–5) that sibling
care by hymenopteran (ant, bee, wasp) workers
evolved from maternal care, which involves pro-
visioning brood by foraging for food and then
feeding them. According to this idea, two prin-
cipal behaviors exhibited by solitary Hymenop-
tera, reproduction (egg-laying) and maternal care
(brood provisioning), became uncoupled during
the early stages of social evolution (6), and these
behaviors eventua lly occurred in separate castes,
queens and workers, respectively (7). Linksvayer
and Wade (8) added a molecular dimension to
this idea by predicting that sibling care and
maternal care behaviors should be regulated by
similar patterns of gene expression.
We used Polistes paper wasps to test Linksvayer
and Wade’s idea. Polistes are primitively eusocial,
which means that although individuals special-
ize as either workers or reproductive individu-
als, these two castes are less distinct than in
advanced eusocial species. In Polistes, both
workers and reproductives display provision-
ing behavior, but at different points in the life
of a colony. Advanced eusocial insects, by con-
trast, have morphologically distinct queen and
worker castes, and in some species, such as the
honey bee, queens no longer exhibit any ma-
ternal care, which precludes comparing the
molecular basis of sibling and maternal care.
Primitively eusocial insects like Polistes afford
the opportunity to explore the molecular basis
of maternal and worker behavior within a sin-
gle species.
We measured brain gene expression in 87
individuals from four distinct behavioral groups
of females from naturally occurring colonies of
the temperate species Polistes metricus (Fig. 1A).
Foundresses are females that establish new col-
onies in the spring, often as solitary individuals.
Foundresses exhibit both reproductive (egg-
laying) and maternal (foraging and brood-
feeding) behavior . After rearing a first generation
of female brood that develop into workers,
successful foundresses become queens and cease
caring for brood. W orkers take over provisioning
the brood—their siblings—by foraging for food
and then feeding them; workers show little, if
any, reproductive behavior . By contrast, queens
focus exclusively on reproductive behavior.
Gynes are reared late in the season; they engage
in no reproductive or maternal care behavior (9).
After successfully mating, gynes overwinter and
then become foundresses (10). We hypothesized
that brain gene expression patterns in P. m et r ic u s
workers and foundresses should be most similar
to each other from among these four groups, be-
cause they both show brood provisioning behav-
ior despite their different reproductive status.
Alternatively, if brain gene expression more
closely reflects reproductive behavior, expression
in foundresses and queens should be most similar
to each other .
Social behavior is a complex and polygenic
trait, so an appropriate test of the idea that ma-
ternal and worker behavior share a common mo-
lecular basis requires analysis of multiple genes
in different pathways. But Polistes wasps, though
venerable models for studies of social evolution
(11, 12), have until recently lacked genomic
sequence information (13). T o provide a ready
source of test genes for quantitative reverse
transcription–polymerase chain reaction analysis,
we used 454 sequencing to obtain 45 megabases
(Mb) in 391,157 cDNA sequence fragments
from the P. m et r i cu s brain transcriptome (14).
We were interested to see whether this low-cost,
high-throughput sequencing method would be
successful for this purpose, despite short se-
1
Department of Entomology and Institute for Genomic
Biology, University of Illinois at Urbana-Champaign, Urbana,
IL 61801, USA.
2
Department of Crop Sciences, University of
Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
3
454
Life Sciences, Branford, CT 06405, USA.
4
Department of Bi-
ology, University of Missouri at St. Louis, St. Louis, MO 63121,
USA.
5
Neuroscience Program, University of Illinois at Urbana-
Champaign, Urbana, IL 61801, USA.
*To whom correspondence should be addressed. E-mail:
amytoth@uiuc.edu
Fig. 1. P. metricus wasp
brain gene expression
analysis tests the predic-
tion that maternal and
worker (eusocial) behavior
share a common molecu-
lar basis. (A) Similarities
and differences in repro-
ductive and brood provi-
sioning status for the four
behavioral groups ana-
lyzed in this study: found-
resses (n = 22), gynes (n =
20), queens (n =23),and
workers (n = 22). Each
individual wasp (total of
87) was assigned to a
behavioral group on the
basis of physiological
measurements (14). (B
to D) Results for 28 genes
selected for their known
involvement in worker
(honey bee) behavior. (B)
Heatmap of mean expres-
sion values by group and
a summary of analysis of
variance (ANOVA) results
for each gene. Genes were
clustered by K-means
clustering (37); those in
red showed significant
differences (ANOVA, P <
0.05; table S1) between
the behavioral groups.
P. metricus gene names
were assigned on the ba-
sis of orthology to honey
bee genes (reference in
parentheses); putative
functions were assigned
on the basis of similarity
to Drosophila melanogas-
ter genes. (C) Results of
linear discriminant anal-
ysis show that foundress
and worker brain profiles
are more similar to each
other than to the other
groups. (D) Results of hierarchical clustering show the same result (based on group mean expression
value for each gene). Four genes (PmVg, Pmg5sd, PmGlyP,andPmRfaBp) were excluded from these
analyses because they showed high levels of expression in tissue adjacent to the brain (fig. S2); results
for all three analyses were similar with and without these four genes (fig. S3).
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quence read lengths (average of 120 bp) and an
estimated 100- to 150-million-year divergence
time between P. metricus and the honey bee,
Apis mellifera (15), the most closely related spe-
cies with a sequenced genome to use as refer-
ence (16).
We generated a map of the honey bee genome
combined with known transcripts and their
relative abundance in the combined bee ex-
pressed sequence tag (EST) data sets (16–18).
P. me t r icu s transcript fragments predicted to
encode proteins orthologous to those encoded
by A. mellifera genes were plotted on the map
according to the number of fragments identified
for a particular locus; matches were found for
39% of all honey bee mRNAs. The relative abun-
dance of P. metricus sequence fragments corre-
sponded well with the abundance of A. mellifera
ESTs for the respective loci (Fig. 2). The com-
bined P. metricus–A. mellifera transcriptome data
set was then used to select the genes for this study .
Prior information allowed us to focus on
genes implicated in honey bee foraging and pro-
visioning behavior, rather than a set of random-
ly chosen genes that might be less informative.
We selected 32 genes (Fig. 1B and table S1)
from the P. metricus EST set that are orthologs
of A. mellifera genes known to be associated in
some way with worker bee behavior, based on
results from studies with microarrays (22 genes)
(19, 20) and candidate genes (10 genes) (21–29).
Twenty-two of the genes have been shown by
microarray analysis to be both differentially
expressed in the brains of honey bees engaged
in foraging or feeding brood [on the list of the
“top 100” genes most consistently associated
with bee foraging behavior (19)] and regulated
by juvenile hormone (20), which also causes
worker bee foraging behavior (30). Five can-
didate genes are differentially expressed in hon-
ey bees engaged in foraging or feeding brood
(21–24, 29), three of which also have been shown
to play causal roles in the regulation of worker
bee foraging behavior (21, 22, 31). Five addi-
tional candidate genes involved in insulin sig-
naling were selected because this pathway is
implicated in honey bee queen-worker caste
determination (25, 26, 32) and worker foraging
behavior (27, 28). Patterns of gene expression in
P. metricus were not used as criteria for gene
selection.
There was a robust association between indi-
vidual wasp brain gene expression and naturally
occurring behavioral differences among the wasp
groups. Leave-one-out cross-validation analysis
(19) resulted in 68, 69, 70, and 47% correct
assignments to the foundress, gyne, queen, and
worker groups, respectively . For the less con-
servative resubstitution method (33), the results
were 89, 100, 100, and 95%. The predictions
from both classification methods were signifi-
cantly better than random (Chi-square tests, P <
0.0001, 25% expected). This honey bee–derived
gene set thus demonstrates extensive brain regu-
lation across the four wasp groups, making it an
informative set to explore the molecular relation-
ship between maternal and worker behavior in
P. metricus.
Sixty-two percent of the genes in the gene set
were differentially regulated in P. m e tr i c u s as a
function of reproductive or provisioning behavior
(Fig. 1B and table S1). Multivariate analysis of
variance showed that brain gene expression
varied significantly with reproduction (F = 3.28,
P = 0.0002) and provisioning (F = 4.76, P <
0.0001), with a signi ficant provi sioning ×
reproduction interaction (F = 2.48, P = 0.002).
Three out of the five insulin-related genes
showed significant associations with provision-
ing and/or reproductive behavior, consistent with
known nutritional effects on behavior and phys-
iology in honey bees and other social insects (34).
Three statistical analyses demonstrated that
brain gene expression for worker wasps was
more similar to that of maternal females (found-
resses) than to that of females not showing
maternal care (queens and gynes). First, K-means
clustering (Fig. 1B and fig. S1) revealed five
clusters of coexpressed genes. The first cluster
contained genes (n = 8) that showed coex-
pression in foundresses and workers compared
to queens and gynes. The second cluster of
genes (n = 7) was mainly characterized by up-
regulation in gynes, and the third (n =6)by
down-regulation in queens, but in both of these
clusters, foundresses and workers also showed
patterns of expression that were similar to each
other (Fig. 1B and fig. S1).
The second statistical analysis, linear discrim-
inant (LD) analysis, also showed similarities be-
tween foundress and worker brain gene expression
(Fig. 1C). A plot of LD1 versus LD2 (which
accounted for 90% of the variation in brain gene
expression across all four groups) revealed group-
specific expression patterns, but foundresses and
workers showed the greatest overlap. This is
consistent with the poorer performance of clas-
sification methods (described above) for those
two groups; overlap in gene expression patterns
made them difficult to distinguish from each
other. Gynes, which engage in neither reproduc-
tive nor provisioning behavior , were the most
distinct group. The third statistical analysis,
hierarchical clustering by group, supported the
patterns found in the other two analyses—brain
gene expression of workers and foundresses was
most similar, and that of gynes was most
divergent (Fig. 1D).
There are marked temporal changes in brain
gene expression as females shift from found-
ress to queen status, i.e., from maternal to repro-
ductive behavior. These findings demonstrate
heterochronic expression of genes associated
with maternal behavior, a form of plasticity that
is considered to be necessary for the evolution of
worker behavior (8). They also reflect the ap-
parent modularity of egg-laying and brood pro-
visioning behavior and their underlying regulatory
networks; this type of modularity also is thought to
be important in the evolution of novel traits (35).
We used the honey bee genome, together with
“next-generation” sequencing technology, to
rapidly bring genomics to the relatively closely
Fig. 2. A representation of
P. metricus brain transcripts
overlaid on a honey bee
genome template (16)
shows wide coverage and
similar transcript abundance
for P. metricus relative to
known honey bee tran-
scripts. P. metricus brain
cDNA sequence fragments
were matched as predicted
proteins to A. mellifera tran-
scripts with experimental
support (known cDNA or
EST sequences). A. mellifera
transcripts (red points, right
of axis) and their closest
P. metricus orthologs from
our survey (blue points, left
of axis) were then mapped
to the corresponding ge-
nomic locus in the A.
mellifera genome. The verti-
cal lines represent A. melli-
fera chromosomes 1 to 16.
The distance of each point from the midline is proportional to the logarithm of the abundance of the
mRNA (the number of sequences for each P. metricus or A. mellifera transcript corresponding to the
A. mellifera gene at that locus) (16– 18). P. metricus orthologs were obtained for a total of 3017 A.
mellifera transc ripts. The P. metricus transcriptome data contained putative orthologs for 39% of
known A. mellifera mRNAs. An additional 252,556 transcript sequence fragments obtained from
P. metricus did not have a clearly orthologous transcript in A. mellifer a.
10 11 12 13 14 15 16
Apis mellifera chromosome
23 4 56 7819
number of
transcripts
10000
1000
100
10
1
Apis mellifera transcript
Polistes metricus transcript
10
6
base pairs
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related wasp P. m et r ic u s; this is an early example
of the utility of 454 sequencing for transcriptomics
(36). Our results demonstrate that it is possible
to use species that have had their genomes se-
quenced as “hubs” to efficiently generate genomic
resources for clusters of related species that might
eachbeespeciallywellsuitedtoaddressparticular
evolutionary problems. This “h ub and spokes”
approach should enable genomics to be deployed
for a broader range of species than is currently
being done, until whole-genome sequencing of
eukaryote genomes becomes routine.
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K. J. Bilof, and S. Buck for assistance in the field;
D. Schejbal and S. Buck for permission to collect wasps at
field sites owned by the University of Illinois;
A. S. Escalante and K. J. Bilof for physiological
measurements; M. T. Henshaw for microsatellite analyses;
R. A. Gibbs for strategic assistance; C. W. Whitfield for
assistance with gene identific ation; R. Rego for brain
dissections and RNA extractions; Y. Li, Y. Lu, and
S. Zhong for assistance with statistical analysis;
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C. W. Whitfield, M. R. Berenbaum, S. A. Cameron,
J. L. Beverly, members of the Robinson laboratory, and
members of the University of Illinois Social Insect
Training Initiative for constructive comments on the
manuscript. Sup ported by the Illinois Sociogenomic
Initiative and NSF grant IOS-0641431 (G.E.R.). The
individual P. metricus sequences and flowgram data have
been uploaded to NCBI Trace Archive, TI range
1888756160 to 1889135944.
Supporting Online Material
www.sciencemag.org/cgi/content/full/1146647/DC1
Materials and Methods
Figs. S1 to S3
Tables S1 and S2
References
18 June 2007; accepted 19 September 2007
Published online 27 September 2007;
10.1126/science.1146647
Include this information when citing this paper.
JMJD6 Is a Histone
Arginine Demethylase
Bingsheng Chang, Yue Chen, Yingming Zhao, Richard K. Bruick*
Arginine methylation occurs on a number of proteins involved in a variety of cellular functions.
Histone tails are known to be mono- and dimethylated on multiple arginine residues where they
influence chromatin remodeling and gene expression. To date, no enzyme has been shown to
reverse these regulatory modifications. We demonstrate that the Jumonji domain–containing 6
protein (JMJD6) is a JmjC-containing iron- and 2-oxoglutarate–dependent dioxygenase that
demethylates histone H3 at arginine 2 (H3R2) and histone H4 at arginine 3 (H4R3) in both
biochemical and cell-based assays. These findings may help explain the many developmental
defects observed in the JMJD6
−/−
knockout mice.
I
ron- and 2-oxoglutarate–dependent dioxy-
genases have been shown to oxidize a variety
of substrates including metabolites, nucleic
acids, and proteins (1). A candidate dioxygenase,
JMJD6, shares extensive sequence and predicted
structural homology with an asparaginyl hydrox-
ylase (2, 3) as well as the JmjC domains found in
several histone lysine demethylases (fig. S1A)
(4–8). Given the predicted conservation of struc-
tural elements and key residues (9–11), it is likely
that JMJD6 retains an analogous catalytic ac-
tivity. Here we report in vitro and in vivo data that
clearly indicate that JMJD6 functions as an ar-
ginine demethylase.
To test whether JMJD6 demethylates the
N-terminal tails of histone H3 or H4, we incu-
bated bulk histones with JMJD6 in the presence
of Fe(II), 2-oxoglutarate, and ascorbate (12). An-
tibodies specific for various methylated sites on
histones H3 and H4 were used to assess demeth-
ylation. Although no lysine demethylation was
observed, a substantial reduction in H3R2me2
and H4R3me2 was observed in the presence of
JMJD6 compared with buffer alone (Fig. 1A).
These effects were site-specific as no changes in
dimethylarginine were seen at positions H3R17
or H3R26. Previously, no enzyme had been
shown to reverse regulatory arginine methylation,
although deiminases can convert methylarginin e
to citrulline via demethylimination (13, 14). How-
ever , the requisite chemistry is analogous to that
demonstrated for demethylation of alkylated ni-
trogens by other dioxygenases (fig. S1C).
To investigate the preference for the substrate
methylation state, we used antibodies specific
for either mono- or dimethylated (symmetric)
H4R3 (Fig. 1B). The recombinant JMJD6 was
able to demethylate H4R3me2 when either het-
erogeneous bulk histones or synthetic peptides
encompassing the N-terminal 30 residues of his-
tone H4 were used as substrates (Fig. 1C). To a
lesser extent, JMJD6 could also demethylate
H4R3me1-containing substrates (Fig. 1C). Mu-
tation of the residues predicted to mediate Fe(II)
binding (mut JMJD6) prevented demethylation
(Fig. 1C).
Department of Biochemistry, University of Texas South-
western Medical Center, 5323 Harry Hines Boulevard,
Dallas, TX 75390–9038, USA.
*To whom correspondence should be addressed. E-mail:
richard.bruick@utsouthwestern.edu
19 OCTOBER 2007 VOL 318 SCIENCE www.sciencemag.org444
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