A Sugar Gustatory Receptor Identified from the Foregut
of Cotton Bollworm Helicoverpa armigera
Wei Xu & Hui-Jie Zhang & Alisha Anderson
Received: 12 September 2012 /Revised: 5 November 2012 /Accepted: 6 November 2012 /Published online: 6 December 2012
#The Author(s) 2012. This article is published with open access at Springerlink.com
Abstract Helicoverpa armigera (Hübner) is one of the
most polyphagous and cosmopolitan pest species, the larvae
of which feed on numerous important crops. The gustatory
system is critical in guiding insect feeding behavior. Here, we
identified a gustatory receptor from H. armigera, HaGR9,
which shows high levels of identity to DmGR43a from
Drosophila melanogaster and BmGR9 from Bombyx mori.
Reverse transcriptase PCR (RT-PCR) revealed HaGR9 is
highly expressed in larval foregut, with little or no expression
in other chemosensory tissues. Membrane topology studies
indicated that, like two previously studied B. mori GRs,
BmGR8 and BmGR53, HaGR9 has an inverted topology
relative to G protein-coupled receptors (GPCRs), an intracel-
lular N-terminus and an extracellular C-terminus. Calcium
imaging studies confirmed HaGR9 is a sugar receptor show-
ing dose-dependent responses to D-galactose, D-maltose, and
D-fructose. This highly-expressed foregut-specific gustatory
receptor may contribute to the regulation of larval feeding
Insects display strong feeding preferences. The gustatory
system plays a critical role in guiding insect feeding behav-
ior. Gustatory stimuli from the environment are recognized
by gustatory receptors (GRs), which are located in the gusta-
tory sensilla distributed throughout the insect body. Despite a
growing body of knowledge about the insect gustatory sys-
tem, little is known about the molecular and cellular mecha-
nisms that underlie recognition of gustatory signals.
Insect GR genes were first identified from Drosophila
melanogaster genome based on a bioinformatics approach
(Clyne et al., 2000). These proteins were found by using
algorithms to search for seven-transmembrane domains, but
they are strikingly different and share no sequence similarity
with vertebrate GRs (Clyne et al., 2000). Furthermore, the
topology is inverted compared to the classic G-Protein
Coupled Receptors (GPCRs) (Benton et al., 2006; Robertson
and Wanner, 2006; Zhang et al., 2011). Insect gustatory
receptors have been classified into “GR43a-like” (Sato et al.,
2011), “CO2” (Jones et al., 2007), “sugar” ( Dahanukar et al.,
2001,2007;Chybetal.,2003; Jiaoetal.,2007,2008;Slone et
al., 2007), and “bitter” clades (Wanner and Robertson, 2008;
Lee et al., 2009). To date, much attention has been paid to the
gustatory receptors of Drosophila (Dahanukar et al., 2001,
2007; Dunipace et al., 2001; Slone et al., 2007; Jiao et al.,
2007, 2008; Gardiner et al., 2008; Lee et al., 2010, 2012;;
on other insect species such as Anopheles gambiae (Hill et al.,
2002), Bombyx mori (Wanner and Robertson, 2008),
Tribolium castaneum (Richards et al., 2008), Apis mellifera
(Robertson and Wanner, 2006), and Acyrthosiphon pisum
(Smadja et al., 2009), the research is extending to a diverse
range of species. However, rarely are studies on gustatory
receptors carried out on serious agricultural pests such as
The cotton bollworm, H. arimgera (Hübner), is one of the
most destructive insect species. The larvae feed on numerous
important cultivated crops such as cotton, peanuts, soybeans,
W. Xu:H.-J. Zhang:A. Anderson (*)
CSIRO Ecosystem Sciences, Black Mountain, Australian Capital
Territory 2601, Australia
J Chem Ecol (2012) 38:1513–1520
and maize. Helicoverpa armigera is distributed widely in
US$ 2 billion annual losses worldwide despite the use of
insecticides (Sharm, 2001). The study of the H. armigera
gustatory system may elucidate the underlying mechanisms
that influence its feeding behavior and help to develop new
insect-control strategies for such polyphagous pests.
In this study, we report the first gustatory receptor,
HaGR9, from H. armigera. Further, we expressed HaGR9
in insect cells, and functionally characterized its topology
and responses to substrates relevant to feeding behavior.
Methods and Materials
Insects and Cell Culture Helicoverpa armigera were fed
artificial food in the laboratory under conditions described
previously (Akhurst et al., 2003). Spodoptera frugiperda
Sf9 cells (Invitrogen, USA) were cultured in Sf-900 II
SFM medium according to the manufacturer’s instructions.
Drosophila melanogaster Schneider’s S2 cells were main-
tained as a suspension culture in Drosophila Schneider’s
medium (Invitrogen, USA) adapting cells from 10 % fetal
bovine serum (FBS, Invitrogen, USA) to 1 % FBS accord-
ing to the Serum Halving Method. Cells were incubated at
28 °C and subcultured to a final density of 1~2 x 106cells/
ml when they reached a density of ~6-10 x 106cells/ml
(Mather and Roberts, 1998).
RNA Isolation, cDNA Synthesis and PCR Approximately
fifty antennae, fifty mouthparts, ten foreguts, five midguts,
and ten hindguts were dissected from mixed sex 5th instars.
Approximately fifty tarsi and antennae were collected from
male and female adults (d 1 – d 5). All collected tissues were
stored immediately in RNA Later (Invitrogen, USA). Total
RNA was purified using RNeasy (Qiagen, USA) or
RNAqueous (Ambion, USA) kits according to the manufac-
turer’s protocol. The purified RNAwas treated with DNase I,
quantified, and qualified by a NanoDrop ND-2000 (Thermo
Scientific, USA), and a 2100 Bioanalyzer (Agilent, USA).
The cDNA was synthesized using a SMART RACE (Rapid
Amplification of cDNA End) cDNA amplification kit
(BD Sciences, Clontech, USA) with SuperScript II re-
verse transcriptase (Invitrogen, USA), according to the man-
an annealing temperature of 55 °C with primers (HaGR9-F
CGCATGCTTTTATACTTAGG and HaGR9-R
TCTCAATTGTCGTATCTTTGG). These primers spanned
the known cDNA sequence and resulted in a 1300 bp
fragment. 5' RACE PCR was performed according to the
SMART RACE cDNA amplification kit manual with the
universal primer and gene-specific primers (HaGR9-1
GCTGACCGTGAAGCCATTGCTGCGAG and HaGR9-2
PCR products were purified using QIAquick gel extraction
(Promega, USA) and sequenced.
Immunocytochemistry Sf9 or S2 cells were subcultured on
poly-L-Lysine-coated coverslips in 6-well plates and trans-
fected with 1 μg plasmid constructs and 6 μl of Fugene HD
transfectionreagent (Promega,USA)in200 μl ofmedium per
well. After 48 h post transfection, immunofluorescence under
permeabilized and non-permeabilized conditions were per-
formed as previously described (Zhang et al., 2011).
Calcium Imaging Sf9 cells were plated into 12-well plates
and left to settle for 20 min before being transfected with
500 ng of plasmid construct (PIB/V5-His vector as control,
HaGR9 or MYC-epitope tagged HaGR9) and 3 μl of Fugene
per well. After 48 h post transfection, calcium imaging and
data analysis were performed using a modification of a previ-
ously described method (Zhang et al., 2011; Anderson et al.,
Tastants The tastants tested were D-fructose, D-galactose,
D-glucose, sucrose, D-maltose, D-trehalose, and myo-inosi-
tol with purities≥99 %. D-galactose was purchased from
Amresco (USA), whereas the others were purchased from
Sigma-Aldrich (USA). The maximum final concentration of
tastants in each well was 50 mM. For dose-dependent cal-
cium imaging studies, D-fructose, D-galactose, and D-
maltose were diluted with HBSS buffer (Zhang et al., 2011).
Molecular Cloning and Expression Profile of HaGR9 Using
DmGR43a aminoacidsequence wesearchedinGenBankwith
Blastp (Altschul et al., 1990). A partial sequence of H. armi-
contained 432 amino acids and lacked the N-terminal se-
quence. We studied its expression profile in several tissues:
foregut, midgut, hindgut, antennae, and mouthparts from 5th-
instars, as well as antennae and tarsi from male and female
of 1300 bp from the larvae foregut (Fig. 1). Aweak band also
appeared in male antennae, larvae antennae, midgut, as well as
the hindgut. Interestingly, there was no expression seen in the
female antennae. However when the number of PCR cycles
was increased from 40 to 50 (data not shown), we were able to
detect a weak band, thus suggesting there is a low level of
expression in the female antennae. The cDNA sample from
foregut was used in 5' RACE reactions to clone the full-length
1514 J Chem Ecol (2012) 38:1513–1520
The full-length sequence was cloned and named HaGR9
(JX970522) because of its 69 % amino acid identity and 78 %
similarity to BmGR9. The predicted protein contains 465
amino acids with seven predicted transmembrane domains
(Split 4.0 Server, http://split.pmfst.hr/split/4/) (Juretic et al.,
2002). It also showed 26 % amino acid identity and 49 %
similarity to DmGR43a. We performed a phylogenetic analy-
sis on HaGR9 with 13 other insect GR43a-like receptors
(Fig. 2). These insect GR43a-like receptors were divided into
4 order-specific groups: Diptera, Coleoptera, Hymenoptera,
and Lepidoptera (Fig. 2). To date, five GR43a-like receptors
have been identified from Lepidoptera insects, Papilio xuthus
(Ozaki et al., 2011), Danaus plexippus (Zhan et al., 2011), B.
mori (Wanner and Robertson, 2008; Sato et al., 2011;),
Heliothis virescens (Krieger et al., 2002) and H. armigera.
than any insect olfactory receptors, the two receptor genes
from H. virescens and D. plexippus reported previously most
likely have been misclassified as odorant receptor 4 (Krieger
et al., 2002; Zhan et al., 2011). The phylogenetic analysis
(Fig. 2) showed that GR43a-like genes are widely distributed
in various insect species, including almost all species with a
sequenced genome. We also found a GR43a-like protein from
pea aphid, A. pisum (XP_003244306), but its sequence is
much longer (756 amino acids) than all the other known
GR43a-like receptors (~300-500 amino acids), and, therefore,
we did not include it in our phylogenetic analysis (Fig. 2).
Topology of HaGR9 The algorithm Split4.0 predicts seven
transmembrane domains for HaGR9 with an intracellular N-
terminus and an extracellular C-terminus. To validate the
topology we used immunofluorescence as previously de-
scribed (Zhang et al., 2011). HaGR9 genes were fused to
double MYC-epitope tags at either the N- or C-termini and
expressed in both S2 cells and Sf9 cells. The native receptor
(HaGR9) was used as a negative control. No positive signals
were found from control cells (Figs.3b, c). Strong green
fluorescence was visualized from cells transfected with either
the N- or C-terminal labelled HaGR9 under permeabilized
conditions (Figs. 3b, c). In contrast, green fluorescence could
be detected only from cells transfected with C-terminally
tagged HaGR9 (HaGR9:MYC) but not from cells transfected
with N-terminally tagged HaGR9 (MYC:HaGR9) under
unpermeabilized conditions. These results indicated that the
N-terminus of HaGR9 is intracellular and the C-terminus is
A A LA LM T T FG MG HG FB
Fig. 1 RT-PCR analysis of HaGR9 gene expression in adult and larval
tissues of Helicoverpa armigera. ♂A, Male adult antennae; ♀A ,
Female adult antennae; LA, Larvae antennae, LM, Larvae mouthpart;
♂T, Male adult tarsi; ♀T, Female adult tarsi; FG, Foregut; MG,
Midgut; HG, Hindgut; FB, Fat body. The actin gene was used as a
control to qualify and quantify cDNA samples
Aedes agypti GR43a
Culex quinquefasciatus GR43a
Anopheles gambiae GR43a
Drosophila melanogaster GR43a
Tribolium castaneum GR20
Nasonia vitripennis GR3
Camponotus floridanus GR43a
Bombus impatiens GR43a
Apis mellifera GR43a
Papilio xuthus GR43a
Bombyx mori GR9
Danaus plexippus OR4
Helicoverpa armigera GR9
Heliothis virescens OR4
Fig. 2 Phylogenetic analysis of GR43a-like receptor genes from
insects. The analysed genes included: Heliothis virescens OR4
(CAD31946) (Krieger et al., 2002), Danaus plexippus OR4
(EHJ77681) (Zhan et al., 2011), Bombyx mori GR9 (NP_001124345)
(Sato et al., 2011) (Wanner and Robertson, 2008), Bombus impatiens
GR43a (XP_003486787), Nasonia vitripennisGR3 (NP_001164386)
(Robertson et al., 2010), Apis mellifera GR43a (XP_001121326),
Drosophila melanogaster GR43a (NP_523650) (Sato et al., 2011),
Tribolium castaneum GR20 (EFA05758), Anopheles gambiae GR43a
(XP_318100), Camponotus floridanus GR43a (EFN61344), Aedes
aegypti GR43a (XP_001658898), Culex quinquefasciatus GR43a
(XP_001842305), and Papilio xuthus GPCR (BAF91710) (Ozaki et
J Chem Ecol (2012) 38:1513–15201515
extracellular (Fig. 3a). The previous study of two BmGRs
(BmGR8 and BmGR53 from the ‘sugar’ and ‘bitter’ clades)
alsoshowedthatthe N-terminuswas insidethe celland the C-
is the same topology as observed for insect odorant receptors,
but reverse to classical GPCRs (Benton et al., 2006). Insect
receptor topologies previously were performed mainly on S2
cells from Drosophila (Smart et al., 2008; Zhang et al., 2011).
Here, we used S2 cells (Fig. 3b) as well as Lepidoptera
derived Sf9 cells (Fig. 3c). Both cell lines showed the same
topology results, indicating both lines are suitable for insect
receptor topology studies.
HaGR9 Can Detect Three Sugars We expressed HaGR9 in
Sf9 cells and performed quantitative calcium imaging to
characterize its specific ligand. Seven sugars, widespread
in plant saps, and belonging to the monosaccharides (D-
glucose, D-fructose, and D-galactose), disaccharides (su-
crose, D-maltose, and D-trehalose), and myo-inositol were
tested at 50 mM in the initial screen. Three of them, D-
galactose, D-maltose, and D-fructose, generated responses
that significantly differed from the control (Fig. 4a). A two-
tailed Student’s t-test indicated that the order of responses of
HaGR9 to three sugars is D-galactose (ΔF00.256, t07.74,
P<0.001)>D-maltose (ΔF00.211, t010.78, P<0.001)>D-
fructose (ΔF00.122, t03.67, P<0.001). Our results also
indicated that Sf9 cells themselves, although transfected
with the empty expression vector, can be activated by the
sugars, which we assume is due to endogenous receptors
expressed in the Sf9 cell membrane. These responses vary
Fig. 3 HaGR9 has extracellular
C-terminus and intracellular
N-terminus indicated by immu-
nofluorescence in both Sf9 and
S2 cells. HaGR9 was expressed
in its native form or fused with
two MYC-epitope tags at either
the N- or C-terminus. a Ex-
pression constructs for native
HaGR9 (controls), N-terminally
MYC-epitope tagged HaGR9
(MYC:HaGR9), and C-
terminally MYC-epitope tagged
HaGR9 (HaGR9:MYC). b, c
Immunofluorescence of HaGR9
in Sf9 and S2 cells under per-
meabilized and unpermeabi-
lized conditions. Green
indicates the location of MYC-
epitope expression directed
Alexa 488 fluorescence. Red
indicates cell nuclear staining
by DAPI. Scale bar05 μm
1516 J Chem Ecol (2012) 38:1513–1520
based on the different sugars (Fig. 4a). Interestingly, cells
transfected with HaGR9 showed reduced responses to tre-
halose when compared with responses from control Sf9
cells (Fig. 4a). We assume that the expression of HaGR9
may reduce or inhibit the expression, localization, or func-
tion of the endogenous taste receptors of Sf9 cells, thus
leading to the lower responses to trehalose (Fig. 4a). Dose-
dependent responses to D-galactose, D-maltose, and D-
fructose were performed (Fig. 4b). Results showed that these
three sugars can cause similar dose-dependent responses from
5 mM to 50 mM concentrations (Fig. 4b) with similar EC50
(21.3±1.1 mM for D-galactose, 16.5±1.3 mM for D-maltose,
and 21.5±1.1 mM for D-fructose).
In order to determine whether the modified GRs used
for topology studies retained functionality, identical cal-
cium imaging experiments were performed (Fig. 4c). Cells
transfected with native HaGR9 and N-terminally labelled
HaGR9 (MYC:HaGR9) both showed significant responses
when compared to empty vector controls (Fig. 3b). However,
there were no statistically significant differences in responses
of C-terminally labelled HaGR9 (HaGR9:MYC) and the
negative control. We cannot rule out the possibility that
Log [Tastants] (mM)
0.51.0 1.5 2.0
* * P<0.01
D-galactose (37.5 mM)
50 mM Tastants
Fig. 4 Responsiveness of HaGR9 to sugars. a Responses of HaGR9 to
seven tastants at 50 mM. The mean responses of cells expressing
HaGR9 are indicated by the dark solid bar. The mean responses of
controls are indicated by the black hollow bars. Error bars indicate the
standard error of the mean. Analysis of the statistical significance
between each response and control was conducted by two-tailed Stu-
dent’s t-test using arcsine transformation. *** P≤0.001, **P<0.01,
*P<0.05. b Log dose-dependent response curves for HaGR9 to
cells have been subtracted. Error bars indicate the calculated error of the
difference between means 0 SE xGr12? xcontrol
epitope tagged HaGR9 to 37.5 mM sugars. Control, empty vector (PIB/
V5-His vector); HaGR9, HaGR9-transfected cells; MYC:GR9, two
MYC-epitope copies fused to HaGR9 N-terminus; GR9:MYC, two
MYC-epitope copies fused to HaGR9 C-terminus
ðÞ. c Response of MYC-
J Chem Ecol (2012) 38:1513–15201517
the labelled HaGR9 renders the protein non-functional
due to the incorrect insertion of the receptor into the mem-
branes. A previous study on BmGR8 showed similar results
where the N-terminallylabelledtagdidnotsignificantlyaffect
the receptor’s function to detect myo-inositol (Zhang et al.,
2011). However the C-terminally tagged receptor showed
barely detectable function. These results indicate that
the C-terminus of GRs is critical for their correct localisation
Our phylogenetic analysis (Fig. 2) demonstrates that GR43a
is a conserved GR that exists in almost all insect species (20-
64 % amino acid identity), and therefore, GR43a may have a
conserved function across insect orders. Our RT-PCR results
(Fig. 1) indicate that HaGR9, a GR43a homologue, is highly
transcribed in the larval foregut of H. armigera, where food
is stored before moving into the midgut for digestion. A
behavioral study on the blowfly, Phormia regina, showed
the involvement of the foregut in the regulation of sugar
taste threshold (Gelperin, 1966), with high sugar concentra-
tions prolonging foregut stimulation and elevating the
threshold for stimulation. Fructose and other sugars were
shown to produce this behavioral effect when acting via the
postulated foregut receptor (Gelperin, 1966). It is likely that
HaGR9 may be a foregut receptor that is involved in the
regulation of foregut stimulation and taste threshold in the
digestive system, as well as in sugar sensing in external
tissues. BmGR9, a GR43a ortholog from B. mori, also was
shown to have high expression in the gut and respond to
fructose significantly in vitro (Sato et al., 2011).
Recent studies in Drosophila show 14 gustatory receptors
are expressed in the Drosophila gut, including GR43a, the
fructose receptor, and GR64a from the sugar receptor family
(Park and Kwon, 2011). The colocalization of the GR-Gal4
drivers with the regulatory peptides neuropeptide F (NPF),
locustatachykinin (LTK), and diuretic hormone 31 (DH31)
also was observed in this study (Park and Kwon, 2011),
providing further evidence that gustatory receptors may be
involved in the detection and regulation of nutrients during
digestion. Gastrointestinal chemosensation is an emerging
field in humans and other mammals, where gustatory recep-
tors expressed in the luminal epithelium are believed to be
acting as nutrient sensors to guide digestive processes, to
protect from harmful substances, and to regulate food up-
take (Sclafani, 2007; Breer et al., 2012). Therefore, an
improved understanding of the intestinal chemosensation
mechanisms may assist in the development of novel treat-
ments of eating disorders, obesity, diabetes, intoxication,
and inflammation (Breer et al., 2012). Insect digestive sys-
tems have many similarities to those of vertebrates in basic
structure, cell types, and development, and may provide a
simple system to study intestinal chemosensation (Park and
To study the topologies of HaGR9, we used both Sf9 and
S2 cell systems. Both gave identical results, showing HaGR9,
a member of the DmGR43a-like subfamily, has an intracellu-
lar N-terminus and an extracellular C-terminus. This result is
consistent with previous studies on other insect gustatory
receptors from the sugar and bitter subfamilies (Zhang et al.,
2011) and also with odorant receptors (ORs) (Benton et al.,
2006; Lundin et al., 2007; Smart et al., 2008), which have
been shown to be an evolutionarily related family (Robertson
etal.,2003). Previous studies suggest thatthe GR gene family
is an ancient chemoreceptor family from which a branch of
OR genes subsequently evolved (Robertson et al., 2003).
Insect ORs are thought to function as odor-gated ion channels
(Sato et al., 2008), although a modulatory role for G proteins
and second messengers is likely to be involved in the down-
stream functions (Benton, 2008; Nakagawa and Vosshall,
2009). Here, we have shown that HaGR9 shares the same
not classical GPCRs and may function in a way similar to
Genetic studies on Drosophila have revealed that coex-
pression of multiple GRs is essential for the detection of
compounds like CO2, caffeine, theophylline, sucrose, D-
glucose, and trehalose (Dahanukar et al., 2001,2007; Moon
et al., 2006; Jones et al., 2007; Jiao et al., 2008 ). However, in
vitro studies with BmGR8 (Zhang et al., 2011), BmGR9 and
DmGR43a (Sato et al., 2011) have shown their responses to
myo-inositol or D-fructose did not require the coexpression of
other GRs, which is consistent with the HaGR9 studies
reported here. We cannot conclude that HaGR9 can function
without a co-receptor in vivo because Sf9 cells may express a
native receptor that can assist HaGR9 in the detection of the
three tested sugars. Orco, a highly-conserved OR co-receptor,
was detected in Sf9 cells in a previous study (Smart et al.,
2008), and is probably involved in the correct functioning of
odorant receptors in the assay. In other in vitro studies using
Xenopus oocytes, BmGR9 and DmGR43a showed a response
only to D-fructose but not to other sugar tastants (Sato et al.,
2011). HaGR9, the orthologous gene of BmGR9 and
DmGR43a, showed responses to D-fructose, D-galactose, and
differences. First, HaGR9 shows 69 % and 26 % identity to
the ligand binding capabilities differ. Second, Sf9 cells and
Xenopus oocytes are different cell systems that may contain
divergent components, leading to different activities and
responses in the assay. To date, four major systems have been
used to study insect chemosensory receptors: the “empty neu-
ron” from Drosophila (Dobritsa et al., 2003), Xenopus oocytes
with patch clamp technologies (Sakurai et al., 2004), human
1518J Chem Ecol (2012) 38:1513–1520
cells (HEK293Tcells; Sato et al., 2008), and Lepidoptera cells
(Sf9 cells) with calcium imaging (Smart et al., 2008; Anderson
Lepidoptera receptors as they are derived from a lepidopteran
species, and therefore, are likely to more closely resemble in
vivo H. armigera receptor function.
In summary, we here identified a novel insect-specific
and insect-conserved gustatory receptor from H. armigera,
and named it HaGR9. HaGR9 shows high homology to
GR43a, and demonstrates high levels of transcription in
the foregut of larvae. Topology study reveals this receptor
has an intracellular N-terminus and an extracellular C-
terminus, which is the same as previous studied insect
GRs and ORs. In addition, HaGR9 shows dose-dependent
responses to D-galactose, D-maltose, and D-fructose.
Further immunohistochemistry studies to localize this recep-
tor to specific cell types may shed light on the function of
sugar receptors in the larval foregut. To study HaGR9 func-
tion in vivo, RNAi (Ozaki, et al., 2011) may be a powerful
technique to inhibit its expression and perform feeding
behavior assay. Studies on agricultural pests like H. armi-
gera gustatory receptors may not only help us understand
the underlying molecular mechanism of insect feeding be-
havior, but also help us develop new strategies to control
this destructive species. For example, the studies on GR
may direct the search for new attractants or repellents
(Lee et al., 2010) from host plants that can be applied
in pest control.
gera, Alexie Papanicolaou for bioinformatics help, Peter Campbell for
RNA preparation. We also thank Alexie Papanicolaou, Bradley
Stevenson, and Mira Dumancic for critically reading the manuscript.
This work was supported by a CSIRO Julius Award (R-00094-01-005)
and a CSIRO Office of the Chief Executive (OCE) post-doctoral
We thank Joel Armstrong for providing H. armi-
Open AccessThis article is distributed under the terms of the Creative
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reproduction in any medium, provided the original author(s) and the
source are credited.
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