royalsocietypublishing.org/journal/rspb
Research
Cite this article: Sadoul B, Alfonso S, Goold
C, Pratlong M, Rialle S, Geffroy B, Bégout M-L.
2022 Transcriptomic profiles of consistent
risk-taking behaviour across time and contexts
in European sea bass. Proc. R. Soc. B 289:
20220399.
https://doi.org/10.1098/rspb.2022.0399
Received: 1 March 2022
Accepted: 14 April 2022
Subject Category:
Behaviour
Subject Areas:
behaviour, molecular biology, physiology
Keywords:
coping style, behavioural syndrome, fish,
personality, anti-predator behaviour,
intra-individual variability
Author for correspondence:
Bastien Sadoul
e-mail: bastien.sadoul@agrocampus-ouest.fr
†
Co-last.
Electronic supplementary material is available
online at https://doi.org/10.6084/m9.figshare.
c.5958649.
Transcriptomic profiles of consistent
risk-taking behaviour across time and
contexts in European sea bass
Bastien Sadoul1,2, Sébastien Alfonso1,3, Conor Goold4, Marine Pratlong5,
Stéphanie Rialle5, Benjamin Geffroy1,† and Marie-Laure Bégout1,†
1
MARBEC, University of Montpellier, CNRS, Ifremer, IRD, Palavas-Les-Flots, France
DECOD, Ecosystem Dynamics and Sustainability, Institut Agro, Ifremer, INRAE, Rennes, France
3
COISPA Technology and Research, Experimental Station for the Study of Sea Resources, Bari, Italy
4
Faculty of Biological Sciences, University of Leeds, LS2 9JT, UK
5
MGX-Montpellier GenomiX, University of Montpellier, CNRS, INSERM, Montpellier, France
2
BS, 0000-0001-5968-3983; SA, 0000-0002-2471-2876; CG, 0000-0002-9198-0889;
MP, 0000-0002-3406-9829; BG, 0000-0001-6120-1103; M-LB, 0000-0003-1416-3479
Bolder individuals have greater access to food sources and reproductive
partners but are also at increased risk of predation. Boldness is believed to
be consistent across time and contexts, but few studies have investigated
the stability of this trait across variable environments, such as varying
stress loads or long periods of time. Moreover, the underlying molecular
components of boldness are poorly studied. Here, we report that boldness
of 1154 European sea bass, evaluated using group risk-taking tests, is consistent over seven months and for individuals subjected to multiple
environments, including a chronically stressful environment. Differences in
risk-taking behaviour were further supported by differences observed in
the responses to a novel environment test: shy individuals displayed more
group dispersion, more thigmotaxic behaviour and lower activity levels.
Transcriptomic analyses performed on extreme phenotypes revealed that
bold individuals display greater expression for genes involved in social
and exploration behaviours, and memory in the pituitary, and genes
involved in immunity and responses to stimuli in the head kidney. This
study demonstrates that personality traits come with an underpinning
molecular signature, especially in organs involved in the endocrine and
immune systems. As such, our results help to depict state–behaviour feedback
mechanisms, previously proposed as key in shaping animal personality.
1. Introduction
For most animals, boldness could be viewed as a risky strategy as it may increase
the probability of being predated. At the same time, risk-taking behaviour of
bold individuals also increases their likelihood of finding more suitable environments (e.g. for food or mating). This trade-off is known to favour bolder
individuals when predation is low and shyer individuals when predation is
high [1]. Risk-taking behaviour is therefore tightly linked to the eco-evolutionary
dynamic of species in a given environment. Animals maximize their fitness
through plasticity (i.e. variation that depends on contexts within individuals)
and selection of adaptive behaviours (i.e. variation over generations within
species). Indeed, it has previously been demonstrated in numerous species that
the environment drives the average behaviour of a population [2,3]. The most
evident examples relate to the effects of differences in predation risks between
two environments, including as a consequence of human presence [4].
Nevertheless, within a given population, individuals differ in their behavioural phenotypes. Some are more prone to take risks than others, and strong
heterogeneity in risk-taking has been related to global population sizes and
larger spatial ranges [5]. Interestingly, these inter-individual differences in
© 2022 The Author(s) Published by the Royal Society. All rights reserved.
2. Material and methods
2
(a) Consistency of group risk-taking behaviour over time
and different environmental conditions
royalsocietypublishing.org/journal/rspb
(i) Experimental animals
All fish used in this study were part of a larger experiment which
led to a first publication [29], providing all details on the rearing
protocol. Briefly, 2053 European sea bass were individually
tagged at 175 ± 3 days post-fertilization (dpf ), using an ISO PIT
tag (8 × 1.4 mm), to ensure individual identification. They were
dispatched in three 1.5 m3 tanks and supplied with water at a
constant temperature of 21°C. At 255 ± 2 dpf and 358 ± 2 dpf, a
total of 288 and 492 fish respectively were randomly extracted
from the tank for two other studies, one being published [29].
From tagging to the end of the experiment (at 462 dpf ), 119
(out of 2053, 5.8%) fish died of unknown reasons.
(ii) Group risk-taking tests
Over the whole experiment (described below) fish were evaluated
in four group risk-taking tests (GRTs), performed following a
previously described protocol [26]. All GRTs were conducted in
a tank of the same size and shape as the rearing tanks, and supplied with water from the same system. The tank was vertically
divided in two parts of equal volume with an opaque screen. In
the middle of the screen, a 12 cm hole was surrounded by a circular RFID-reading antenna (DORSET, The Netherlands) connected
to a computer. The hole was blocked with a door that could be
opened without being seen by the fish.
Fish were fasted for 24 h prior to the start of the experiment.
They were then lightly anaesthetized (100 ppm of Benzocaine,
150 g l−1, E1501, Sigma, Saint Louis, MO, USA) and transferred
between 10.00 and 11.00 on one side of the tank that we covered
with a black tarp in order to get complete darkness. The other
side was not covered in order to create a riskier zone. After 2 h of
habituation, the door was opened to allow the fish to move freely
to the other side of the tank; PIT tags were read by the antenna
and the time of first passage (Latency to exit (s)) was stored. In
order to avoid a too long starvation period, we censored the test
to 20 h. After the test was completed, fish were anaesthetized
(300 ppm of Benzocaine) and were measured for length (cm) and
weight (g) and transferred back into their original rearing tank.
(iii) Experimental protocol
At 255 ± 4 dpf, the first risk-taking test (GRT1) was conducted for
each rearing tank, providing a risk-taking phenotype for the 1748
fish available (figure 1). At 309 dpf, fish from all three tanks were
transferred in six tanks of 1 m3 with equal number of fish from
each initial rearing tank (figure 1). At 336 dpf, and for a period
of three weeks, three tanks were stressed as previously detailed
[29], while the three other tanks were kept under normal rearing
conditions (figure 1). Briefly, stressors consisted in random light
flashes, chasings and confinements randomly programmed over
the three weeks.
At 358 ± 1 dpf, a subsample from each tank was extracted for
the analyses published in another paper [29]. The remaining fish
(n = 1187) were transferred to three 1 m3 tank making sure that
previously stressed and control fish were equally represented.
Two weeks later, at 373 ± 1 dpf, fish from each tank were evaluated in a new risk-taking test (GRT2) performed for each tank
(figure 1). A total of 1187 individuals were phenotyped at
GRT2. The third GRT (GRT3) was performed six weeks later at
423 ± 3 dpf, on each tank for the 1181 remaining fish. At
443 dpf, all fish were then transferred in a unique 5 m3 tank,
and at 463 dpf, the last risk-taking test was performed at once
in a 5 m3 tank (GRT4) on the 1154 remaining fish. All fish were
then euthanized and a random subsample (n = 606) was sexed
Proc. R. Soc. B 289: 20220399
risk-taking behaviour are relatively consistent over time and
contexts [6], and being bold or shy is thus considered as a
personality trait in animals [7]. Personality traits have been
observed for a wide range of terrestrial and aquatic species
and linked to fitness in a given environment [8]. The emergence of personalities were previously proposed to be linked
to key life-history traits, such as metabolism and growth,
through a state–behaviour feedback [9,10]. In this framework,
individuals that are more prone to take risks also exhibit lower
hypothalamo–pituitary–interrenal/adrenal axis responses,
but stronger hypothalamo–sympathetic activity [11]. These
interconnections between behaviour and stress physiology
are part of the concept of coping styles [11] and might lead
individuals to consistently differ in behaviour.
Yet, it remains unclear to what extent personality traits
are governed by the genetic architecture of an individual
and/or depend on the environment it will experience
[12–14]. In addition, we still lack information regarding the
molecular mechanisms underlying these risk-taking-related
behaviours, although some efforts have been made to elucidate the genetic component of traits, e.g. boldness: [15].
Most studies of animal personality focus on short time
periods or one life stage. This focus provides a limited view
of how personality interacts with physiology, life stage,
experience and the environment [16]. Understanding molecular mechanisms behind key behavioural phenotypes, such as
the boldness/shyness continuum, is of primary importance
in behavioural ecology.
In this study, we characterize the boldness of European
sea bass (Dicentrarchus labrax) by assessing the consistency
of risk-taking-related behaviour across multiple environments, challenges and time. Then, we highlight molecular
pathways in the brain, the pituitary and the head kidney
associated with these divergent behavioural phenotypes.
These three organs were chosen because (i) the brain centralizes perceptions of the environment and controls behaviour
and physiology [17], (ii) the pituitary secretes hormones regulating multiple processes including growth, sexual maturity,
stress response and energetic balances [18] and (iii) the head
kidney, analogous to the adrenal gland in mammals, plays a
central role in the immune system and stress regulation by
secreting cortisol and catecholamines, the major stress
hormones in fish [19,20]. The European sea bass was used
as a model species, because of its ecological and economic
importance [21]. Previous work demonstrated strong interindividual variation in metabolism [22], stress response
[23], feeding behaviour [24] and coping style [25] of the
species. Consistency in risk-taking behaviour over three
months has also been demonstrated in European sea bass
[26] but has never been challenged by environmental variation, especially by extreme events such as a chronic stress
period. Also, within a population, correlations exist between
molecular phenotypes related to behaviour and coping
capacities [27], but no conserved transcriptomic signature
for boldness has been shown in the sea bass whole brain
[28]. The present study aims (i) to investigate the longterm consistency in risk-taking behaviour of individuals
from a large sea bass population (n = 1154), (ii) to demonstrate that extreme individuals (i.e. very shy or very bold)
contrastingly respond to a different behavioural test and
then (iii) to depict molecular differences in gene expression
across specific organs (brain, pituitary and head kidney) in
those extreme individuals.
G
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1187
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478
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1181 96
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Figure 1. Experimental protocol assessing consistency of risk-taking behaviour over time and environments. Fish were reared in replicated tanks (N = number of
rearing tanks). A total of four GRTs were performed on each rearing tank. Colour of the tank illustrates a different rearing condition. Until 309 dpf, juveniles were
reared in 1.5 m3 tanks (dark green tank), they were then transferred in 1 m3 tanks (light green tanks) and half of the tanks were stressed (blue tanks) from 336 to
358 dpf. At the end of the stress period, fish were mixed and transferred to other 1 m3 tanks (yellow tanks) until 443 dpf when they were all transferred in a
unique 5 m3 tank (orange tank). (Online version in colour.)
(figure 1) indicating that the population was composed of 54% of
females (329 females versus 277 males).
(iv) Statistical analyses of group risk-taking test data
We analysed the GRT data with hierarchical Bayesian regression
with a lognormal likelihood function in Stan [30] and cmdstanr
[31] in R v. 4.0.5 [32]. We included data from 1748 fish who participated in GRT1 and who were assigned a chronic stress group.
Missing data due to fish extracted from the experiment (see previous section) were imputed in the Bayesian model rather than
being discarded. Censored values were accounted for via imputation with a lower bound of 20 h. Fish were categorized as bold
or shy at the first GRT (at 255 dpf ) based on whether they were
above (shy) or below (bold) the median latency in their tank. We
included GRT as an observation-level predictor and bold/shy
(bold = 875), chronic stress (yes = 874) and sex (female = 598,
male = 494) as individual-level predictors. Six-hundred-andfifty-five fish were missing sex assignment, which was accounted
for by marginalizing out of the sex variable for those fish [33].
Four models were run and compared using the expected log
predictive density (ELPD) estimated via Pareto-smoothed importance sampling leave-one-out cross-validation (ELPD-LOO [34])
in the loo package [35]. Model 1 included random intercepts (i.e.
personality) for individuals, model 2 extended model 1 with
random slopes by GRT time (i.e. behavioural plasticity) for
individuals, model 3 extended model 2 by including a random
intercept of initial tank and model 4 extended model 3 by
modelling heterogeneous residual variances by individual
(i.e. behavioural predictability). From these models, we calculated
behavioural repeatability (intraclass correlation coefficients; ICCs)
at each GRT time point (i.e. conditional repeatability [36]) as well
as the coefficient of variation for predictability as a measure of
behavioural predictability effect size [37].
We summarize the parameters and results using the posterior mean and 95% highest density interval (HDI; the most
probable values).
(b) Group behavioural response to novel environment
and hypoxia challenge
exits or no exit at all) or bold behavioural type (three early
exits) were selected and challenged in novel environment and a
hypoxia challenge at 429 ± 2 dpf (mean ± s.e., 91.0 ± 6.6 g and
194 ± 4.8 cm). This test was performed in groups of eight fish
with similar behaviour (i.e. n = six trials*eight individuals per behavioural type, i.e. bold versus shy). The trials to monitor the
group behavioural response to a novel environment and hypoxia
were carried out according to the protocol described in [29] and
detailed in the electronic supplementary material, SMM1.
(ii) Statistical analyses of behavioural data in the novel
environment test
Measured variables (time spent in the periphery, velocity and
inter-individual distances) during the novel environment and
hypoxia challenge were analysed independently before and
after the hypoxia challenge. A linear mixed model was fitted
with risk-taking behavioural phenotype (bold or shy), time (categorical) and their interaction as fixed effects and the day of
experiment and trial as random effects. A Tukey post hoc test
was completed with the glht function from the multcomp package [38] to test significant differences between the levels of a
significant fixed effect.
(c) Transcriptomic signatures between bold and shy
individuals
(i) Samplings
At 478 ± 2 dpf, i.e. two weeks after GRT4 (figure 1), fish were
caught from their rearing tank and euthanized using 1500 ppm
of Benzocaine. Sampling of the whole brain, pituitary and head
kidney was performed on five individuals per behavioural phenotype chosen among the individuals also tested in the novel
environment test (i.e. 49 days after the novel environment test).
The 30 samples collected (five individuals × two behavioural
phenotypes × three tissues) were flash-frozen in liquid nitrogen
and kept at −80°C until further analyses. Individuals were
chosen to be equally represented for each sex (two shy males,
three shy females, three bold males and two bold females).
(i) Behavioural tests in a novel environment
(ii) RNA sequencing and analyses
Among fish screened during GRT1, GRT2 and GRT3, two subsamples of 48 individuals displaying consistent shy (three late
Details on RNA extraction, sequencing and analyses are provided
in the electronic supplementary material, SMM2. Briefly,
Proc. R. Soc. B 289: 20220399
no. fish
in the test
373
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3. Results
The average ICC for between-individual differences was 0.38
(95% HDI: [0.30, 0.47]) and was highest at GRT1 (GRT1: 0.44,
HDI: [0.38, 0.50]; GRT2: 0.33, HDI: [0.29, 0.37]; GRT3: 0.41,
HDI: [0.36, 0.46]; GRT4: 0.36, HDI: [0.31, 0.42]). The ICC for
between-tank differences was very low (0.003, HDI: [0.000,
0.012]). The model with the highest ELPD was model 3
(ELPD-LOO: −9832.8; model 1 = −10118.0, model 2 = −10071.8,
model 4 = −9835.5) including behavioural predictability.
The coefficient of variation for behavioural predictability was
relatively high at 0.57 (HDI: [0.51, 0.64]).
Fish had generally lower latencies to leave the sheltered
areas at GRT1 and GRT2 compared to GRTs 3 and 4, but
there were interactions between GRT and fish types (figure 2).
Fish categorized as bold (see estimates in figure 2) at GRT1
had significantly lower latencies (between 3 and 5 h) at the
following GRTs than shy fish. This was repeatable across
the rearing tanks (electronic supplementary material,
figure S1). Female fish had a tendency for lower latencies than
male fish across the GRTs, but the difference was only significant at GRT3 and GRT4 (approximately 2–3 h quicker).
Chronic stress had no credible influence on latency to leave
the shelter.
(b) Group behavioural response to novel environment
and hypoxia challenge
After introduction in the novel environment, both behavioural phenotypes (bold and shy) showed first a low swimming
velocity which gradually increased to reach a plateau after
30 min. Bold individuals showed a tendency for higher
swimming velocity after 10 min compared to shy individuals
( p = 0.057; figure 3a). At introduction in the novel environment, shy individuals showed increased dispersion in the
tank (figure 3b; p < 0.05) and spent more time in the periphery (figure 3c; p < 0.05) than bold individuals. After 20 min,
both behavioural phenotypes showed a similar group behaviour pattern which became constant after 30 min (figure 3c).
Both behavioural phenotypes displayed a same response to
the hypoxia challenge for the three behavioural measures
(figure 3).
(c) Transcriptomic signatures between bold and shy
individuals
In the pituitary and the head kidney, 556 and 141 genes were
DE between bold and shy individuals respectively.
In the brain, only six genes were DE and four of them
were annotated (nr2e3, glipr2, dsg2 and pla2g1b). In the
4. Discussion
(a) Consistency of group risk-taking behaviour over time
and different environmental conditions
Mean latency to exit the covered side of the GRT tank greatly
varied over time, indicating that environment and/or age
impact risk-taking behaviour. Although we are unable to
identify causes of this variation, it supports that risk-taking
behaviour is relatively plastic and can be affected by biotic
or abiotic changes [25,40]. Nevertheless, within our studied
population, individuals showed heterogeneous risk-taking
behaviour which was moderately consistent over time and
environmental conditions. This is particularly interesting
given that repeatability was assessed over almost seven
months and was challenged by varying the rearing
conditions and stress loads. Consequently, risk-taking behaviour can be considered plastic over time and environmental
conditions for the whole population but, at the same time,
environmental conditions are not sufficient to trigger long
lasting changes in individual risk-taking within a population.
In this study, a GRT was performed two weeks after
the end of the chronic stress protocol. The absence of
differences in risk-taking behaviour between the chronically
stressed individuals and controls suggest that the measured
behaviour is resilient (i.e. significant change but quick
recovery) or resistant (little change) to different stress
loads. Stress is known to affect behaviour, including risktaking [41,42]. Behavioural measures are also generally
considered as reliable markers of welfare [29,43,44]. Nevertheless, to our knowledge, no previous studies have
investigated long-term post-stress differences in risk-taking
behaviour in fishes; behavioural differences being observed
only during the stress protocol or right after. Yet, some
events occurring during fish life history impacted boldness
[45,46], which was not the case in our study, suggesting
that this measured behaviour, performed in group, is resilient
toward stressors.
4
Proc. R. Soc. B 289: 20220399
(a) Consistency of group risk-taking behaviour over time
and different environmental conditions
pituitary, the biological process ‘Behaviour’ (GO:0007610)
was, out of level 2 biological processes, the most differentially
represented within the genes DE between bold and shy
individuals (electronic supplementary material, table S1).
Most of these behaviour-related genes were upregulated
in bold individuals (figure 4a). All DE genes related to
social behaviour, exploration behaviour and memory were
upregulated in bold individuals (figure 4b–d). Only casp3,
trpv1 and arrdc3 were found downregulated in shy
individuals (figure 4a).
In the head kidney, nine biological processes out of all
level two biological processes were overrepresented among
DE genes, with ‘immune system process’ being the most significant (GO:0002376). Most DE genes related to immune
system processes were upregulated in bold individuals
(figure 5a). All DE genes related to immune system development and leucocyte activation were found to be upregulated
in bold individuals except for pawr (figure 5b,c).
After adjustment of the p-values to lower false discovery
rates, six biological processes were still significant in the
head kidney, confirming the true biological signal underpinning transcriptomic differences (electronic supplementary
material, table S1 and figure S2).
royalsocietypublishing.org/journal/rspb
after libraries were constructed, validated and quantified, they
were sequenced in equimolar amounts using a HiSeq 2500 (Illumina, San Diego, CA, USA). Reads were then aligned to the
Dicentrarchus labrax genome (NCBI, reference GCA_000689215.1)
with a set of gene model annotations [39]. Differentially expressed
(DE) genes between bold and shy were identified within each
organ, while the effect of sex was accounted for. Gene ontology
(GO) analysis was then performed to categorize each gene
within a biological process.
(a)
GRT1 (255 dpf)
GRT3 (423 dpf)
GRT4 (463 dpf)
–4.89 (HDI: (–6.31, –3.34))
–4.74 (HDI: (–5.65, –3.79))
–3.25 (HDI: (–4.71, –1.92))
latency (h)
30
20
10
0
bold
GRT1 (255 dpf)
bold
shy
bold
shy
bold
shy
GRT2 (373 dpf)
GRT3 (423 dpf)
GRT4 (463 dpf)
–1.02 (HDI: (–0.45, 2.46))
–0.14 (HDI: (–1.04, 0.74))
0.3 (HDI: (–1.1, 1.66))
25
latency (h)
20
15
10
5
0.11 (HDI: (–0.56, 0.77))
0
no
(c)
yes
GRT1 (255 dpf)
no
yes
no
yes
no
yes
GRT2 (373 dpf)
GRT3 (423 dpf)
GRT4 (463 dpf)
–1.56 (HDI: (–3.58, 0.45))
–2.41 (HDI: (–3.8, –1.06))
–2.28 (HDI: (–4.42, –0.31))
25
latency (h)
20
15
10
5
0.02 (HDI: (–0.76, 0.82))
0
female
male
female
male
female
male
female
male
Figure 2. Latencies (in hours) at the four GRTs by at 255 dpf by (a) (estimates not shown for GRT1 because boldness was a direct function of latencies),
(b) chronically stressed between GRT1 and GRT2, and (c) sex. Points show the posterior mean and vertical line segments represent the 95% HDI. Text annotations
illustrate the posterior differences between each estimate and text in bold font highlights significant differences (i.e. the 95% HDI do not include zero). (Online
version in colour.)
We observed moderate consistency in the latency to
leave the initial area over a relatively long period of time
(ICC = 0.38), and for whether fish left the initial area at
all (ICC = 0.35). Previous studies have also demonstrated
consistency in fish behaviour overtime within a population
[15,40,47,48], but only few exceeded 45 days [26,47,49]. Longterm consistency in personality traits have, nevertheless, been
studied in multiple mammal species. Multiple of these studies
observed changes once puberty or sexual maturation occurs
[50,51]. To our knowledge, only one study investigated boldness
consistency in fishes over maturation and sex change and
observed a strong loss of consistency once sea bream (Sparus
aurata, protandric species) became adults [47]. Our study
investigated behaviour from 255 to 462 dpf, while sea bass
experienced puberty (at least for males), suggesting that repeatable risk-taking behaviour occurs in European sea bass over
important ontogenetic transitions. Altogether, our results
advocate for resilient risk-taking behaviour over time and
environmental conditions and further consolidate the bold-shy
continuum as a personality trait in European sea bass.
(b) Group behavioural response to novel environment
and hypoxia
Both bold and shy fish showed similar expected behavioural
responses (i.e. increasing velocity and cohesion and reducing
time in the periphery) during the acclimation period in the
novel environment test [29,43]. Differences between bold
and shy groups were observed at the very beginning of the
acclimation period. We identified higher thigmotaxic
behaviour for shy individuals compared to bold, classically
indicative of anxiety-like behaviour [52]. Shy individuals
also showed less cohesion than bold individuals at the beginning of the acclimation period. Based on the high cohesion
values observed at the end of the acclimation period for
both groups, we suspect that values at the start of the
Proc. R. Soc. B 289: 20220399
(b)
shy
5
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GRT2 (373 dpf)
(b)
velocity (cm s–1)
mean interindividual distance (cm)
8
p = 0.057
6
behaviour
4
bold
shy
0
25
100
bold
shy
35
30
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25
125
(c)
0
25
50
75
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100
125
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time (min)
100
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(d)
200
behaviour
oxygen saturation (%)
time spent in the periphery (%)
behaviour
Proc. R. Soc. B 289: 20220399
50
75
time (min)
6
bold
shy
75
50
25
150
100
50
*
0
25
50
75
time (min)
100
125
0
Figure 3. Group behaviour in a novel environment before and during a hypoxia challenge depending on fish behaviour in the GRTs (bold in orange, n = 48; shy in
grey, n = 48). (a) Mean velocity (cm s−1), (b) inter-individual distance (cm) and (c) time spent in the periphery (%) are illustrated during a 60 min acclimation
period to the novel environment and during a hypoxia challenge (starting at the red line) obtained by reducing the oxygen saturation down to 20% (d). Data are
represented as mean ± s.e.m. The difference in behaviour between the two fish groups at a single time point is illustrated with an asterisk ( p < 0.05). (Online
version in colour.)
experiment are a marker of fear in shy groups. The tendency
for higher activity in bold animals during the acclimation
period could also be characteristic of fearless individuals
[53,54]. Results from the two behavioural tests are therefore
consistent. In response to the hypoxia challenge, both
bold and shy fish decreased their swimming activity and
enhanced the distance between individuals. These behaviours allow to, respectively, reduce energetic costs and
maximize oxygen uptake [55]. These results suggest that
both phenotypes respond similarly, in their behaviour, to a
life-threatening challenge.
(c) Transcriptomic signatures between bold and shy
individuals
Bold and shy individuals showed only limited differences
in the expression of genes in the whole brain. This contrasts with a previous study highlighting 246 DEGs
between behaviours in the whole brain [28], while we only
observed 6. Nevertheless, in Rey et al. [28], the deployed
risk-taking test was different and involved an hypoxia condition, forcing individuals to leave the sheltered area, and
this was demonstrated to phenotype for a different behaviour
in sea bass [15].
Gene expressions in the pituitary were the most significantly different between bold and shy individuals. The
pituitary gland of bony fishes is composed of seven
royalsocietypublishing.org/journal/rspb
(a)
endocrine-involved cell types [18] under the control of the
hypothalamus in the brain. They release in the circulatory
system hormones playing key roles in many biological processes, such as stress response, behaviour or growth [18].
Interestingly, multiple genes related to the ‘behaviour’ were significantly different between individuals categorized as bold or
shy. Individuals categorized as bold overall showed an
increased expression of genes related to social, exploration
behaviours and memory.
For instance, the gene dlg4 was upregulated in bold individuals. The knock out of this gene leads to reduced social
behaviour and increased anxiety behaviour in mice [56].
This correlates well with the significant behaviour observed
during the novel environment test, with bold animals showing increased grouping (lower inter-individual distance) and
lower anxiety (lower time spent in the periphery) than shy
individuals. Shy individuals had also reduced expression of
jph3, encoding a protein of the junctophilin family, that was
previously demonstrated to have an active role in exploratory
behaviour in mice [57]. This higher exploration tendency for
bold individual is consistent with behavioural responses
both in GRT and novel environment test, and consistent
with classic scheme of features underlying divergent coping
style, i.e. proactive versus reactive individuals [10].
Genes related to memory and learning capacities were generally downregulated in shy individuals (e.g. egr1, npas4). In
some cases, bolder and faster explorers also are fast learners
(a)
(b)
behaviour
7
social behaviour
no. copies
0
behaviour
bold
shy
1.0
(c)
nr
xn
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np
as
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–1
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gn
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dl
g4
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gr
id
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exploration behaviour
no. copies
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behaviour
bold
shy
10.0
1.0
(d)
slc
4a
10
dl
g4
0.1
jp
h3
1
10.0
memory
1000.0
no. copies
100.0
behaviour
bold
shy
10.0
1.0
cy
p7
b1
eg
r1
jp
h3
np
as
se 4
rp
in
slc f1
8a
sn 2
ap
25
th
ad
cy
8
ap
p
0.1
gene
Figure 4. Differences between bold and shy individuals in the expression of genes related to the biological process ‘behaviour’ in the pituitary of European sea bass.
(a) Heatmap of all genes DE in the GO ‘behaviour’. (b–d ) Raw expressions on a log-scale for the two behavioural phenotypes of genes involved in social behaviour,
exploration behaviour and memory (transparent triangles or rectangles for bold or shy individuals, respectively). Mean and standard error are also illustrated. (Online
version in colour.)
[58]. At the opposite, it can also be argued that shy individuals
are more efficient in reverse learning and display more elaborated memory processes [59]. Links between cognition and
animal personality, including boldness, could be influenced
by many factors and are still debated [59,60]. In European sea
bass, previous study observed increased expression of egr1, a
gene involved in neurogenesis, of individuals categorized as
shy in an individual novel environment test [25], while the
opposite has been observed in the present study. Differences
may be explained by the fact that the GRT and individual
novel environment are measuring different behavioural
responses, potentially due the context, isolation versus group
testing [25]. These differences could also be due to difference
in sampling time (after the experiment versus control) and
thus differences in fish stress state, or due to the difference in
brain parts sampled. This example illustrates that multiple
sampling in various context are a key to understand the
inter-individual variability.
In the head kidney, many genes were also DE between
the two groups. The head kidney is central in the stress
axis regulation, by releasing cortisol and catecholamines
into the blood. The analogue adrenal gland in mammals
was previously demonstrated to show differences in the
expression of genes related to glucocorticoid receptor signalling between two clear behavioural phenotypes in pigs [61].
While in our study a significant enrichment for ‘response to
stimulus’ was observed, none of the DE genes were part of
the GO ‘glucocorticoid receptor signalling pathway’
(GO:0042921). In teleosts, the head kidney is also involved
in the immune system, by producing and maturing white
blood cells and hosting antibody-producing cells [19,20].
The genes that are part of the ‘immune system processes’
were the most different between the two groups, with individuals categorized as bold showing overall increased gene
expression. Although the link between behaviour and
immune system is well documented in mammals, this link
Proc. R. Soc. B 289: 20220399
th
lypd1
slc4a10
nlgn3
snap25
slc8a2
cyp7b1
cntnap4
pum1
ctnnd2
astn1
app
lamb1
sobp
adcy8
serpinf1
apba1
npas4
jun
nr1d2
mbd5
nrxn2
jph3
egr1
egr2
grid1
dlg4
strbp
spg11
apba1
2
royalsocietypublishing.org/journal/rspb
100.0
casp3
trpv1
arrdc3
(a)
immune system process
8
(b)
irf4
2
3000
prkcb
klhl6
1
2
no. copies
pik3cd
1000
behaviour
bold
shy
300
tnrc6c
no
tc
h2
pi
k3
r1
tn
rc
6c
gene
Ipin1
krf13
(c)
leukocyte activation
traf2
pik3r1
1000
tsc22d3
tagap
no. copies
ddit4
behaviour
bold
shy
100
dhx58
rpl30
pawr
irf
4
itg
av
no
tc
h2
pa
wr
pi
k3
cd
pi
k3
r1
pr
kc
b
10
trim25
gene
Figure 5. Differences between bold and shy individuals in the expression of genes related to the biological process ‘immune system process’ in the head kidney of
European sea bass. (a) Heatmap of all genes DE in the GO ‘immune system process’. (b,c) Raw expressions on a log-scale for the two behavioural phenotypes of
genes involved in immune system development and leucocyte activation (transparent triangles or rectangles for bold or shy individuals, respectively). Mean and
standard error are illustrated. (Online version in colour.)
has been investigated in only a few fish studies reporting distinct immune capacities between behavioural phenotypes
[53,62]. Whether the higher expression of immune genes
observed for bold European sea bass in our study translates
to increased immune functions, later affecting responses to
pathogens, still needs to be demonstrated.
Ethics. Experiments were authorized by ethics committee agreement
APAFIS#10745 and all procedures involving animals were in accordance with the ethical standards of the institution and followed the
recommendations of Directive 2010/63/EU.
Data accessibility. All data and models used are available in the electronic
supplementary material [66]. Raw sequencing data are available
under the GEO accession no. GSE195636.
Authors’ contributions. B.S.: conceptualization, data curation, formal
5. Conclusion
Our study demonstrates that inter-individual differences in
risk-taking behaviour are consistent over time and environmental conditions. Differences observed in the GRT also
translated into differences in a novel environment test in
smaller groups, where bold individuals adapted faster to a
novel environment than shy fish. Finally, inter-individual
differences also correlated with transcriptomic changes in
the pituitary and head kidney, mostly through differences
in behaviour- and immune-related genes. These results
highlight the importance of investigating inter-individual
differences in behaviour and physiology for the ecology of
species as they probably explain dispersal and/or migration
at the population scale [63,64] and are as such at the forefront
of the response to global changes [65].
analysis, methodology, visualization, writing—original draft;
S.A.: data curation, methodology, software and writing—review
and editing; C.G.: data curation, methodology and writing—review
and editing; M.P.: data curation and formal analysis; S.R.: data curation, formal analysis and writing—review and editing; B.G.: data
curation, formal analysis, investigation and writing—review and
editing; M.-L.B.: conceptualization, funding acquisition, project
administration, supervision and writing—review and editing.
All authors gave final approval for publication and agreed to be
held accountable for the work performed therein.
Conflict of interest declaration. We declare we have no competing interests.
Funding. This work was funded by the ERANET COFASP project SUSHIFISH (ANR-15-COFA-0002-01) and ERANET ANIHWA WINFISH
(grant no. ANR-14- ANWA-0008). MGX acknowledges financial support from France Genomique National infrastructure funded as part
of ‘Investissement d’Avenir’ program managed by Agence Nationale
pour la Recherche (contract ANR-10-INBS-09).
Acknowledgements. We also thank Peter Biro for stimulating discussions
regarding statistics. Finally, authors are also grateful to Emilie
Levavasseur for the European sea bass drawing in figure 1.
Proc. R. Soc. B 289: 20220399
notch2
kl
f1
3
ets1
irf
4
100
–1
et
s1
itgav
royalsocietypublishing.org/journal/rspb
immune system development
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