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1
Genetic variation in the social environment affects behavioral 1
phenotypes of oxytocin receptor mutants in zebrafish 2
Ribeiro, D. 1, Nunes, A.R.
1, Teles, M.C.
1, Anbalagan, S.
2,3, Blechman, J.
2, 3
Levkowitz, G.2, Oliveira, R.F.
1,4,5,* 4
1Instituto Gulbenkian de Ciência, Oeiras, Portugal;
2Weizmann Institute of Science, 5
Rohovot, Israel; 3 ReMedy-International Research Agenda Unit, Centre of New 6
Technologies, University of Warsaw, Warsaw, Poland. 4ISPA – Instituto 7
Universitário, Lisboa, Portugal; 5Champalimaud Research, Lisboa, Portugal; 8
* E-mail: [email protected] 9
10
Abstract 11
Oxytocin-like peptides have been implicated in the regulation of a wide range of 12
social behaviors across taxa. On the other hand, the social environment, which is 13
composed of conspecifics genotypes, is also known to influence the development of 14
social behavior, creating the possibility for indirect genetic effects. Here we used a 15
knockout line for the oxytocin receptor in zebrafish to investigate how the genotypic 16
composition of the social environment (Es) interacts with the oxytocin genotype (G) 17
of the focal individual in the regulation of its social behavior. For this purpose, we 18
have raised wild-type or knock-out zebrafish in either wild-type or knock-out shoals 19
and tested different components of social behavior in adults. GxEs effects were 20
detected in some behaviors, highlighting the need to control for GxEs effects when 21
interpreting results of experiments using genetically modified animals, since the 22
social environment can either rescue or promote phenotypes associated with specific 23
genes. 24
25
Keywords: social genetic effects, indirect genetic effects, GxE interaction, oxytocin, 26
social behavior, zebrafish 27
28
29
30
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2
Introduction 31
Social genetic effects (aka indirect genetic effects) occur when the phenotype of an 32
organism is influenced by the genotypes of conspecifics. Previous work has 33
highlighted the major potential evolutionary consequences of social genetic 34
effects(1,2), with evidence for such effects to be present both in interactions between 35
related (e.g. mothers and offspring(3,4)) and unrelated individuals (e.g. sexual 36
displays(5), aggression (6–8)). More recently the importance of social genetic effects 37
for health and disease has also been recognized (9), which may explain the 38
pervasiveness of the social environment as a mortality risk in humans (10,11). 39
Interestingly, the potential consequences of social genetic effects for the interpretation 40
of research results using genetically modified organisms (GMO) has been greatly 41
neglected. GMOs have been widely used in behavioral neuroscience to investigate the 42
causal role of candidate genes and behavioral phenotypes. Typically Knock-in and 43
Knock-out transgenics and mutants have been used to causally link the gain or loss of 44
behavioral function to a specific gene (12). In recent years the development of 45
genome editing techniques, such as CRISPR-Cas9-and TALEN- induced mutations, 46
have increased the interest in this approach and opened the door to studying the 47
genetic basis of behavior in non-model organism (13). However, most studies using 48
GMO in behavioral neuroscience have ignored the potential contribution of the 49
environment to the behavioral phenotype studied. This is because it has been assumed 50
that if the genetic background of these mutants is identical and their environment has 51
been kept constant, any phenotypic differences must come from the genetic 52
manipulation. However, when GMOs are incrossed or visually screened at a very 53
young age (e.g. using reporter genes, GFP) and thereafter raised and housed together 54
until used in experiments, changes in their behavior might be affected by divergent 55
social environments experienced by these mutants. In other words, modified behavior 56
might be as a result of growing with their peer mutants, rather than the canonical 57
social environment provided by wild type conspecifics. Such problem is particularly 58
relevant when studying social behavior. For example, mutant Drosophila males that 59
lack the fruitless gene (fruM
) do not express courtship behavior when held in isolation, 60
but acquire the potential to court females when grouped with other flies (14), 61
suggesting the occurrence of a combined gene (G) and social environment (Es) effect 62
(aka, GxEs). Thus, given the rising interest in the study of social behavior in model 63
organisms from worms to higher vertebrates, an assessment of potential gene by 64
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social environment effects on the behavioral phenotype of interest in GMOs used in 65
social neuroscience is crucial). 66
Despite the wide variety of species-specific social behaviors, a wealth of 67
evidence has implicated the paralog nonapeptides vasopressin (VP) and oxytocin 68
(OXT) and their receptors in the regulation of different aspects of social behavior 69
across vertebrates(15,16), suggesting a genetic toolkit role for these nonapeptides in 70
social behavior. Nonapeptides are an ancestral neuropeptide family found both in 71
vertebrates and invertebrates, that derived from a VP-like peptide, and that evolved 72
along two parallel clades of VP- and OXT-like peptides derived from the duplication 73
of the VP gene in early jawed fish (ca. 500 Mya). Both peptides have been implicated 74
in the regulation of behavior and physiology across different taxa, with VP being 75
more involved in aggression and agonistic behaviors and OXT-like peptides 76
consistently acting in affiliative behaviors and species-specific social behaviors across 77
diverse taxa (i.e. sexual behavior, social interactions) (17,18). Despite this wealth of 78
evidence on the direct genetic effects of OXT on social behavior, social genetic 79
effects of OXT genotypes have never been studied. 80
In this study we aimed to provide a proof of principle for GxEs effects in 81
behavioral phenotypes observed in GMO by assessing the occurrence of such effects 82
in a knockout line for the OXT receptor in zebrafish, a commonly used model species 83
in behavioral neuroscience (19). For this purpose, we studied the GxEs interaction in 84
the effects of the OXT gene (oxtr) in different aspects of social behavior, namely: 85
social preference (i.e. social approach); social habituation; social recognition; and 86
shoaling behavior. For this purpose, we have raised individual zebrafish of the WT 87
(oxtr(+/+)
) or knock-out genotype (oxtr(-/-)
) in different social environments (i.e. oxtr(+/+)
88
shoal or oxtr(-/-)
shoal; Figure 1A). 89
90
Results and Discussion 91
Adult zebrafish, like many other social animals, express a tendency to approach and 92
interact with conspecifics (social preference, Figure 1B) (20). Here we show that there 93
was no significant effect of either genotype, environment or GxEs interaction on 94
social preference (Table 1; Figure 1C). However, when fish were presented for a 95
second time to a shoal to measure social habituation (i.e. expected reduction in social 96
preference), we found a GxEs interaction, where oxtr(-/-)
individuals raised in oxtr(-/-)
97
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shoals express enhanced social habituation (F1,44 = 5.642, p = 0.022; Figure 1D). In 98
contrast, when we tested social recognition, which is a form of social memory needed 99
for individuality in social interactions (i.e. differential expression of social behavior 100
depending on identity of interacting individual), that is known to be modulated by 101
oxytocin both in mammals and zebrafish (21,22), we observed that oxtr(-/-)
individuals 102
exhibit a deficit in acquisition and retention of social recognition irrespective of the 103
social environment (oxtr(-/-)
or oxtr(+/+)
) in which they were raised (F1,44 = 7.600, p = 104
0.008; Figure 1F). 105
Given that social behavior of zebrafish mainly occurs in the context of 106
shoaling we have also investigated two shoaling behavior parameters: (1) social 107
integration – how well the focal individual integrates in the shoal, as measured by its 108
average distance to the centroid of the shoal (Figure 1G,H); and (2) social influence – 109
how the focal individual affects the shoaling behavior of the remaining shoal 110
members, by measuring the shoal dispersion as defined by the perimeter of the other 111
shoal members (Figure 1I,J). A GxEs interaction was found for social integration: 112
oxtr(-/-)
individuals raised in oxtr(-/-)
shoals exhibit a significantly lower social 113
integration than oxtr(-/-)
individuals raised in oxtr(+/+)
shoals; in contrast, oxtr(+/+)
114
individuals exhibit high levels of social integration irrespective of the shoal type in 115
which they were raised (Table 1; Figure 1H). Finally, the presence of a single WT 116
(oxtr(+/+)
) individual in a oxtr(-/-)
shoal was enough to increase its dispersion, whereas 117
the presence of a single oxtr(-/-)
individual in a oxtr(+/+)
shoal did not affect its 118
dispersion (Table 1; Figure 1J). 119
In summary, we show that distinct components of social behavior are 120
differentially affected by the genetic composition of the social environment versus the 121
oxtr genotype of the focal individual. Social recognition exhibited a pure effect of the 122
genotype. However, clear GxEs interactions were observed in the cases of social 123
habituation and social integration, and social influence had a major contribution of the 124
social environment. Thus, we demonstrated that genetic variation in the social 125
environment interacts with individual genotype during the developmental acquisition 126
of social behavior. In other words, the social environment can revert phenotypes 127
associated with specific genes. These results are in line with reported interactions 128
between the social environment and oxytocin receptor genotype in the determination 129
of social behavior phenotypes in human populations (23–25). Our results suggest that 130
more caution is needed in the interpretation of studies using transgenic or mutant 131
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individuals that are raised in cohorts of the same genotype, and that some phenotypes 132
observed in transgenic or mutant lines may in fact result from GxES interactions. 133
134
Materials and Methods 135
Zebrafish lines and maintenance 136
Zebrafish were raised and bred according to standard protocols and all experimental 137
procedures were approved by the host institution, Instituto Gulbenkian de Ciência, 138
and by the National Veterinary Authority (DGAV, Portugal; permit number 139
0421/000/000/2013). OXTR mutant zebrafish line (ZFIN ID: ZDB-ALT-190830-1) 140
was generated and provided by Dr. Gil Levkowitz (Weizmann Institute of Science) 141
using a TALEN-based genome editing system. The characterization of this line has 142
been described in Nunes et al. (26). 143
All the experimental groups were formed at 4 days post-fertilization, based on 144
the genotype of the progenitors, before they imprint for olfactory and visual kin 145
recognition (27,28). To evaluate genotype-environment effects, fish were raised in 146
groups according to the experimental design in Figure 1A and both female and males 147
tested in adulthood (3 months old). 148
149
Genotyping 150
At 3 months old, before behavioural screenings, genomic DNA was extracted from 151
adult fin clips using the HotSHOT protocol (29). The genomic region of interest was 152
amplified by PCR and sequenced to identify the focal fish in each group. The 153
following primers were used: sense 5’-TGCGCGAGGAAAACTAGTT-3’, antisense 154
5’-AGCAGACACTCAGAATGGTCA-3’. 155
156
Behavioural assays 157
Video acquisition 158
Fish were in a tank placed on top of an infrared lightbox and video-recorded either 159
from above (shoal preference and social recognition tests) or laterally (group 160
behaviour tests). Video acquisition was done with software Pinnacle Studio 14 (Corel 161
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Corporation, Ottawa, Canada). Shoal preference, social habituation and social 162
recognition analyses were performed with EthoVision video tracking system (Noldus 163
Information Technologies, Wageningen, The Netherlands) and group behaviour 164
analyses were done with the open source FIJI image-processing package (30). 165
166
Social preference and social habituation 167
The social preference test assesses the individual’s sociability by observing the 168
interactions between conspecifics (22): a focal fish was placed in a central 169
compartment (30x15x10 cm) of a three-compartment tank, separated by transparent 170
and sealed partitions. A shoal of fish was placed in one of the lateral compartments 171
(15x10x10 cm), while the other contained only water. To avoid any side bias the 172
stimuli were balanced across trials. After an acclimatization period (10 min), the focal 173
fish was released from a start box and allowed to explore the tank, while its behaviour 174
was video-recorded for 10 min. The time spent by the focal fish near (less than two 175
body lengths) each compartment was quantified and used to calculate the social 176
preference score (SP = Time near shoal/ [Time near shoal + Time near empty]). A 177
score above 0.5 indicates a preference for the shoal. 178
The social preference test was performed twice, with 24 hrs in between, and social 179
preference scores of both tests were used to calculate the habituation index (Hab. 180
Score = 1- [SPTrial2]/[SPTrial1 + SPTrial2]). A score above 0.5 reveal a decrease in 181
preference to associate with conspecifics. 182
183
Social recognition 184
The social recognition assay to evaluate short-term (i.e. 10 min retention) social 185
memory was adapted from the procedure already developed in our lab for long-term 186
(i.e. 24h retention) social memory in zebrafish (31), and has already been used 187
successfully in a previous study (22). A focal fish was placed for 10 min in the central 188
compartment of a three-compartment tank, separated by transparent and sealed 189
partitions, to acclimatize. The focal fish was allowed to interact visually across 190
partitions with 2 novel conspecifics for 10 min. After, both stimuli were removed, one 191
was placed in the same compartment (familiar conspecific stimulus), while a novel 192
conspecific was placed in the other compartment (novel conspecific stimulus). In a 193
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second 10 min interaction the time spent by the focal fish near each compartment 194
(termed novel cue or familiar cue) was quantified and used to measure the preference 195
for the novel (Disc.Score= Time near Novel/[Time near Novel + Time near 196
Familiar]). A discrimination score of 0.5 indicates no preference between novel or 197
familiar conspecifics. 198
199
Shoaling behaviour 200
Shoaling behaviour is a common behaviour present in fish models and allows to 201
determine complex interactions between individuals. Both focal fish and social 202
partners were recorded in the home tanks (3.5L tank). Focal fish were tagged with fin 203
clips for easy identification. The behaviours were video-recorded from side view for 204
10 min. Two components of shoaling behaviour were analysed in time bins of 8 205
seconds: (1) focal fish distance to the group centroid (social integration); and (2) the 206
dispersion of the remaining shoal members as measured by their perimeter (social 207
influence). 208
209
Data analysis 210
Data were analysed using SPSS 25.0. All data sets were tested for departures from 211
normality with Shapiro-Wilks test. Two factor univariate ANOVA were used for 212
comparing multiple groups. All data sets were corrected for multiple comparisons. 213
Tukey’s Test comparisons were used as post-hocs. Graphs were performed with 214
GraphPad software. 215
216
Ethical Approval 217
All experiments were performed in accordance with the relevant guidelines and 218
regulations for the care and use of animals in research and approved by the competent 219
Portuguese authority (Direccao Geral de Alimentacao e Veterinaria, permit 220
0421/000/000/2017). 221
222
223
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224
Acknowlegdements 225
We thank IGC Fish Facility for assistance on fish maintenance and Peter McGregor 226
for comments and discussion on an earlier version of the manuscript. The authors 227
declare no conflicts of interest related to this work. This work was funded by a 228
Fundação para a Ciência e a Tecnologia (FCT) research grant (PTDC/BIA-229
ANM/0810/2014) and a FEDER grant (Lisboa-01-0145-FEDER-030627) awarded to 230
RFO. ARN and MST were supported by Post-doc fellowships from Fundação para a 231
Ciência e a Tecnologia (FCT, ARN: SFRH/BPD/93317/2013). Congento LISBOA-232
01-0145-FEDER-022170, co-financed by FCT (Portugal) and Lisboa2020, under the 233
PORTUGAL2020 agreement (European Regional Development Fund). The authors 234
declare no competing interests. 235
236
Author contributions 237
R.F.O. and D.R. designed the research; D.R. and A.R.N. performed the research; 238
D.R., A.R.N. and M.S.T. carried out the molecular lab work; A.S., J.B. and G.L. 239
developed and validated the mutant zebrafish line; D.R. and M.S.T. carried out the 240
statistical analyses; D.R. and R.F.O. wrote the paper and all authors critically revised 241
the manuscript. 242
243
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Figure legends 330
331
Figure 1. The contribution of the social environment (Es) and genotype (G) to the 332
expression of behavioral phenotypes in zebrafish was assessed by raising oxytocin 333
receptor mutant fish and wild types (focal fish marked with *) in shoals of either 334
mutants or wild types (A). Social preference, measured by the time spend near a shoal 335
in a choice test (B), showed no effect of either G or Es (C). Social habituation, which 336
consisted on a consecutive social preference test exhibited a GxEs effect (D). Social 337
recognition, measured as the discrimination between a novel and a familiar 338
conspecific (E), shows a pure G effect (F). Social integration, measured as distance to 339
the centroid of the shoal (G), showed a GxEs effect (H). Social influence, measured by 340
the cohesion of the remaining shoal members (I), also showed a GxEs effect (J). Data 341
is presented as mean ± standard error of the mean (SEM). Sample sizes are 9 for 342
heterogeneous groups (i.e. focal individual with different genotype from the 343
remaining individuals in the shoal; mutant focal in WT shoals and WT focal in mutant 344
shoals) and 15 for homogeneous groups (i.e. focal individual with the same genotype 345
of the remaining individuals in the shoal; mutant focal in mutant shoals and WT focal 346
in WT shoals). Different letters indicate significant differences (p<0.05) between 347
treatments as assessed by Two-Way ANOVA followed by post-hoc tests (see Table 348
1). 349
350
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Tables 351
352
Table 1. Two-way ANOVA for ‘genotype’, ‘environment’ and their interaction 353
(‘genotype’ x ‘environment’) was analysed for each behavioural essay. * indicates p < 354
0.05, ** indicates p < 0.01, *** indicates p < 0.001. 355
Social Preference
d.f. Mean Squares F Significance
Genotype (G) 1 0.023 1.731 0.195
Environment
(E) 1
0.050 3.788 0.058
G x E 1 0.001 0.049 0.825
Error 44 0.013
Habituation
d.f. Mean Squares F Significance
G 1 0.058 13.927 0.001 **
E 1 0.008 1.936 0.171
G x E 1 0.024 5.642 0.022 *
Error 44 0.004
Social Recognition
d.f. Mean Squares F Significance
G 1 0.213 7.600 0.008 **
E 1 0.005 0.189 0.666
G x E 1 0.001 0.041 0.841
Error 44 0.028
Social Competence
d.f. Mean Squares F Significance
G 1 39.486 24.370 <0.001 ***
E 1 12.565 7.755 0.008 **
G x E 1 12.811 7.907 0.007 **
Error 44 1.620
Social Group Dispersion
d.f. Mean Squares F Significance
G 1 174.366 4.309 0.044 *
E 1 657.221 16.240 <0.001 ***
G x E 1 122.980 3.039 0.088
Error 44 40.469
356
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A B C
D E F
G H I J
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