1. Introduction
Social interactions (SIs) include a wide range of interactions among individuals and play a fundamental role in their lives. These interactions can be complex and diverse, often involving a range of behaviors, such as mating, aggression, dominance, vocalizations, body language, etc. [
1]. Over time, as individuals within the group interact with each other, specific patterns and dynamics emerge. Disruption in normal social behavior and SI is a common feature of various human diseases and conditions, such as neurodevelopmental disorders [
2]. An example of neurodevelopmental disorder which is characterized by impaired SI is fragile X syndrome (FXS).
FXS is caused by a full mutation (>200 CGG trinucleotide repeats) in the Fragile X Messenger Ribonucleoprotein 1 (
FMR1) gene and is the most common monogenic cause of inherited intellectual disability and autism spectrum disorder [
3]. Among other symptoms, individuals with FXS experience a wide range of challenges related to SI including challenges in maintaining eye contact, shyness, social anxiety, social withdrawal, and social avoidance [
4,
5]. Impairment of SI often causes daily struggles that significantly impact the ability of FXS individuals to engage in typical daily activities [
6].
Animal models have played a crucial role in advancing the understanding of FXS. Commonly used animal models include
Fmr1 knock-out (KO) mice,
Fmr1 KO zebrafish, and the
Drosophila melanogaster (
D. melanogaster) model of FXS, where the only ortholog of human
FMR1 (
dFMR1) is mutated [
7,
8,
9,
10,
11].
D. melanogaster, commonly known as the fruit fly, has a well-characterized nervous system and genetic manipulations can be performed to mimic the genetic mutations associated with FXS.
D. melanogaster represents a valuable model for understanding the molecular and neurological aspects of the syndrome [
9,
10]. It is well known that phenotypes in
dFMR1 mutants closely resemble the phenotypes in individuals with FXS. For example,
dFMR1 mutants display arrhythmic circadian rhythm, abnormal locomotor activity and learning and memory deficits, similar to the symptoms of FXS [
12,
13,
14]. Despite
D. melanogaster being an excellent model for studying FXS, there are currently limited data on SI in this model. On the other hand, there are studies that described SI in other fruit fly models. For example, SI impairments were described in
orco,
lush, and
or65a mutants [
15,
16,
17]. Exploring and collecting more information on SI in the
D. melanogaster model of FXS could significantly enhance FXS research and contribute to the preclinical evaluation of drug effects in this condition.
The aim of this study was to investigate, analyze and describe SI in the FXS model of D. melanogaster using the novel chamber and a Python data processing pipeline based on social network analysis (SNA).
4. Discussion
The current study is among the first studies that describe in detail impaired SI in the FXS model of D. melanogaster. According to our results, dFMR1B55 flies demonstrated hypoactivity, fewer connections in their networks, a lower ability to efficiently transmit information due to fewer alternative pathways for information transmission, a higher variability in the number of interactions they achieved among themselves, and tended to stay near the boundaries of the testing chamber. However, they mostly interacted with individuals who had a similar number of interactions. In addition, the distances among them were longer and the spread of information was slower. Interestingly, our results suggested that there were individual dFMR1B55 flies in the network that played important roles as intermediaries connecting different parts of the network and higher modularity suggests that a dFMR1B55 network can be divided into distinct communities with more connections within the community than outside of it. On the other hand, two groups of flies (wild-type and dFMR1B55) exhibited similar local connectivity patterns. Briefly, dFMR1B55 flies achieved a lower total and average number of SIs, and exhibited alterations in various SIN measures compared to wild-type flies. These alterations suggest changes in mobility, connectivity, and overall network organization in dFMR1B55 flies.
In contrast to previous studies that researched SI in
dFMR1 mutants, all presented results obtained in the current study were based on SNA as a powerful statistical tool. SNA has been used in the last 20 years to analyze the collective animal behaviors and to identify patterns of SI in groups (reviewed in: [
30]). There has also been a growing interest in utilizing SNA to examine the social behavior of
D. melanogaster [
15,
21,
24,
26,
30,
31,
32,
33,
34]. In addition, our study, for the first time, provides results of SI in the FXS model of fruit flies obtained using a combination of novel tools: the Drosophila Shallow Chamber and the open-source Python data processing pipeline for analysis of SI in
D. melanogaster. Specifically, a validation of SNA in research with fruit flies, as a method that was chosen and used in the current study, was recently published [
25].
Only a few studies focused on SI in the
D. melanogaster model of FXS (reviewed in: [
9]). Dockendorf
et al. (2002) showed that
dFMR1B55 mutants exhibit altered courtship and mating behavior [
12]. Male flies failed to advance to more intricate phases of courtship beyond following and tapping, resulting in shorter time spent in courtship activities [
12,
35]. These findings may resemble the loss of interest to engage in SI, which is frequently present in humans with FXS [
12,
36]. In addition, Bolduc et al. (2010) used a different methodology to study SI in
dFMR1B55 and
dFMR13 mutants [
37]. Specifically, they used two chambers separated by a plastic mesh to research parameters related to SI and demonstrated that both
dFMR1 mutants and control groups tended to stay near the boundaries of the testing chamber. This behavior, known as thigmotaxis, has been well-documented and observed in fruit flies [
37,
38]. Our results are in accordance with previous observations and confirmed the presence of thigmotaxis in
dFMR1B55 using a novel Shallow Chamber. The Shallow Chamber was developed by Maze Engineers to study groups of flies using a design to avoid the flies from obscuring one another. In addition, Bolduc et al. (2010) showed hypoactivity in both
dFMR1 mutants, which is consistent with our data [
37]. Hypoactivity was also previously found in
dFMR1 larvae [
39]. Furthermore, the likelihood of SI, measured by the interfly distance, was lower in
dFMR1B55 than in wild-type flies [
37]. This is in line with our findings:
dFMR1B55 flies had a lower number of interactions with greater interfly distance, shown through lower closeness centrality. Moreover, it was shown that
dFMR1 mutants and wild-type flies display different spatial distributions within the chamber. Wild-type flies were uniformly distributed, while
dFMR1B55 flies preferred the chamber interior [
37]. Interestingly,
dFMR13 mutants showed a phenotype with shared characteristics of both
dFMR1B55 and wild-type flies. Our results also indicate a uniform distribution of wild-type flies in the social network. Lower betweenness centrality and lower modularity primarily mean that each fly is equally important in the information transmission process and that wild-type SINs are more uniform than
dFMR1B55 SINs. Additionally, researchers previously noticed that
dFMR1B55 flies made frequent, irregular stops. Such a behavior was described as an arrhythmic phenotype by Dockendorff and colleagues in 2002 and Bolduc and colleagues in 2010 [
12,
37]. This behavior, described as a basic form of dyspraxia, agrees with a decreased receptive response observed in
dFMR1B55 flies [
12,
37]. While the SNA that was used in our study does not provide information regarding the regularity and duration of individual stops out of SI, other results in our study are consistent with limited data of SI in the FXS model of
D. melanogaster. However, our results provide more data about their mutual interactions, as well as their role in the social network over time.
The importance of the current research on SI in the
Drosophila FXS model is based on the fact that the social network structure is highly influenced by genotype [
15,
24,
31,
32]. Different
D. melanogaster strains exhibit differences in SINs, indicating the potential influence of genes on SIN structure [
31]. Wice and Saltz (2021) analyzed five commonly studied SIN measures (in-strength, out-strength, clustering coefficient, and betweenness centrality) in 40 randomly chosen inbred lines of flies [
24]. They confirmed that individual’s genotype was a significant indicator for all network measures examined [
24]. The authors calculated broad-sense heritability, a genetic parameter used to estimate the proportion of phenotypic variation due to genetic factors [
40]. Betweenness centrality displayed the highest broad-sense heritability, with genotype accounting for about 17% of the variation in this network measure [
24]. Additionally, Wice and Saltz (2023) demonstrated that SIN measures depend on the individual’s genotype and also on the genotypes of other individuals within the network [
24,
32]. Furthermore, Alwash et al. (2021) investigated the characteristics of SI in flies mutated in
for, a pleiotropic gene regulating several metabolic, physiological and behavioral phenotypes [
31]. They demonstrated that the positions within the group are inherited and that the flies form SINs are robust over time [
31]. These SINs are characteristic strain-dependent social networks and are known as group phenotype [
31]. Here, we described in details that the genotype that was in our focus (
dFMR1 mutants) plays a crucial role in SI and suggest that
dFMR1 models could be used in various biomedical and pharmacological studies which are based on SI impairment.
Apart from genetic factors, variation in SINs could be due to other factors, such as social experience. Jezovitz et al. (2021) compared the results of studies that focused on social networks in
D. melanogaster, and observed that, despite methodological differences, studies agree that isolated flies exhibit distinctly altered SINs [
30]. Isolated flies form SINs with increased global efficiency and lower betweenness centrality and these characteristics are recorded regardless of fly age [
15,
23,
30,
33]. Moreover, significant variability across all SIN measures in isolated flies could suggest that a lack of social experience results in less predictable networks [
30]. Although, in the current study,
dFMR1 mutants were not isolated, variability of a few SIN measures was also observed in these mutants. These findings suggested that variability of some SIN measures is influenced by other factors in addition to isolation. Thus, further behavioral and molecular studies are needed to identify more details in SI impairments in
dFMR1 mutants, as an excellent model for pharmacological screening studies.