Supergenes on steroids.

Article Figures and data Abstract Editor's evaluation Introduction Results Discussion Materials and methods Data availability References Decision letter Author response Article and author information Metrics Abstract In white-throated sparrows, two alternative morphs differing in plumage and behavior segregate with a large chromosomal rearrangement. As with sex chromosomes such as the mammalian Y, the rearranged version of chromosome two (ZAL2m) is in a near-constant state of heterozygosity, offering opportunities to investigate both degenerative and selective processes during the early evolutionary stages of ‘supergenes.’ Here, we generated, synthesized, and analyzed extensive genome-scale data to better understand the forces shaping the evolution of the ZAL2 and ZAL2m chromosomes in this species. We found that features of ZAL2m are consistent with substantially reduced recombination and low levels of degeneration. We also found evidence that selective sweeps took place both on ZAL2m and its standard counterpart, ZAL2, after the rearrangement event. Signatures of positive selection were associated with allelic bias in gene expression, suggesting that antagonistic selection has operated on gene regulation. Finally, we discovered a region exhibiting long-range haplotypes inside the rearrangement on ZAL2m. These haplotypes appear to have been maintained by balancing selection, retaining genetic diversity within the supergene. Together, our analyses illuminate mechanisms contributing to the evolution of a young chromosomal polymorphism, revealing complex selective processes acting concurrently with genetic degeneration to drive the evolution of supergenes. Editor's evaluation In this important paper, the authors generate and analyze new genome and gene expression data to understand better the evolution of the white-throated sparrow supergene region, which contains 1000 genes and determines whether a bird has a tan or a white stripe. The study convincingly illustrates how the cessation of recombination that results from a chromosomal inversion can become a source of evolutionary novelty. The lack of recombination can result in the accumulation of deleterious variation leading to degeneration, but it can also (as here) facilitate genomic diversification and adaptation. The results will be of interest to a broad array of researchers studying genome architecture and phenotypic diversity and evolution. https://doi.org/10.7554/eLife.79387.sa0 Decision letter Reviews on Sciety eLife's review process Introduction Supergenes comprise closely linked genetic variants that are maintained due to suppressed recombination (Charlesworth, 2016; Thompson and Jiggins, 2014). Their evolution presents an interesting paradox, in that the suppression of recombination that occurs inside supergenes reduces the efficacy of natural selection, leading to genetic degeneration. At the same time, supergenes are associated with dramatically divergent, adaptive phenotypes. These divergent phenotypes, which include classic examples of Batesian mimicry and self-incompatibility in flowering plants (Charlesworth, 2016; Thompson and Jiggins, 2014; Otto and Lenormand, 2002) and striking polymorphisms in social behavior (Wang et al., 2013; Huang et al., 2018; Yan et al., 2020; Martinez-Ruiz et al., 2020; Farrell et al., 2013; Küpper et al., 2016; Lamichhaney et al., 2016), have long inspired both theoretical and empirical studies of their evolution. Recent genome-scale studies have illuminated wide-ranging impacts of supergene evolution on complex phenotypes across diverse taxa (e.g. Schwander et al., 2014; Pearse et al., 2019; Hager et al., 2022; Joron et al., 2011; Kunte et al., 2014; Kess et al., 2019; Lundberg et al., 2017; Roberts et al., 2009; Sanchez-Donoso et al., 2022; Funk et al., 2021). Currently, the mechanisms by which functionally divergent supergene haplotypes evolve in the face of multiple evolutionary forces remain poorly understood, presenting a critical gap in knowledge. One notable example of a supergene associated with social behavior is found in white-throated sparrows (Zonotrichia albicollis), in which a large supergene co-segregates with parental behavior and aggression (Tuttle, 2003; Maney et al., 2015; Horton et al., 2013; Tuttle et al., 2016; Sun et al., 2018; Merritt et al., 2020). White-throated sparrows occur in two alternative plumage morphs, white- and tan-striped (Lowther, 1961). These morphs differ not only in their plumage coloration, but also in their social behavior, with white-striped birds exhibiting increased aggression and more frequent extra-pair copulations, and tan-striped birds engaging in more parental care compared with birds of the white-striped morph (Tuttle, 2003; Maney et al., 2015; Maney, 2008; Horton et al., 2012). These alternative morphs are linked to a large (~100 Mbp, >1 k genes) rearrangement on the second largest chromosome, called ZAL2m, so named because the rearranged chromosome is metacentric. White-striped birds are heterozygous for ZAL2m and the sub-metacentric chromosomal arrangement, ZAL2, whereas tan-striped birds are homozygous for ZAL2 (30, 31). In addition to highly divergent social behavior, this relatively young supergene (estimated to have arisen 2–3 million years ago Tuttle et al., 2016; Thomas et al., 2008; Huynh et al., 2010), is also associated with a remarkable disassortative mating system that maintains the ‘balanced’ morph frequencies in the population. Almost all breeding pairs consist of one bird of each morph, earning the species the moniker ‘the bird with four sexes’ (Campagna, 2016). Breeding pairs consisting of two individuals of the same morph are estimated to occur less than 1% of the time (Tuttle et al., 2016; Thorneycroft, 1966) and only six ZAL2m homozygotes (i.e. ‘super-white’ birds) have ever been identified (Horton et al., 2013; Tuttle et al., 2016; Thorneycroft, 1975; Falls and Kopachena, 2020) out of thousands of birds karyotyped or genotyped. Given that ZAL2m exists in a near-constant state of heterozygosity, it is in a state of suppressed recombination, similar to the Y and W sex chromosomes in mammals and birds, respectively. The suppression of recombination on ZAL2m is expected to reduce the efficacy of natural selection, leading to reduced genetic diversity and the degeneration of the chromosome (Barton and Charlesworth, 1998; Charlesworth, 2012). On the other hand, the tight linkage of alleles within the ZAL2m supergene may contribute to adaptive phenotypes (Tuttle et al., 2016; Sun et al., 2018; Maney et al., 2020). Therefore, this system provides a unique opportunity to investigate the evolution of a supergene underlying social and mating behavior (Tuttle et al., 2016; Sun et al., 2018; Merritt et al., 2020; Sun et al., 2021). Here we aim to better understand the evolutionary forces shaping the ZAL2 and ZAL2m chromosomes. Our goal was to address two unanswered questions. First, to what extent has ZAL2m degenerated? Early analyses of the rearrangement (Davis et al., 2011) did not show signals of degeneration, such as pseudogenization or the accumulation of repetitive sequences. However, Tuttle et al., 2016 found a weak signal of excess non-synonymous polymorphism for genes inside the rearranged region on ZAL2m and reduced allelic expression for ZAL2m genes, which could be consistent with functional degradation of ZAL2m (Tuttle et al., 2016). (Sun et al., 2018) similarly found a slightly higher number of non-synonymous substitutions and an increased ratio of non-synonymous to synonymous substitution rates (dN/dS) on ZAL2m compared with ZAL2. (Sun et al., 2018) also found reduced expression of ZAL2m alleles in brain tissue, perhaps suggesting that the accumulation of deleterious mutations has led to reduced expression of genes from ZAL2m. Their additional finding of reduced accumulation of mutations in functional regions suggested that ZAL2m has, in fact, experienced purifying selection to remove deleterious alleles. Thus, while there is some evidence that ZAL2m has degenerated, these results have been inconsistent and somewhat inconclusive. Second, what are the selective forces shaping the genomic landscapes of both ZAL2 and the ZAL2m supergene? The signals of both purifying and positive selection have been relatively weak in previous genomic analyses of ZAL2 and ZAL2m (Tuttle et al., 2016; Sun et al., 2018). Yet, by definition, ZAL2m must contain variation that underlies the differences between the white- and tan-striped morphs (Fisher, 1931; Bull, 1983; Charlesworth and Charlesworth, 1980; Rice, 1987b; Rice, 1987a). There is already some evidence that this variation affects behavior; allelic differences in the promoter region of the gene encoding estrogen-receptor alpha (ESR1) are likely to alter expression (Merritt et al., 2020), and the expression of this gene was shown to be necessary for aggressive behavior typical of the white-striped morph (Merritt et al., 2020). ZAL2m is also associated with differential expression of a key neuromodulator, vasoactive intestinal peptide (Horton et al., 2020), known to be causal for aggression in songbirds (Goodson et al., 2012). Investigations of young heteromorphic sex chromosomes suggest that the accumulation of sexually antagonistic genes (i.e. genes that are beneficial to one sex and harmful to the other) may in fact drive the evolution of sex chromosomes (Bachtrog, 2004; Bachtrog, 2006; Zhou and Bachtrog, 2012). For example, positive selection at a small number of antagonistic alleles was shown to be a potent force shaping evolution of the young Y chromosomes in Drosophila miranda (Bachtrog, 2004) even in the face of degeneration of other genes elsewhere on the chromosome. In white-throated sparrows, evidence of positive selection on both ZAL2 and ZAL2m has been quite limited (Tuttle et al., 2016; Sun et al., 2018). Nonetheless, the discovery of ZAL2- and ZAL2m-specific alleles that benefit the tan- and white-striped morphs, respectively (Merritt et al., 2020; Horton et al., 2020), suggests that antagonistic selection likewise contributes to the evolution of both ZAL2 and ZAL2m. In addition to antagonistic selection, balancing selection may be implicated in the evolution of ZAL2m. The negative assortative mating system in white-throated sparrows, which maintains the chromosomal polymorphism, is a canonical example of balancing selection (Huynh et al., 2010). However, balancing selection is also a way of maintaining advantageous genetic diversity in populations, which may be especially critical in the context of a non-recombining chromosome. Indeed, balancing selection appears to be more common in self-fertilizing (selfing) vs non-selfing species, which are likewise characterized by reduced genetic diversity, increased linkage disequilibrium, and reduced efficacy of selection (Glémin, 2021; Glémin et al., 2019; Delph and Kelly, 2014; Gaut et al., 2015). Therefore, balancing selection may maintain multiple alleles inside non-recombining regions of chromosomes like ZAL2m. Previous studies have been limited in the extent to which they could test directly for degeneration, adaptive changes on ZAL2m, and selection at the genome level. These limitations stemmed from low sample sizes of sequencing data, the reduced intraspecies variability, and a low-quality ZAL2m assembly that prevented detection of long-range haplotypes (Tuttle et al., 2016; Sun et al., 2018; Thomas et al., 2008). Here, we overcome these challenges by analyzing extensive genomic, transcriptomic, and population data, providing insight into the evolution of young supergenes. Results Novel and extensive genomic and population data from white-throated sparrows To better understand the evolutionary history of the ZAL2m chromosomal rearrangement, we generated additional sequence data from a rare, ‘super-white’ (ZAL2m homozygote) bird (Horton et al., 2013; Sun et al., 2018). We generated variable fragment size libraries consisting of 150 bp paired-end reads (insert size of 300 bp and 500 bp) and 125 bp mate pair reads (insert size of 1 kb, 4–7 kb, 7–10 kb, and 10–15 kb). We performed whole-genome sequencing of an additional 62 birds (49 white-striped birds and 13 tan-striped birds sampled from a variety of locations around the U.S.) (Materials and methods, Supplementary file 1). White-striped birds, which are heterozygous for the rearrangement (ZAL2/2m), were sequenced at higher coverage than tan-striped birds (ZAL2 homozygotes) so that we could obtain sufficient reads to separate ZAL2 and ZAL2m alleles in white-striped individuals (average mean depth coverages were 41.5 × vs 28.4 × for white- and tan-striped birds, respectively, Supplementary file 2). Genomic variants were called according to the guidelines of Genome Analysis Toolkit (GATK) (ver. 4.1) (Materials and methods), leading to the discovery of a total of 11,382,994 single nucleotide polymorphisms (SNPs). None of the samples showed evidence of family relationships when we computed relatedness estimates between individuals. Consequently, we used all samples in the subsequent analyses. We found a significantly higher number of polymorphic sites within white-striped birds than tan-striped birds exclusively for ZAL2/2m chromosomal regions (Figure 1—figure supplement 1). Nucleotide diversity of the rearranged region of the ZAL2/2m chromosomes was elevated in white-striped birds compared with tan-striped birds, suggesting distinctive patterns between the two plumage morphs (Figure 1a). Figure 1 with 2 supplements see all Download asset Open asset Genomic data from newly sequenced tan- and white-striped birds. (A) Nucleotide diversity of macro-chromosomes for tan-striped (TS) and white-striped (WS) birds. White-striped birds (ZAL2/2m) show elevated nucleotide diversity for the ZAL2/2m inverted (INV, i.e. rearranged) regions (ZAL2/2m inv), while TS birds (ZAL2/2) show overall reduced nucleotide diversity for the inverted regions compared with other chromosomes. Note that panel (A) shows the comparison across morph. The comparison across the ZAL2 and ZAL2m alleles is shown in Figure 2a. (B) Scatterplots of eigenvector 1 (PC1) and eigenvector 2 (PC2) from principal component analysis of all single-nucleotide variants (left panel). (C) Principal component analysis (PCA) excluding single nucleotide polymorphisms (SNPs) on the ZAL2 chromosomes (right panel). The sex chromosomes and the ZAL3 chromosome (which includes an additional chromosomal inversion) were excluded from both PCA analyses. Note that ‘location’ here refers to the site of collection or capture of the bird: Georgia (GA), Illinois (IL), or Maine (ME). Breeding locations for GA and IL birds are unknown. Figure 1—source data 1 Nucleotide diversity between tan- and white-striped birds. Figure 1B and C: Supplementary file 1 (PCAs performed using variant call format (vcf) data from whole genome sequencing). https://cdn.elifesciences.org/articles/79387/elife-79387-fig1-data1-v1.txt Download elife-79387-fig1-data1-v1.txt Among the total SNPs identified, 12.6% (N=1,439,991) resided on scaffolds we have previously assigned to the ZAL2/2m chromosome (Sun et al., 2018). Principal component analysis (PCA) of these ZAL2/2m SNPs revealed distinct clusters corresponding to the morphs (Figure 1b). The first principal component (PC1), which explained 6.7% of the variation in the data, clearly separated tan- and white-striped birds, with the lone super-white individual (ZAL2m/2m homozygote) as a clear outlier. In contrast, other available phenotypic information, including sex and geographic origin of samples, did not show meaningful variation with the principal components, and other PCs had little explanatory power (Figure 1b). Tests for admixture also failed to identify significant population substructures by geographical origin of samples (Figure 1—figure supplement 2). This lack of population structure is unsurprising, as 35 of the 63 samples (56%) were from birds that were migrating, and, thus, the breeding location of these birds is unknown. Features of the ZAL2m chromosome consistent with reduced efficacy of natural selection and low levels of recombination We examined several genomic features of the ZAL2m chromosome using the additional genomic resources we generated. We first performed a de novo genome assembly of the super-white bird, employing newly generated sequence data, to study the ZAL2m chromosome with an assembly derived entirely from a bird homozygous for the ZAL2m chromosome. The total assembly size was 1058 Mbp (N50 length of 3.1 Mbp, longest scaffold 27 Mbps), comparable to that of the ZAL2/2 reference assembly (1052 Mbp, N50 scaffold length of 4.86 Mbp, longest scaffold 45 Mbp) (see Supplementary file 3 for more details). There were 160 putatively ZAL2m-linked scaffolds (Materials and methods), with a total length (110.99 Mbp) comparable with that of ZAL2-linked scaffolds from the reference assembly (108.5 Mbp Tuttle et al., 2016). Despite this similarity in total length, however, the average length of the individual ZAL2m-linked scaffolds was significantly shorter than scaffolds on other chromosomes in the super-white assembly (p<0.001, Mann-Whitney U-test). It was also shorter than the average scaffold length on the ZAL2 chromosome in the ZAL2/2 reference assembly (Figure 2a). We did not observe such a pattern in the other chromosomes of similar size when comparing between the two assemblies (Figure 2a). This result was consistent with the presence of repetitive DNA sequences on ZAL2m causing more assembly breaks compared with the ZAL2/2 reference genome. We found evidence that the ZAL2m chromosome contained more repeat elements and was especially enriched for long terminal repeat elements (2.4 Mbp vs 2.1 Mbp) and interspersed repeats (5.8 Mbp vs 5.5 Mbp), compared with the ZAL2 chromosome. The number of these repeat elements is likely to be underestimated, given that the ZAL2m assembly is highly fragmented. Additionally, we found that ZAL2 and ZAL2m had accumulated a higher proportion of structural variants (insertions and deletions) compared with other chromosomes (Figure 2b). Figure 2 with 1 supplement see all Download asset Open asset Genetic divergence between ZAL2 and ZAL2m chromosomes. (A) The scaffolds for the ZAL2m chromosome in the super-white (SWS) assembly tend to be fragmented compared with those for the ZAL2 chromosome in the tan-striped (TS) assembly. ** p<0.001 (Mann-Whitney U-test); ns, not significant (B) Fraction of structural variants (SV), both insertion and deletion events, for the 4 largest chromosomes, using the tan-striped assembly as a reference. The fraction of SV is computed as a total base affected by variants divided by the length of the chromosome. (C) Number of fixed mutations derived in ZAL2 and ZAL2m in protein-coding regions (D) Sliding window (window size of 20 genes with step size of 5 genes) analysis of the ratio of nonsynonymous to synonymous nucleotide diversity (πN/πS) within the ZAL2 and ZAL2m chromosomes. The ZAL2m outlier region is highlighted (colored background). (E) Site frequency spectrum of polymorphic sites. (F) Decay of linkage disequilibrium. (G) Proportion of the ZAL2m alleles expressed for each tissue set. The proportion of the ZAL2m alleles expressed is less than the null hypothesis of 0.5 for all tissues except nestling AMV using false discovery rate (FDR) correction. Hyp, hypothalamus; AMV, ventromedial arcopallium. Figure 2—source data 1 Scaffold length. https://cdn.elifesciences.org/articles/79387/elife-79387-fig2-data1-v1.txt Download elife-79387-fig2-data1-v1.txt Figure 2—source data 2 Structural variant proportions. https://cdn.elifesciences.org/articles/79387/elife-79387-fig2-data2-v1.txt Download elife-79387-fig2-data2-v1.txt Figure 2—source data 3 Variant information. https://cdn.elifesciences.org/articles/79387/elife-79387-fig2-data3-v1.txt Download elife-79387-fig2-data3-v1.txt Figure 2—source data 4 Haplotype phased nucleotide diversity data. https://cdn.elifesciences.org/articles/79387/elife-79387-fig2-data4-v1.txt Download elife-79387-fig2-data4-v1.txt Figure 2—source data 5 Minor alleles. https://cdn.elifesciences.org/articles/79387/elife-79387-fig2-data5-v1.txt Download elife-79387-fig2-data5-v1.txt Figure 2—source data 6 Linkage disequilibrium. https://cdn.elifesciences.org/articles/79387/elife-79387-fig2-data6-v1.txt Download elife-79387-fig2-data6-v1.txt Figure 2—source data 7 RNAseq allele specific expression data in long format. https://cdn.elifesciences.org/articles/79387/elife-79387-fig2-data7-v1.txt Download elife-79387-fig2-data7-v1.txt Figure 3 with 5 supplements see all Download asset Open asset Genetic diversity and patterns of divergence across the rearranged region of the ZAL2m chromosome and in the ZAL2m outlier region. (A) Tajima’s D and nucleotide diversity across the ZAL2 and ZAL2m chromosomes. The ZAL2m outlier region is highlighted (colored background). (B) Phylogenetic tree of randomly selected regions (left panel) and the ZAL2m outlier region (right panel). The ZAL2m chromosome shows multiple haplotype structures and has longer branch lengths within the population compared with ZAL2 chromosomes. (C) Single nucleotide polymorphism (SNP) genotype plot of a scaffold inside the ZAL2m outlier region (Scaffold NW_005189516.1, 1900001–1950001). The plot shows two haplogroups. Major allele SNPs (A, same genotype as the super-white ZAL2m/2m genome) are represented in purple, and minor allele SNPs (a, different from the super-white genome) in red. Tan that there were fixed SNPs to ZAL2 vs ZAL2m in data. (D) Genetic divergence for a of the rearrangement. between the ZAL2 chromosome and 1 is in between ZAL2 and 2 in and between and in Figure data 1 for scaffold Figure Supplementary file 1 D and nucleotide diversity from variant call format (vcf) data from whole genome sequencing). Download Figure data 2 for scaffold Download Figure data 3 data for scaffold Download Figure data 4 between ZAL2m haplotypes and ZAL2. Download using the large of newly generated population genomic data, we examined patterns of SNPs on ZAL2m alleles from those on ZAL2 haplotype using fixed differences between the two chromosome (Materials and We found that the total number of genetic variants was reduced on the ZAL2m alleles compared with ZAL2 alleles vs SNPs and vs on ZAL2m and ZAL2, respectively, after excluding The mean nucleotide diversity was similarly reduced on the ZAL2m chromosome compared with the ZAL2 chromosome vs for ZAL2m vs ZAL2, of the genetic variants on the ZAL2 and ZAL2m alleles showed evidence of only weak genetic degeneration. The ratio of non-synonymous to synonymous fixed differences inside the rearranged region was but elevated for compared with fixed differences (Figure which is consistent with positive selection or purifying selection on ZAL2m. We found that the ratio of non-synonymous to synonymous nucleotide diversity (πN/πS) was significantly increased on ZAL2m compared with ZAL2 × Mann-Whitney Figure The minor allele site frequency spectrum for the ZAL2m synonymous and non-synonymous sites showed a large proportion of variants and an of allele frequency as the minor allele (Figure also suggesting reduced efficacy of purifying selection on ZAL2m. the rates of the two chromosomes are the ratio of population size between the ZAL2m and ZAL2 can be by the ratio of nucleotide diversity of synonymous sites between the ZAL2 and ZAL2m. The proportion of between ZAL2m and ZAL2 is which is than the expected ratio of the ZAL2m chromosome is as frequent as the ZAL2, given ‘balanced’ morph frequencies in the This proportion suggests that of ZAL2m has than expected from the size in the consistent with the of reduced We that the linkage between variants on ZAL2m the classic with (Figure at some of these results are consistent with but not entirely recombination on the ZAL2m chromosome. We examined whether degeneration inside the rearranged region on ZAL2m has in reduced expression of the alleles by the ZAL2m supergene To we used multiple large RNAseq from a variety of tissues in birds sampled from different geographic locations and of (see Materials and methods, 1). As and consistent with what was previously (Sun et al., we found evidence of reduced expression of the ZAL2m alleles in of tissue (Figure We for an between the number of accumulated mutations and in the promoter on ZAL2m and allelic bias in expression of the ZAL2m alleles within each tissue, which genetic degeneration within or genes to reduced expression of ZAL2m. We found evidence that allelic bias in gene expression was associated with the rate of non-synonymous fixed differences the size was small (Figure supplement 1a). the rate of synonymous fixed differences Figure supplement the number of fixed differences within 1 of the site Figure supplement were associated with allelic Thus, the overall in expression of the alleles by the ZAL2m supergene is associated with an increased number of non-synonymous fixed nucleotide changes within The limited and weak of the however, that the pattern of gene expression may have been affected also by other for example selection as nucleotide selection at more such as differences between ZAL2 and ZAL2m in DNA or (see Sun et al., 2021). 1 of sequencing data size early in the breeding et al., 2015; Sun et al., early in the breeding from during the breeding and during in on long or to breeding vs at two time during the et al., of balancing selection on the ZAL2m chromosome the of genetic diversity was overall reduced on ZAL2m, it was elevated in one region corresponding to 5 This region, to as the ZAL2m outlier region 2 and includes at protein-coding genes that are as single genes across 13 species 2). On nucleotide diversity in ZAL2m across this region was which is higher than the mean nucleotide diversity of ZAL2m and even the nucleotide diversity in the corresponding region within ZAL2 by 2 of protein-coding genes inside the ZAL2m outlier region.

A chromosomal inversion predicts the expression of sex steroid-related genes in a species with alternative behavioral phenotypes

Despite growing interest in animal social networks, surprisingly little is known about whether individuals are consistent in their social network characteristics. Networks are rarely repeatedly sampled; yet an assumption of individual consistency in social behaviour is often made when drawing conclusions about the consequences of social processes and structure. A characterization of such social phenotypes is therefore vital to understanding the significance of social network structure for individual fitness outcomes, and for understanding the evolution and ecology of individual variation in social behaviour more broadly. Here, we measured foraging associations over three winters in a large PIT-tagged population of great tits, and used a range of social network metrics to quantify individual variation in social behaviour. We then examined repeatability in social behaviour over both short (week to week) and long (year to year) timescales, and investigated variation in repeatability across age and sex classes. Social behaviours were significantly repeatable across all timescales, with the highest repeatability observed in group size choice and unweighted degree, a measure of gregariousness. By conducting randomizations to control for the spatial and temporal distribution of individuals, we further show that differences in social phenotypes were not solely explained by within-population variation in local densities, but also reflected fine-scale variation in social decision making. Our results provide rare evidence of stable social phenotypes in a wild population of animals. Such stable social phenotypes can be targets of selection and may have important fitness consequences, both for individuals and for their social-foraging associates.

Article Figures and data Abstract eLife digest Introduction Materials and methods Data availability References Decision letter Author response Article and author information Metrics Abstract The zebrafish was used to assess the impact of social isolation on behaviour and brain function. As in humans and other social species, early social deprivation reduced social preference in juvenile zebrafish. Whole-brain functional maps of anti-social isolated (lonely) fish were distinct from anti-social (loner) fish found in the normal population. These isolation-induced activity changes revealed profound disruption of neural activity in brain areas linked to social behaviour, social cue processing, and anxiety/stress. Several of the affected regions are modulated by serotonin, and we found that social preference in isolated fish could be rescued by acutely reducing serotonin levels. eLife digest Socialising is good for people’s mental health and wellbeing. The connections and relationships that we form can make us more resilient and healthier. Researchers also know that prolonged periods of social isolation, and feeling lonely, can be detrimental to our health, especially in early childhood. The paradox is that loneliness often results in an even lower desire for social contact, leading to further isolation. But not everyone craves social contact. Some people prefer to be alone and feel more comfortable avoiding social interaction. Zebrafish display the same social preferences. This, along with their transparent brains, makes them a useful model to study the links between social behaviour and brain activity. Like humans, zebrafish are social animals, with most fish taking a strong liking to social interactions by the time they are a few weeks old. A small number of ‘loner’ fish, however, prefer to avoid interacting with their siblings or tank mates. And so, if loneliness quells the desire for more social contact, the question becomes, does isolation turn otherwise social fish into loners? Here, Tunbak et al. use zebrafish to study how social isolation changes brain activity and behaviour. Social fish were isolated from others in the tank for a few days. These so-called ‘lonely fish’ were then allowed back in contact with the other fish. This revealed that, after isolation, previously social fish did avoid interacting with others. With this experimental set-up, Tunbak et al. also compared the brains of lonely and loner fish. When fish that prefer social interaction were deprived of social contact, they had increased activity in areas of the brain related to stress and anxiety. These lonely fish became anxious and very sensitive to stimuli; and their brain activity suggested that social interaction became overwhelming rather than rewarding. Positively, the lonely fish quickly recovered their normal, social behaviour when given a drug that reduces anxiety. This work provides a glimpse into how human behaviour could be affected by lengthy periods in isolation. These results suggest that humans could feel anxious upon returning to normal life after spending a long time alone. Moreover, the findings show the impact that social interaction and isolation can have on the young, developing brain. Introduction Social preference behaviour, the drive for individuals to identify and approach members of their own species (Rogers-Carter et al., 2018; Winslow, 2003), is a fundamental component of all social behaviour. We previously found that most zebrafish develop a strong social preference by 2–3 weeks of age (Dreosti et al., 2015), yet we also found a small number (~10%) of ‘loner’ fish that were averse to social cues. A similar diversity of individual social preferences has been found in many species, including humans (Sloan Wilson et al., 1994). Loneliness, undesired isolation from social interaction, has been linked to a reduction in social preference (Engeszer et al., 2004; Shams et al., 2018). We therefore asked whether the socially-averse loner fish found in the normal population would show a similar behavioural phenotype and neuronal activity to socially-averse lonely fish raised in isolation. To answer this question, we compared the behavioural and functional responses of isolated fish to controls during viewing of conspecifics. This comparison found that isolation induces patterns of brain activity that are not present in the normal population. We then asked if we could rescue the aversive behaviour of isolated fish. Since some of the highly activated areas in isolated fish are serotoninergic, we used Buspirone, a 5HT1A receptor agonist. These findings will have important implications for how we understand and treat the impact of social isolation. Prolonged periods of social isolation are particularly detrimental to humans during early development. However, even brief periods of social isolation have been shown to impact mental and physical health. We therefore tested two models of social isolation, Full (fish raised completely without social interaction) and Partial (fish isolated for 48 hr prior to behavioural testing). Each experiment comprised two sessions, 15 min of acclimation to the chamber followed by 15 min of exposure to two size matched sibling fish that were not isolated. To quantify social preference, we calculated a visual preference index (VPI) that compares the amount of time fish spend in the chamber nearest the conspecifics versus the opposite chamber where they are visually isolated from social cues (see Materials and methods). Full social isolation (Fi) caused a significant decrease in social preference relative to normally raised sibling controls (C) (Figure 1A, left and middle panel: C vs Fi, p=8.3e−8, Mann-Whitney). Specifically, there was an increase in the number of individuals that had a large negative VPI. We therefore decided to divide the fish into three sociality groups: a) anti-social (-S) fish with VPIs below −0.5; b) pro-social (+S) fish with VPIs above +0.5; c) non-social fish with −0.5 < VPI < +0.5. Fish that underwent Partial isolation (Pi), exhibited an intermediate, yet highly significant, change in social preference (Figure 1A, right panel: C vs Pi, p=2.5e−8, Mann-Whitney). Figure 1 with 1 supplement see all Download asset Open asset Isolation alters social preference behavior and swimming activity. (A) Histograms of all the VPIs during the social cue period across different conditions: controls (C, left), full isolation (Fi, middle), and partial isolation (Pi, right). For visual clarity, red bars highlight strong pro-social fish (+S, VPIs > 0.5), blue bars anti-social fish(-S, VPIs < -0.5), and gray non-social fish (ns, -0.5 < VPI < +0.5). (B) Swarm plots comparing the activity levels of fish during the acclimation period expressed as percent time moving (C, n=380; Fi, n=47; Pi, n=157). Mean and standard errors are shown. (C) Swarm plots comparing the activity levels of anti-social (left) and social (fish) fish during visual social cue exposure for each rearing condition (C (-S), n=39; Fi (-S), n=21; Pi (-S), n=53) or (C (+S), n=193; Fi, n=11; Pi (+S), n=57). (D) Time projection through the video of a pro-social control, C(+S), and a fully isolated, Fi(+S), fish during social cue exposure. The dashed lines mark the division between the social cue side (SC) and the side without social cues (No SC) that was used to calculate VPI. Video 1 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg Example of a control and a fully isolated +S fish video during social cue presentation. Two minutes of behaviour is shown in 20 s (6x playback acceleration). The control fish shows a strong social preference for the social cue and has a stereotypical social phenotype (left). The test fish spends most of its time watching the social cue with a 45-degree angle and synchronizing its bout motion with the other two conspecifics. The fully isolated fish spends long periods of time as well on the side of the conspecifics. Its behaviour, however, its characterized by long pauses while watching the conspecifics (right). As previously reported (Zellner et al., 2011), we found that fish raised in isolation were significantly less active than their normally raised siblings during the acclimation period (Figure 1B: C vs Fi, p=9.0e−6; C vs Pi, p=2.8e-9 Mann-Whitney) and during the social viewing session (Figure 1—figure supplement 1A: left C vs Fi, p=0.0001; C vs Pi, p=0.004 Mann-Whitney). We then divided fish into groups based on their social preference. Interestingly, anti-social fully and partially isolated fish showed very similar movement activity compared to anti-social controls during the acclimation (Figure 1C left: C (-S) vs Fi (-S), p=0.17 Mann-Whitney; C (-S) vs Pi (-S) p=0.23 Mann-Whitney) and during the social viewing session (Figure 1—figure supplement 1B left: C (-S) vs Fi (-S), p=0.48 Mann-Whitney; C (-S) vs Pi (-S) p=0.10 Mann-Whitney). The pro-social isolated fish, which also exhibited a reduction in activity relative to controls during the acclimation session (Figure 1C right: C (-S) vs Fi (-S), p=8.0e−5 Mann-Whitney; C (-S) vs Pi (-S) p=1.0e−7 Mann-Whitney), instead showed similar activity relative to controls during social viewing (Figure 1—figure supplement 1B right: C (-S) vs Fi (-S), p=0.02 Mann-Whitney; C (-S) vs Pi (-S) p=0.14 Mann-Whitney). In addition, we noticed that all isolated fish behaved qualitatively differently, exhibiting prolonged periods of quiescence (freezing) even when observing conspecifics (Figure 1D and Video 1). Freezing is a hallmark of anxiety-like behaviour observed in many species, and reported in zebrafish exposed to stressors (Giacomini et al., 2015; Shams et al., 2018), including periods of social isolation (Egan et al., 2009; Shams et al., 2017). In order to quantify freezing behaviour, we measured the percentage of time spent in continuous periods (>3 s) without motion (Figure 1—figure supplement 1C-D). We found that both fully and partially isolated fish exhibited significantly more freezing than controls during the acclimation period (Figure 1—figure supplement 1C left: C vs Fi, p=3.4e−16 Mann-Whitney; C vs Pi, p=2.8e−5 Mann-Whitney), and that this increase relative to controls persisted for fully isolated fish during social viewing, but was reduced in partially isolated fish, perhaps representing some recovery during the 15 min of social interaction (Figure 1—figure supplement 1D left: C vs Fi, p=6.3e−13 Mann-Whitney; C vs Pi, p=0.03 Mann-Whitney). When we compared freezing behaviour of groups with similar social preference, we found, as expected, that anti-social fish exhibited increased freezing during social viewing regardless of rearing condition. However, pro-social fully isolated fish also showed increased freezing during social viewing, suggesting that they were not engaged in typical social interaction, but rather remained immobile on the side with the conspecifics (Figure 1—figure supplement 1D right). The behavioural similarities between anti-social isolated (lonely) and anti-social control (loner) fish led us to hypothesize that isolation might simply predispose fish to the same anti-social state found in the normal population. If this is the case, neural activity of anti-social isolated and anti-social control fish should be similar when presented with social cues. To test this hypothesis, we performed whole-brain two-photon imaging of c-fos expression, an immediate early gene whose expression is associated with increased neural activity (Herrera and Robertson, 1996), in juvenile brains following testing in the social preference assay. Dissected brains were imaged with the dorsal surface down (bottom-up) to achieve clear views of the ventral brain structures that have been previously implicated in the social brain network (Figure 2A, also see Materials and methods). Volumes of 1.5 mm x 1.5 mm x 700 µm, with a voxel size of 1 × 1×3 µm, were acquired from 135 zebrafish brains across all experimental groups and registered to a reference brain (Marquart et al., 2017). These c-fos whole-brain functional maps were first normalised to a background intensity level (see Materials and methods) and then used to compare the neural activity patterns of different test groups. We compared the average activity map for each rearing/sociality condition with the average map acquired from similarly raised sibling fish that were placed in the behavioural assay for 30 min without any social cues (nsc, no social-cue). The resulting normalised difference stacks (e.g. (+S - nsc)/nsc) allowed us to identify changes in neural activity associated with exposure to a visual social cue (Figure 2A). Figure 2 Download asset Open asset Functional maps of the social brain in normal and isolated fish. (A) Schematic of the custom-built two-photon microscope used for acquiring whole-brain volumes of dorsal-down mounted fish brains (top panel). Horizontal sections of pro-social control fish (C(+S)) responses at increasing imaging depth (lower panels). Images are average differences between (C(+S)) and siblings not presented with a social cue. Positive values (white) indicate increased cFos expression in socially preferring fish, while negative values (black) indicate decreased expression. Scale bar is 200μm. The intensity scale bar is shown in B, C(+S) row. (B) Region analysis of two different brain areas that have been implicated in social behavior: caudal hypothalamus and preoptic area. A schematic of the anatomical regions and corresponding DAPI staining is shown (top panel) with two sub-regions highlighted in green. Images showing changes in cFos activation in these areas for pro- (+S) and anti-social (-S) controls, fully isolated, and partially isolated fish are shown. Images are horizontal sections of the average difference between each test group and their corresponding sibling group not presented with a social cue. Scale bar is 100μm. Intensity scale bar is shown for each group. (C) Average image of TH1, TH2, Slc6a4b, and DAT expression in the same section of the caudal hypothalamus as 2B (n=3 each). Scale bar is 100μm. (D) Summary graphs showing the change in cFos activation for four different brain areas calculated by using the average difference images shown in (B) and using 3D masks (a single plane of each area of the masks is shown in green in B). Positive values indicate increases in cFos expression; asterisks mark significant changes relative to no social cue siblings. D=dorsal and V=ventral caudal hypothalamus; Pa=ventrolateral preoptic area, PM=dorsal preoptic area. Several brain areas showed strong activation or inhibition in normally raised fish upon social cue exposure. We focused on areas that have been reported as social brain areas (O'Connell and Hofmann, 2011) and show differences between our experimental groups (Figure 2B: C (+S and -S)). The caudal hypothalamus was differentially activated in pro- vs. anti-social control fish. A dorsal sub-region was significantly activated in pro-social controls (Figure 2B and D: dHc - C (+S) vs C (nsc), p=0.007, Mann-Whitney), whereas it was inhibited, along with the adjacent ventral sub-region, in anti-social controls (Figure 2B and D: vHc - C (-S) vs C (nsc), p=0.003, Mann-Whitney). The caudal hypothalamus is known to express high levels of serotonin and dopamine, as well as glutamate and histamine (Filippi et al., 2010; Kaslin and Panula, 2001). Furthermore, a segregation into distinct dorsal and ventral areas of the caudal hypothalamus has already been shown for some of these markers, such as tyrosine hydroxylase 1 and 2, (Th1 and Th2) (Yamamoto et al., 2010) and we confirmed these previous results with immunostaining (Figure 2C left), as well as for the dopamine and serotonin transporters, DAT and slc6a4b (Figure 2C right) (Filippi et al., 2010; Lillesaar, 2011). Changes in serotonin and dopamine levels have been widely documented in response to social interaction (Scerbina et al., 2012), viewing social cues (Saif et al., 2013), and social isolation (Huang et al., 2015; Shams et al., 2018; Shams et al., 2015). While the serotoninergic system has been linked to stress and arousal (Backström and Winberg, 2017), the dopamine circuitry has been shown to regulate the reward system underlying social behaviour (Teles et al., 2013). Since the caudal hypothalamus expresses both of these neurotransmitters, and our data demonstrate a pattern of activation/inhibition that is distinct for pro- and anti-social fish, then this area could be crucial in regulating social preference. The second social brain area we investigated was the preoptic area. Our data showed a similar activation pattern for anti-social and pro-social fish characterised by a small increase in the dorsal preoptic area (dPa) and a small decrease in the ventral preoptic area (vPa). However, only anti-social control fish showed a significant change in the ventral area (Figure 2B and D: C (-S) vs C (nsc), vPa p=0.003, Mann-Whitney). The activation of the preoptic area during social behaviour is consistent with previous literature in a number of species (O'Connell and Hofmann, 2011). This area has been shown to express several neuropeptides involved in social behaviour such as arginine/vasotocin and oxytocin (Heinrichs et al., 2009; Herget and Ryu, 2015). It was recently shown that oxytocin does not seem to be responsible for social interaction (Ribeiro et al., 2019) as mutants for oxytocin receptors shows no alteration in social preference, but rather reduced social recognition. Furthermore, injections of oxytocin do not have any effect on shoaling and interaction (Langen et al., 2015). The neuropeptide vasotocin, instead, has been shown to have a specific effect on reducing social interaction (Langen et al., 2015) and not shoaling behaviour. This neuropeptide has also been shown to be involved in aggression (Teles et al., 2016) and stress by stimulating cortisol release. We then compared the brain activity maps of anti- and pro-social control fish with fully and partially isolated fish. As described previously, anti-social control (loner) fish showed a behavioural phenotype very similar to anti-social isolated (lonely) fish. Therefore, we investigated whether their brain activity maps were also similar following the presentation of a social cue. Contrary to our hypothesis, c-fos functional maps of anti-social fully isolated fish (Figure 2B: Fi (-S)) revealed a completely different activity profile than their anti-social sibling controls (Figure 2B: C (-S)). The ventral sub-region of the caudal hypothalamus (vHc) of Fi (-S) fish was not inactivated, while the preoptic area was strongly activated in both the dorsal (dPa) and the ventral (vPa) regions, but significantly only in the dorsal (Figure 2B and D: Fi (-S) vs Fi (nsc), p=0.006 dPa; p=0.07 vPa, Mann-Whitney). Furthermore, the pro-social fully isolated fish (Figure 2B: Fi (+S)), who exhibited an increase of freezes and reduced motility compared to control fish when viewing conspecifics, showed a similar activation to pro-social controls in the caudal hypothalamus, but increased activity in the dorsal preoptic area. Interestingly, the preoptic area was activated differently in pro-social and anti-social isolated fish, with only the dorsal preoptic area strongly activated in the pro-social group (Figure 2B and D: Fi (+S) vs Fi, p=0.04 vPa, p=0.002 dPa, Man-Whitney). These data suggest that long social isolation causes abnormal neural responses during viewing of social cues. Furthermore, anti- and pro-social fish exposed to a brief isolation for only 48 hr prior to testing, showed similar functional activity changes to fully isolated fish, albeit less strong (Figure 2B and D: Pi (-S) vs Pi (nsc), p=0.18 dHc; p=0.28 vHc; p=0.04 vPa; p=0.04 dPa, Mann-Whitney; Figure 2B and D: Pi (+S) vs Pi (nsc), p=0.17 dHc; p=0.05 vHc; p=0.007 vPa; p=0.006 dPa). Together with the behavioural data, this finding supports the idea that short term isolation is enough to induce brain activity changes similar to those observed following complete isolation, and strikingly different than those observed in anti-social controls. We were next interested in understanding why social isolation promotes social aversion instead of increasing the drive for social interaction. An important clue was found in the pattern of brain activity changes that were unique to isolated fish. When we directly compared the normalised c-fos functional brain maps of isolated and control fish that were not exposed to social cues during the assay (Figure 3A), we found a significant increase in two interesting areas; one associated with visual processing, the optic tectum, [McDowell et al., 2004]), and one involved in stress responses, the posterior tuberal nucleus (Ziv et al., 2013). Figure 3 Download asset Open asset Changes in baseline brain activity following isolation. (A) Images of two areas that show strong c-fos activation in fully isolated fish independent of social stimuli (optic tectum and posterior tuberal nucleus (PTN)). Schematics of the horizontal sections and corresponding DAPI image are shown in the left panels. One plane of the 3D regions used for analysis is Images of isolated fish c-fos neuronal calculated as average differences between fully isolated (Fi) fish and normally raised fish without social cues are shown in the right panels. Scale bar (B) Summary graphs showing the normalised c-fos expression in the optic tectum and 3D masks for each experimental social cue (nsc), pro-social (+S) and anti-social (-S) for all the controls fully isolated and partially isolated fish. In pro-social control fish, viewing social cues in a significant increase of neuronal activity in the optic tectum (Figure C (+S) vs C (nsc), p=0.004 Mann-Whitney). However, in fully isolated fish, there was already increased neuronal activity in the optic tectum in the of social cues (Figure Fi vs C (nsc), Mann-Whitney), suggesting that isolation increases visual as previously reported in humans et al., 2015). This increased of fully isolated fish not presented with social cues was in partially isolated fish (Figure Pi vs C (nsc), Mann-Whitney). However, a increase in activity was observed when pro-social partially isolated fish conspecifics, that some visual had (Figure Pi (+S) vs C (+S), Mann-Whitney). In addition, increased activity was also present in both fully and partially isolated anti-social fish (Figure Fi (-S) vs C (-S), Pi (-S) vs C (-S), Mann-Whitney), even these fish the chamber with visual to conspecifics. We also observed activity increases in the posterior tuberal an area associated with stress responses in zebrafish et al., et al., 2013). Full isolation caused a significant increase in posterior tuberal nucleus activity in the of social cues (Figure Fi vs C (nsc), Mann-Whitney) and in both anti-social and pro-social fish exposed to social cues (Figure Fi (+S) vs C (+S), Fi (-S) vs C (-S), Mann-Whitney). partial isolation, posterior tuberal nucleus activity was not increased in the of social cues (Figure Pi vs C (nsc), Mann-Whitney), only in pro-social fish (Figure Pi (+S) vs C (+S), but significantly in anti-social fish (Figure Pi (-S) vs C (-S), these results from the optic tectum and posterior tuberal we that isolation to social However, when this results in an increase of stress and levels during social viewing that to an aversion for social To test our that reducing could the anti-social behaviour observed in isolated we acutely control and partially isolated fish with Buspirone, an of the 5HT1A The of isolation was by the behavioural and functional phenotype of partial isolation relative to and full isolation, which would us to more both and negative of on The of was by the changes in activity observed in the caudal hypothalamus of isolated fish, and by the that the caudal hypothalamus and the preoptic area strongly express one of the two of the 5HT1A receptor et al., has been shown to in humans, and zebrafish et al., 2009; and 2010; et al., and While it is not fully how reduces it has been shown to social interaction of and et al., 2011), of zebrafish and 2012), and social in humans et al., et al., Its to the of social isolation in zebrafish has not been We first tested the of exposure to in control fish, as expected, we observed a small significant increase in social preference relative to controls, however, a population of anti-social fish remained (Figure supplement C vs C Mann-Whitney). We then partially isolated fish with 30 and (Figure supplement of the drug was in both to the anti-social phenotype caused by isolation (Figure Pi vs Pi 30 and Mann-Whitney). Figure with 1 supplement see all Download asset Open asset social preference in isolated fish. (A) of VPIs during the social cue period in partially isolated fish with 30 and of For visual clarity, the bars are as in Figure (B) VPI values calculated in time for controls (C, partial isolated (Pi, blue and Pi with green how fish normal social preference the first (C) of time moving calculated in for the same fish as B, lines indicate standard When we then compared the time of this phenotype by the VPIs for each the 15 min of the behavioural experiment (Figure We found that the isolated fish with Buspirone, while would normal social preference behaviour the first min of exposure to social cues (Figure C vs Pi first Mann-Whitney). In the VPIs of isolated fish remained significantly lower than controls the We next compared the time of movement activity (Figure and found that it increased quickly the first min of the social viewing the activity of isolated fish with was already at the level of controls from the of the social viewing session (Figure C vs Pi first Mann-Whitney), which that the recovery of normal movement as a of reduced the recovery of normal social preference. Therefore, impact on the of recovery of social preference that it may do by reducing perhaps at the level of the preoptic caudal area, to the to social stimuli acquired during the isolation In our results demonstrate that lonely fish, which have been isolated from social cues and show anti-social behaviour, have a completely different functional response to social stimuli than loner fish, anti-social fish found in the normal population. In addition, the functional changes caused by social deprivation are consistent with an increase in resulting from to social similar to the of isolation on We could in zebrafish with an drug that on the Zebrafish will a for the impact of loneliness on brain and different for or even its Materials and methods or or three

Consistent individual differences in social phenotypes have been observed in many animal species. Changes in demographics, dominance hierarchies or ecological factors, such as food availability or disease prevalence, are expected to influence decision-making processes regarding social interactions. Therefore, it should be expected that individuals show flexibility rather than stability in social behaviour over time to maximize the fitness benefits of social living. Understanding the processes that create and maintain social phenotypes requires data encompassing a range of socioecological settings and variation in intrinsic state or life-history stage or strategy. Using observational data spanning up to 19 years for some individuals, we demonstrate that multiple types of social behaviour are repeatable over the long term in wild chimpanzees, a long-lived species with complex fission–fusion societies. We controlled for temporal, ecological and demographic changes, limiting pseudo-repeatability. We conclude that chimpanzees living in natural ecological settings have relatively stable long-term social phenotypes over years that may be independent of life-history or reproductive strategies. Our results add to the growing body of the literature suggesting consistent individual differences in social tendencies are more likely the rule rather than the exception in group-living animals.

BackgroundRASopathies are a group of disorders that result from mutations in genes coding for proteins involved in regulating the Ras-MAPK signaling pathway, and have an increased incidence of autism spectrum disorder (ASD). Legius syndrome is a rare RASopathy caused by loss-of-function mutations in the SPRED1 gene. The patient phenotype is similar to, but milder than, Neurofibromatosis type 1—another RASopathy caused by loss-of-function mutations in the NF1 gene. RASopathies exhibit increased activation of Ras-MAPK signaling and commonly manifest with cognitive impairments and ASD. Here, we investigated if a Spred1-/- mouse model for Legius syndrome recapitulates ASD-like symptoms, and whether targeting the Ras-MAPK pathway has therapeutic potential in this RASopathy mouse model.MethodsWe investigated social and communicative behaviors in Spred1-/- mice and probed therapeutic mechanisms underlying the observed behavioral phenotypes by pharmacological targeting of the Ras-MAPK pathway with the MEK inhibitor PD325901.ResultsSpred1-/- mice have robust increases in social dominance in the automated tube test and reduced adult ultrasonic vocalizations during social communication. Neonatal ultrasonic vocalization was also altered, with significant differences in spectral properties. Spred1-/- mice also exhibit impaired nesting behavior. Acute MEK inhibitor treatment in adulthood with PD325901 reversed the enhanced social dominance in Spred1-/- mice to normal levels, and improved nesting behavior in adult Spred1-/- mice.LimitationsThis study used an acute treatment protocol to administer the drug. It is not known what the effects of longer-term treatment would be on behavior. Further studies titrating the lowest dose of this drug that is required to alter Spred1-/- social behavior are still required. Finally, our findings are in a homozygous mouse model, whereas patients carry heterozygous mutations. These factors should be considered before any translational conclusions are drawn.ConclusionsThese results demonstrate for the first time that social behavior phenotypes in a mouse model for RASopathies (Spred1-/-) can be acutely reversed. This highlights a key role for Ras-MAPK dysregulation in mediating social behavior phenotypes in mouse models for ASD, suggesting that proper regulation of Ras-MAPK signaling is important for social behavior.

Differences in behavioural phenotype between parental deletion and maternal uniparental disomy in Prader–Willi syndrome: an ERP study

Activity of the hypothalamic–pituitary–gonadal axis differs between behavioral phenotypes in female white-throated sparrows (Zonotrichia albicollis)

Human life history (LH) strategies are theoretically regulated by developmental exposure to environmental cues that ancestrally predicted LH-relevant world states (e.g., risk of morbidity–mortality). Recent modeling work has raised the question of whether the association of childhood family factors with adult LH variation arises via (i) direct sampling of external environmental cues during development and/or (ii) calibration of LH strategies to internal somatic condition (i.e., health), which itself reflects exposure to variably favorable environments. The present research tested between these possibilities through three online surveys involving a total of over 26,000 participants. Participants completed questionnaires assessing components of self-reported environmental harshness (i.e., socioeconomic status, family neglect, and neighborhood crime), health status, and various LH-related psychological and behavioral phenotypes (e.g., mating strategies, paranoia, and anxiety), modeled as a unidimensional latent variable. Structural equation models suggested that exposure to harsh ecologies had direct effects on latent LH strategy as well as indirect effects on latent LH strategy mediated via health status. These findings suggest that human LH strategies may be calibrated to both external and internal cues and that such calibrational effects manifest in a wide range of psychological and behavioral phenotypes.

Arid1b is a high confidence risk gene for autism spectrum disorder that encodes a subunit of a chromatin remodeling complex expressed in neuronal progenitors. Haploinsufficiency causes a broad range of social, behavioral, and intellectual disability phenotypes, including Coffin-Siris syndrome. Recent work using transgenic mouse models suggests pathology is due to deficits in proliferation, survival, and synaptic development of cortical neurons. However, there is conflicting evidence regarding the relative roles of excitatory projection neurons and inhibitory interneurons in generating abnormal cognitive and behavioral phenotypes. Here, we conditionally knocked out either one or both copies of Arid1b from excitatory projection neuron progenitors and systematically investigated the effects on intrinsic membrane properties, synaptic physiology, social behavior, and seizure susceptibility. We found that disrupting Arid1b expression in excitatory neurons alters their membrane properties, including hyperpolarizing action potential threshold; however, these changes depend on neuronal subtype. Using paired whole-cell recordings, we found increased synaptic connectivity rate between projection neurons. Furthermore, we found reduced strength of excitatory synapses to parvalbumin (PV)-expression inhibitory interneurons. These data suggest an increase in the ratio of excitation to inhibition. However, the strength of inhibitory synapses from PV interneurons to excitatory neurons was enhanced, which may rebalance this ratio. Indeed, Arid1b haploinsufficiency in projection neurons was insufficient to cause social deficits and seizure phenotypes observed in a preclinical germline haploinsufficient mouse model. Our data suggest that while excitatory projection neurons likely contribute to autistic phenotypes, pathology in these cells is not the primary cause.

Endocrine correlates of alternative phenotypes in the white-throated sparrow ( Zonotrichia albicollis)

The White-throated Sparrow is a passerine bird of the American sparrow family Emberizidae, which has been studied extensively as it maintains a 100Mb inversion polymorphism on chromosome 2 via disassortative mating. The inverted arrangement is maintained in a near constant state of heterozygosity. Approximately half of the population is homozygous for the ZAL2 arrangement, whereas the other half of the population has the heterozygous condition. The exceptional inversion polymorphism (ZAL2m) in the white-throated sparrow (Zonotrichia albicollis) is linked to variation in plumage, social behavior and mate choice, and is maintained in the population by negative assortative mating.

Coevolution of Genome Architecture and Social Behavior

The social behavior of male rats administered an adult-onset calorie restriction regimen

Here we employed the partner preference test (PPT) to examine how naked mole-rat non-breeding individuals of different behavioral phenotypes make social decisions. Naked mole-rats from six colonies were classified into three behavioral phenotypes (soldiers, dispersers, and workers) using a battery of behavioral tests. They then participated in a 3 h long PPT, where they could freely interact with a tethered familiar or tethered unfamiliar conspecific. By comparing the three behavioral phenotypes, we tested the hypothesis that the PPT can be used to interrogate social decision-making in this species, revealing individual differences in behavior that are consistent with discrete social phenotypes. We also tested whether a shorter, 10 min version of the paradigm is sufficient to capture group differences in behavior. Overall, soldiers had higher aggression scores toward unfamiliar conspecifics than both workers and dispersers at the 10 min and 3 h comparison times. At the 10 min comparison time, workers showed a stronger preference for the familiar animal’s chamber, as well as for investigating the familiar conspecific, compared to both dispersers and soldiers. At the 3 h time point, no phenotype differences were seen with chamber or investigation preference scores. Overall, all phenotypes spent more time in chambers with another animal vs. being alone. Use of the PPT in a comparative context has demonstrated that the test identifies species and group differences in affiliative and aggressive behavior toward familiar and unfamiliar animals, revealing individual differences in social decision-making and, importantly, capturing aspects of species-specific social organization seen in nature.