Animals display remarkable variation in social behavior.  However, outside of rodents, little is known about the neural mechanisms of social variation, and whether they are shared across species and sexes, limiting our understanding of how sociality evolves. Using coral reef butterflyfishes, we examined gene expression correlates of social variation (i.e., pair bonding vs. solitary living) within and between species and sexes.  In several brain regions, we quantified gene expression of receptors important for social variation in mammals: oxytocin (OTR), arginine vasopressin (V1aR), dopamine (D1R, D2R), and mu-opioid (MOR). We found that social variation across individuals of the oval butterflyfish, Chaetodon lunulatus, is linked to differences in OTR,V1aR, D1R, D2R, and MOR gene expression within several forebrain regions in a sexually dimorphic manner. However, this contrasted with social variation among six species representing a single evolutionary transition from pair bonded to solitary living. Here, OTR expression within the supracommissural part of the ventral telencephalon was higher in pair bonded than solitary species, specifically in males. These results contribute to the emerging idea that nonapeptide, dopamine, and opioid signaling is a central theme to the evolution of sociality across individuals, although the precise mechanism may be flexible across sexes and species.

Field collections                                                            

Specimens were collected from wild populations at Lizard Island, in the northern Great Barrier Reef (GBR), Australia in 2013. Collections were made in May-July, outside of peak reproductive periods, capturing social behavior independent of reproductive activity. Since we were interested in adults, only individuals that were within 80% of the asymptotic size for each species were used, as these are more likely to be reproductively mature. Individuals were haphazardly encountered on snorkel, allowed to acclimate to observer presence for 3 min, and level of selective affiliation with another conspecific was observed for 3 min. Individuals displaying a high level of affiliation selectively with another conspecific were considered pair bonded, while those displaying a low and indiscriminate level of affiliation with other conspecific were considered solitary. Collected sample sizes are as follows: C. lunulatus males = 11, females = 10; C. baronessa males = 7, females = 6; C. vagabundus males = 6, females = 6; C. plebeius females = 10; C. rainfordii males = 2, females = 11; C. trifascialis males = 2, females = 12.  Behavioral characterization, collection, and sexing of focal fishes has been described previously (Nowicki et al., 2018). Brains were dissected, embedded in optimal cutting temperature compound (VWR), frozen and transported in liquid nitrogen, and stored at -80 ͦC.

Gene expression

Frozen brains were coronally sectioned at 100 µm on a cryostat, and brain regions were identified using a Chaetodon brain atlas and isolated using micropunches (Fig. 2 inset). RNA from each brain region was extracted and reverse transcribed to cDNA, purified, and stored at -20 °C. The Chaetodon lunulatus transcriptome was sequenced, from which cloning and exon-exon flanking qPCR primers for target genes and 18S endogenous control were designed and optimized. Quantitative PCR was then performed on each sample using a reaction mixture and qPCR cycling instrument (CFX380) that was recommended by the enzyme manufacturer (see Manuscript Supplementary Methods S1 for detailed methods, Table S1 for primer sequences, and Table S2 for cycling parameters).  Not all regions of each brain were measured for gene expression due to insufficient tissue available.

References

Nowicki, J. P., O’Connell, L. A., Cowman, P. F., Walker, S. P., Coker, D. J., & Pratchett, M. S. (2018). Variation in social systems within Chaetodon butterflyfishes, with special reference to pair bonding. PloS one, 13(4).

These data sets pertain to the manuscript, "Gene expression correlates of social evolution in coral reef butterflyfishes", submitted for publication in Proceedings of the Royal Society, B (manuscript no. RSPB-2020-0239).