Social behaviors are diverse in nature, but it is unclear how conserved genes, brain regions, and cell populations generate this diversity. Here we investigate bower-building, a recently-evolved social behavior in cichlid fishes. We use single nucleus RNA-sequencing in 38 individuals to show signatures of recent behavior in specific neuronal populations, and building-associated rebalancing of neuronal proportions in the putative homolog of the hippocampal formation. Using comparative genomics across 27 species, we trace bower-associated genome evolution to a subpopulation of glia lining the dorsal telencephalon. We show evidence that building-associated neural activity and a departure from quiescence in this glial subpopulation together regulate hippocampal-like neuronal rebalancing. Our work links behavior-associated genomic variation to specific brain cell types and their functions, and suggests a social behavior has evolved through changes in glia.
Social behaviors are essential for survival and reproduction and vary strongly among individuals, species, and heritable brain diseases. The molecular and cellular bases of this variation are poorly resolved, and discovering them is necessary to understand how neural circuit and behavioral functions-and dysfunctions-vary in social contexts. Here we integrate single nucleus RNA-sequencing (snRNA-seq) with comparative genomics and automated behavior analysis to investigate the neurobiology of castle-building, a recently-evolved social, spatial, goal-directed, and repetitive construction behavior in Lake Malawi cichlid fishes. We simultaneously control for and analyze two biological variables correlated with castle-building behavior: quivering, a courtship behavior, and relative gonadal mass. We find signatures of building-, quivering-, and gonadal-associated neuronal excitation, gene expression, and neurogenesis in distinct cell populations. Converging lines of evidence support the involvement of estrogen, TrkB, and CCK signaling systems, and specific pallial excitatory neuronal subpopulations, in castle-building behavior. We show additional evidence that castle-building has evolved in part through genomic divergence in a gene module that is selectively expressed in stem-like quiescent radial glial cells (RGCs) lining the ventricular zone of the pallium. This RGC subpopulation exhibits signatures of a building-associated departure from quiescence, which in turn is associated with neuronal rebalancing in the putative fish homologue of the hippocampus. Our work supports an unexpected role for glia and neurogenesis in the evolution of social behavior, and more broadly shows how snRNA-seq can be used to systematically profile the cellular bases of previously unstudied social behaviors in new species systems.
The telencephalon has undergone remarkable diversification and expansion throughout vertebrate evolution, exhibiting striking differences in structural and functional complexity. Nevertheless, fundamental features are shared across vertebrate taxa, such as the presence of distinct regions including the pallium, subpallium, and olfactory structures. Teleost fishes have a uniquely 'everted' telencephalon, which has made it challenging to compare brain regions in fish to those in other vertebrates. Here we combine spatial transcriptomics and single-nucleus RNA-sequencing to generate a spatially-resolved transcriptional atlas of the cichlid fish telencephalon. We then compare cell-types and anatomical regions in the cichlid telencephalon with those in amphibians, reptiles, birds, and mammals. We uncover striking transcriptional similarities between cell populations in the fish telencephalon and subpallial, hippocampal, and cortical cell populations in tetrapods. Ultimately, our work lends new insights into the organization and evolution of conserved cell-types and regions in the vertebrate forebrain.
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