PROJECT SUMMARY Autism spectrum disorder (ASD) is caused by both environmental and genetic factors, with the genetic contribution estimated at 60-80%. Dozens of genes that increase risk for ASD have been identified, most based on de novo mutations, but these mutations are predicted to account for only 15-20% of ASD cases. Thus, the majority of the genetic contribution to ASD is predicted to result from common and rare inherited variation, but few such genes have been identified. Recently, using whole genome sequencing, we reported genome wide evidence for >60 ASD risk genes, 26 of them novel for ASD, with signals derived from inherited and de novo protein truncating or missense mutations. The functions of most of these genes are unknown, so a crucial and necessary next step is to explore their impact on neurodevelopment and neuronal function using a model organism. The current pace of translating genetic risk factors into phenotypes, mechanisms and therapies is limited in part by inefficiencies with in vivo mammalian model systems, which makes them impractical for creating and behaviorally testing large numbers of mutant lines. Here, we leverage the zebrafish, which occupies a niche as a vertebrate model with features amenable to both in vivo screening and mechanistic understanding, including a conserved yet small vertebrate brain, behaviors relevant to ASD, and cost-effectiveness relative to mammalian models. While the zebrafish cannot recapitulate ASD and has limitations for modeling a human disorder, an emerging literature supports the notion that it is a useful model to study the functions of genes that contribute to ASD risk. Rather than assess ASD-risk genes one at a time, we will accelerate progress towards mechanistic understanding via high-throughput assays and analyses. In the parent grant, we proposed to use whole-brain calcium imaging to study neuronal network properties of zebrafish ASD risk gene mutants at the larval stage of development. This diversity supplement application describes an experimental and conceptual career development plan for a graduate student whose experimental goals are to (1) establish a system for brain-wide calcium imaging of juvenile zebrafish during presentation of virtual social cues, and (2) use this system to identify neuronal network properties of zebrafish ASD risk gene mutants compared to wild-type controls in response to social cues. This experimental plan directly relates to the parent grant by characterizing brain states in response to social cues at the juvenile stage of development, when zebrafish first show social behaviors. These experiments are separate from, yet synergize with, the experiments described in the parent grant. Together, the parent grant and diversity supplement have the potential to identify neuronal mechanisms that explain the behavioral phenotypes observed in zebrafish that contain mutations in ASD risk genes.