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Background and aimsSYNGAP1-related disorder (SYNGAP1-RD) is a prevalent genetic form of Autism Spectrum Disorder and Intellectual Disability (ASD/ID) and is caused byde novoor inherited mutations in one copy of theSYNGAP1gene. In addition to ASD/ID, SYNGAP1 disorder is associated with comorbid symptoms including treatment-resistant-epilepsy, sleep disturbances, and gastrointestinal distress. Mechanistic links between these diverse symptoms andSYNGAP1variants remain obscure, therefore, our goal was to generate a zebrafish model in which this range of symptoms can be studied. MethodsWe used CRISPR/Cas9 to introduce frameshift mutations in thesyngap1aandsyngap1bzebrafish duplicates (syngap1ab) and validated these stable models for Syngap1 loss-of-function. BecauseSYNGAP1is extensively spliced, we mapped splice variants to the two zebrafishsyngap1aandbgenes and identified mammalian-like isoforms. We then quantified locomotory behaviors in zebrafishsyngap1ablarvae under three conditions that normally evoke different arousal states in wild-type larvae: aversive, high-arousal acoustic, medium-arousal dark, and low-arousal light stimuli. ResultsWe show that CRISPR/Cas9 indels in zebrafishsyngap1aandsyngap1bproduced loss-of-function alleles at RNA and protein levels. Our analyses of zebrafish Syngap1 isoforms showed that, as in mammals, zebrafish Syngap1 N- and C-termini are extensively spliced. We identified a zebrafishsyngap1α1-like variant that maps exclusively to thesyngap1bgene. Quantifying locomotor behaviors showed thatsyngap1abmutant larvae are hyperactive compared to wild-type but to differing degrees depending on the stimulus. Hyperactivity was most pronounced in low arousal settings, and hyperactivity was proportional to the number of mutantsyngap1alleles. LimitationsSyngap1loss-of-function mutations produce relatively subtle phenotypes in zebrafish compared to mammals. For example, while mouseSyngap1homozygotes die at birth, zebrafishsyngap1ab−/−survive to adulthood and are fertile, thus some aspects of symptoms in people withSYNGAP1-Related Disorder are not likely to be reflected in zebrafish. ConclusionOur data support mutations in zebrafishsyngap1abas causal for hyperactivity associated with elevated arousal that is especially pronounced in low-arousal environments. 

The vertebrate brain is highly conserved topologically, but less is known about neuroanatomical variation between individual brain regions. Neuroanatomical variation at the regional level is hypothesized to provide functional expansion, building upon ancestral anatomy needed for basic functions. Classically, animal models used to study evolution have lacked tools for detailed anatomical analysis that are widely used in zebrafish and mice, presenting a barrier to studying brain evolution at fine scales. In this study, we sought to investigate the evolution of brain anatomy using a single species of fish consisting of divergent surface and cave morphs, that permits functional genetic testing of regional volume and shape across the entire brain. We generated a high-resolution brain atlas for the blind Mexican cavefishAstyanax mexicanusand coupled the atlas with automated computational tools to directly assess variability in brain region shape and volume across all populations. We measured the volume and shape of every grossly defined neuroanatomical region of the brain and assessed correlations between anatomical regions in surface fish, cavefish, and surface × cave F₂hybrids, whose phenotypes span the range of surface to cave. We find that dorsal regions of the brain are contracted, while ventral regions have expanded, with F₂hybrid data providing support for developmental constraint along the dorsal-ventral axis. Furthermore, these dorsal-ventral relationships in anatomical variation show similar patterns for both volume and shape, suggesting that the anatomical evolution captured by these two parameters could be driven by similar developmental mechanisms. Together, these data demonstrate thatA. mexicanusis a powerful system for functionally determining basic principles of brain evolution and will permit testing how genes influence early patterning events to drive brain-wide anatomical evolution.