Brain bilateral asymmetry – insights from nematodes, zebrafish, and Drosophila.

Stereotyped brain asymmetry, also known as laterality, is widespread and multiscale, encompassing asymmetrical molecular function, left–right (LR) asymmetrical neurons or circuits, structural differences, and lateralized behaviors. Brain laterality can modulate a wide variety of behaviors and cognitive functions, including sleep, memory, food preference, fear, and anxiety. Long-term memory is among the most evolutionary conserved cognitive traits influenced by brain lateralization. The mechanisms and molecular pathways governing brain laterality vary widely among model organisms. The current limited understanding of the diversity of asymmetries hinders the definition of common principles and conserved symmetry-breaking mechanisms. The concordance between body and brain LR asymmetry can be strong, partial, or completely absent, raising questions about the emergence and coordination of asymmetry between the brain and body. Chirality is a fundamental trait of living organisms, encompassing the homochirality of biological molecules and the left–right (LR) asymmetry of visceral organs and the brain. The nervous system in bilaterian organisms displays a lateralized organization characterized by the presence of asymmetrical neuronal circuits and brain functions that are predominantly localized within one hemisphere. Although body asymmetry is relatively well understood, and exhibits robust phenotypic expression and regulation via conserved molecular mechanisms across phyla, current findings indicate that the asymmetry of the nervous system displays greater phenotypic, genetic, and evolutionary variability. In this review we explore the use of nematode, zebrafish, and Drosophila genetic models to investigate neuronal circuit asymmetry. We discuss recent discoveries in the context of body–brain concordance and highlight the distinct characteristics of nervous system asymmetry and its cognitive correlates. [ABSTRACT FROM AUTHOR]