Hox Activity Levels Governs the Evolution of Homologous Behaviors.

Central neuronal network rewiring is a key mechanism for the evolution of distinct behaviors. However, elucidating the genetic processes that have taken place to reconfigure circuits has rarely been performed. Here, taking advantage of the diversification of locomotor behaviors along the body of a single animal, we demonstrate how changes in the levels of the conserved developmental Hox genes govern the assembly of distinct circuits generating specific motor patterns. We show how these genetic changes occur during the late stages of development, refine the neuronal morphologies of a subset of neurons and lead to their incorporation into different circuits ultimately generating new function and behavior. Our work therefore uncovers a mechanism whereby evolutionary selected deviations in the expression levels of specification genes, refines connectivity in an otherwise unchanged network allowing for the evolution of a new behavior while ensuring system stability.