Evolution of the gene regulatory network of body axis by enhancer hijacking in amphioxus.

A central goal of evolutionary developmental biology is to decipher the evolutionary pattern of gene regulatory networks (GRNs) that control embryonic development, and the mechanism underlying GRNs evolution. The Nodal signaling that governs the body axes of deuterostomes exhibits a conserved GRN orchestrated principally by Nodal, Gdf1/3, and Lefty. Here we show that this GRN has been rewired in cephalochordate amphioxus. We found that while the amphioxus ortholog exhibited nearly no embryonic expression, its duplicate , linked to , was zygotically expressed in a similar pattern as . Consistent with this, while mutants showed defects in axial development, mutants did not. Further transgenic analyses showed that the intergenic region between and could drive reporter gene expression as that of the two genes. These results indicated that has taken over the axial development role of in amphioxus, possibly through hijacking enhancers. We finally demonstrated that, to compensate for the loss of maternal expression, Nodal has become an indispensable maternal factor in amphioxus and its maternal mutants caused axial defects as mutants. We therefore demonstrated a case that the evolution of GRNs could be triggered by enhancer hijacking events. This pivotal event has allowed the emergence of a new GRN in extant amphioxus, presumably through a stepwise process. In addition, the co-expression of and achieved by a shared regulatory region may have provided robustness during body axis formation, which provides a selection-based hypothesis for the phenomena called developmental system drift.