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 morphogenesis during embryonic development, and the underlying molecular mechanism of the GRNs evolution. The Nodal signaling that governs the body axes of deuterostomes exhibits a conserved GRN orchestrated principally by Nodal, Gdf1/3 and Lefty. Despite that the GRN retains a conserved function in deuterostomes, the expression patterns of Gdf1/3 and Nodal are derived in cephalochordate amphioxus, implying a specific rewiring of the GRN in this lineage. Here we examined the regulatory mechanism and evolution of this GRN in amphioxus by functional genetic manipulations. We found that while the amphioxus Gdf1/3 orthologue shows nearly no expression during embryogenesis, its duplicate Gdf1/3-like linking to Lefty is zygotically expressed in a similar pattern as Lefty . Mutant and transgenic analyses revealed that Gdf1/3 is no longer crucial for amphioxus axial development. Instead, Gdf1/3-like assumes this responsibility, likely through hijacking Lefty enhancers. We also showed that amphioxus Nodal has become an indispensable maternal factor to compensate for the loss of maternal Gdf1/3 expression. We therefore demonstrated a case that the evolution of an ancestral GRN could be triggered by enhancer hijacking events. This pivotal event has allowed the emergence of a new GRN in the extant amphioxus, presumably evolving through a stepwise process. The co-expression of Gdf1/3-like and Lefty achieved by shared regulatory region may have provided developmental robustness during body axis formation in extant amphioxus, which provides a selection-based hypothesis for the phenomena called developmental system drift.