Role of <i>klf2</i> in innovation of the Pmar1-HesC double-negative gate in echinoderms

The diversity of organism morphology is heavily dependent on the evolution of gene regulatory networks. However, the mechanistic basis on which change can be incorporated within these systems remains poorly understood. Evolution of the echinoderm hesC gene as a novel upstream mesoderm regulator found uniquely in the euechinoid sea urchin lineage is an intriguing example of gene regulatory network modification. To reveal how new components can be incorporated within upstream pathways, we investigated the starfish mesoderm regulatory pathway, which lacks the derived hesC mesoderm regulation. We show that starfish HesC is able to perform the function of endogenous HesC in the context of sea urchin embryogenesis. Furthermore, we revealed that both starfish and euechinoids employ the klf2 gene as an upstream mesoderm regulator within a similar regulatory apparatus as the euechinoid hesC. It can be speculated that klf2 plays a crucial role in buffering the recruitment of hesC into a similar regulatory position through a gradual exchange of players, which may be a prevalent mechanism for drastic regulatory innovations.