New tools in the box: An evolutionary synopsis of chromatin insulators

• Insulator proteins play a fundamental role in genome regulation and architecture.
• We review these proteins and their function in an evolutionary context.
• We find evidence of dynamic and ongoing evolution of insulator proteins in multiple lineages.
• Evolutionary changes in insulator binding sites modify gene expression and phenotype.
• We suggest that the evolution of animals and their rich gene regulatory repertoire is linked to the evolution of insulator proteins.

Despite progress in understanding genome organization and gene expression during the last decade, the evolutionary pathways that led to the intricate patterns of gene expression in different cells of an organism are still poorly understood. Important steps in this regulation take place at the level of chromatin, where the (epi)genomic environment of a gene determines its expression in time and space. Although the basic mechanisms of gene expression apply to all eukaryotes, multicellular organisms face the additional challenge of coordinating gene expression during development. In this review we summarize and put into evolutionary context current knowledge about chromatin insulators, an important class of regulatory factors mediating these tasks. Our interpretation of historical and recent findings points to a dynamic and ongoing evolution of insulator proteins characterized by multiple instances of convergent evolution, gene loss, and binding site changes in different organisms. The idea of two autonomously evolving insulator functions (as a barrier element and an enhancer blocker) further suggests that the evolution of metazoans and their enhancer-rich gene regulatory repertoire might be connected to the radiation of enhancer blocking insulators. Although speculative at the moment, such coevolution might create tools for complex gene regulation and therefore influence the evolutionary roadmaps of metazoans.