The origin and evolution of cell types

Key Points A new evolutionary definition of cell types is presented. Cell types are evolutionary units defined by common descent rather than phenotypic similarity, and characterized by their ability to evolve gene expression programmes independently of each other. The evolutionary diversification of cell types is driven by genomic individuation, which increases the capacity of sister cell types to establish and maintain distinct gene expression programmes. Core regulatory complexes (CoRCs) of terminal selectors determine cell type identity. CoRCs are the molecular agents that enable cell type-specific gene expression. Apomeres are new cellular modules or variants of modules in incipient sister cell types that implement cell type-specific structure and functions. Concerted evolution is characteristic for related cell types that share some genomic information, such as genes or shared enhancers. The evolutionary lineage of cell type individuation is different from the developmental lineage. Serial sister cell types that develop from distinct developmental regions are closely related in evolution but differ in developmental lineage. In this article, the authors review the current understanding of the molecular mechanisms underlying cell type identity and discuss how new phenotypic features of cell types evolve. They explain how evolutionary lineage differs from developmental lineage and highlight how an evolutionary view of cell type identity can facilitate research in comparative cell biology. Cell types are the basic building blocks of multicellular organisms and are extensively diversified in animals. Despite recent advances in characterizing cell types, classification schemes remain ambiguous. We propose an evolutionary definition of a cell type that allows cell types to be delineated and compared within and between species. Key to cell type identity are evolutionary changes in the 'core regulatory complex' (CoRC) of transcription factors, that make emergent sister cell types distinct, enable their independent evolution and regulate cell type-specific traits termed apomeres. We discuss the distinction between developmental and evolutionary lineages, and present a roadmap for future research.