HOW CAN A CHARACTER BE “THE SAME UNDER EVERY VARIETY OF FORM AND FUNCTION”? A GENE REGULATORY PERSPECTIVE (Invited Presentation)

Richard Owen’s influential definition of homology as “the same organ in different animals under every variety of form and function” poses an interesting puzzle. What does it really mean to be “the same under any variety of form and function”? This means that similarity among characters is exactly not required for them to be homologous. Lankaster’s evolutionary interpretation of homology does not really help either, since it only adds a historical dimension to the same basic idea: two organs in two species are homologous if they are derived from the same organ in the most recent common ancestor. Note that this definition does not tell us more than Owen’s about what it means to be “the same”. This problem got further complicated with the accumulation of data from evolutionary developmental biology, where different molecular signaling pathways are involved in the development of clearly homologous characters. In this contribution I will discuss how a simple model of developmental control can solve the problem of character identity. It is proposed that character identity is based on the activation of a core regulatory network, aka Character Identity Network ChIN, which controls the expression of effector genes, which in turn produce the morphological or physiological phenotype. This model implies that character identity, understood as the ability to differentially regulate effector genes, is mechanistically decoupled from the production of the phenotype, i.e. the phenotype under every variety of form and function. This model explains the conceptual distinction between character identity and character states by the mechanistic dissociation between ChIN activity and the set of effector genes regulated by the ChIN. I will illustrate these concepts with respect to cell type identity and evolution. Then I want to discuss how this concept needs to be modified to be applicable to multicellular characters.