CELLS, FOSSILS, AND EMBRYOS: USING PHYLOGENETIC COMPARATIVE METHODS TO UNDERSTAND HOW SPECIALIZED CELL TYPES HAVE CHANGED IN DEEP TIME

Cell types have diversified in structure and function since the dawn of multicellularity, though how cell type identity is maintained and yet labile in evolutionary time is of great interest. The fossil record can be used to inform our understanding of when particular cell types evolved. For instance, the fossil record tells us the stereom endoskeleton of echinoderms is an ancient structure that extends far back into deep time. Studies from developmental biology tell us a specialized cell type called skeletogenic cells synthesizes this structure in larval and adult echinoderms. Uniting these two datasets will inform our understanding of how cell types change in deep time, and thus may contribute to our understanding of how the great diversity of extant cell types arose in evolution. Here, we employ phylogenetic comparative methods to track cell type identity of the echinoderm skeletogenic cell by integrating evolutionary time with developmental spatial gene expression data. By framing our results in the context of cell type evolution, we show that, during the evolution of the echinoderm skeletogenic cell type, cell type identity was maintained in spite of differential transcription factor usage. Our results support an evolutionary scenario whereby the larval skeletal structures arose one time during the evolution of eleutherozoan echinoderms.