Meeting Abstract
Cell types have diversified prodigiously since the dawn of multicellularity. During development, cell types are specified by cohorts of transcription factors interacting with cell-type specific genomic regulatory elements, which together approximate cell type identity. During evolution, cell type identity has been rigid enough to maintain highly conserved cell types but also labile enough to generate novel cell types. Hence understanding the evolution of cell type identity is key to explaining how ancestral cell types diversified to give rise to what we see today. Here, we tracked cell type identity of the echinoderm larval skeletogenic cell, a highly conserved and widely studied cell type that is characteristic of adult echinoderms and also early development of numerous echinoderm taxa. We used phylogenetic comparative methods to frame developmental spatial gene expression data in the context of cell type evolution. Our results show that, during the evolution of the echinoderm skeletogenic cell type, cell type identity was maintained in spite of differential transcription factor usage, suggesting that genomic regulatory information is critical to the identity of this cell type. We conducted ancestral state reconstruction to ask whether the most recent common ancestor of eleutherozoan echinoderms possessed a larval skeletogenic cell. Our analysis provided support for the presence of this cell type in larval development of the eleutherozoan MRCA, suggesting that all extant echinoderm larval skeletogenic cells are homologous and descend from a common ancestral cell type that was likely acquired in early development in stem eleutherozoans.
