Fig. 1

The main molecular actors of epithelial cell features in bilaterians and their presence/absence in non-bilaterian metazoans. A Schematic representation (modified from [99] of typical bilaterian epithelial cells and of proteins involved in (i) adherens junctions, (ii) the three polarity complexes and (iii) the basement membrane (Review in [116]. (i) In detail, cell adhesion can be Nectin- and E-cadherin-based and connect via numerous proteins to the actin cytoskeleton: Nectins via Afadin and Integrin; E-cadherins via δ-, β- and α-catenins to the cell adhesion complex proteins Vinculin, VASP, Formin, Arp2/3. In addition, Rap, Rac, CDC42, FAK and IqGAP1 regulate the changes and organization of the actin skeleton [99]. (ii) The three major polarity complexes namely the apically located PAR3/PAR6/aPKC complex and the CRUMBS (CRB)/PALS1/PATJ complex and along with a lateral SCRIBBLE (SCR)/DLG/LGL complex [7]. (iii) The basement membrane of bilaterians involves mainly type IV collagen, laminins, perlecan and nidogen; laminins interact with integrins to establish cell–matrix adhesion [47]. The nomenclature chosen in the scheme is according to human proteins. B Presence/absence of epithelial histological features and epithelial genes in non-bilaterians mapped on a schematic representation of consensual phylogenetic relationships among metazoans according to the literature (for review see [70, 116, 126]. “Partially present” means that either only part of the genes were found in the considered taxa or that some species of the taxa lack the genes or features