Fig. 6

Summary of the origin of skeletogenic cells and the evolution of the molecular signature in deuterostomes. a Model of main hypothesis of sclerocyte origin and differentiation at early stages of regeneration (stage 2/3 and 4/5): new sclerocyte precursors detach from the aboral coelomic cavity (ACC) epithelium and differentiate while moving towards the epidermis together with summary of regulatory (red), differentiation (black), and signalling (blue) gene expression of the ACC, the dermis, and the epidermis. Schematics are based on data shown in Fig. 2 and Additional file 1 Fig. S2; gene expression data is based on Figs. 3 and 4 and Additional file 2 Figs. S5 and S6 combined with already published data (References in Table S2). b Summary of molecular signature of different skeletal elements at the proximal side of late stages of regenerating arms. Gene expression data is based on proximal differentiating skeletal elements shown in Fig. 5 and Additional file 2 Figs. S5 and S6 together with published data. AP, aboral arm plate; LP, lateral arm plate; OP, oral arm plate; Sp, spine; V, vertebra. c Phylogenetic tree with conserved role of skeletogenic genes between the brittle star Amphiura filiformis (present work), Strongylocentrotus purpuratus, and vertebrates. Genes encoding for transcription factors are in red, and the differentiation genes are in black. For details and relative references, see Additional file 2 Table S2. For this summary figure, we considered the gene present if we could find data on a member of the gene family; therefore, it is not a strict 1:1 orthology, considering also the two rounds of genome duplication occurred at the base of vertebrate evolution