It has been considered that the adult skeleton originally existed in the common ancestor of echinoderms, and the developmental process was recruited to early development during echinoderm diversification, leading to the innovation of the larval skeleton 8. All five groups of extant echinoderms (echinoids, holothuroids, ophiuroids, asteroids, and crinoids ) possess adult skeletons, whereas larval skeletons are formed in only echinoids, ophiuroids, and holothuroids (see Koga et al. The co-option of pre-existing gene regulatory networks (GRNs) is considered to be key to the evolution of morphological novelties 1, and together with novelties in insects, such as beetle horns 2, 3, butterfly eyespots 4, and treehopper helmets 5, the calcitic larval skeleton in the echinoderm is one of the best-studied experimental models of this process 6. Based on these data, we provide a model for the adult skeletogenic GRN in the echinoderm and propose that the upstream regulatory system changed from the feeding-TOR-Vegf pathway to a homeobox gene-system during co-option of the skeletogenic GRN. In the larval skeletogenesis of sea urchins, the homeobox gene pmar1 activates skeletogenic regulatory genes, but in starfish, localized expression of the pmar1-related genes phbA and phbB was not detected during the adult skeleton formation stage. Treatment with inhibitors and gene knockout using transcription activator-like effector nucleases (TALENs) suggest that the feeding-nutrient sensing pathway activates Vegf signaling via target of rapamycin (TOR) activity, leading to the activation of skeletogenic regulatory genes in starfish. Expression patterns of skeletogenesis-related genes ( vegf, vegfr, ets1/2, erg, alx1, ca1, and clect) suggest that adult skeletogenic cells develop from the posterior coelom after the start of feeding.

To explore the evolutionary changes that occurred during co-option, we examined the mechanism for adult skeletogenesis using the starfish Patiria pectinifera. The larval skeleton of the echinoderm is believed to have been acquired through co-option of a pre-existing gene regulatory network (GRN) that is, the mechanism for adult skeleton formation in the echinoderm was deployed in early embryogenesis during echinoderm diversification.

Skin gills tarfish