The retinoic acid receptor regulates development of a key evolutionary novelty - the molluscan shell

The shells of molluscs are iconic structures of invertebrate exoskeletons supporting and protecting their soft body parts. The shell matrix molecules are synthesized and secreted from a shell-producing tissue, the shell gland. Shell gland cells develop at the early trochophore stage of the larvae. To date, the molecular signalling pathways by which the shell gland forms and starts to secrete the shell remains elusive. Here we demonstrate in the Pacific oyster Crassostrea gigas and the limpet Nipponacmea fuscoviridis, that the retinoic acid receptor (RAR), is crucial for inducing shell gland formation. RAR is expressed in both species in the shell gland at the late gastrula to early trochophore stages prior to the first production of shell. Suppression of the RAR by chemical inhibitors or gene-knock-down lead to a complete loss of the larval shell. Transcriptomic and in situ hybridisation analyses revealed that the developmental regulatory genes that are normally expressed in the shell gland, including engrailed, are down-regulated in the RAR-suppressed embryos. Using the RAR functional assay carried out on zebrafish embryos, we also revealed that the oyster RAR cannot transduce the RA signal in zebrafish, indicating that the molluscan RAR is clearly different from vertebrate RARs in its binding capacity to the RA. Our finding represents a key example of adaptive evolution of developmental "toolkit" genes for the origin of a major novel trait, the molluscan shell, in animals.