A rapidly evolving secretome builds and patterns a sea shell

Here we show that over 25% of the genes expressed in the mantle of the vetigastropod Haliotis asinina encode secreted proteins, indicating that hundreds of proteins are likely to be contributing to shell fabrication and patterning. Almost 85% of the secretome encodes novel proteins; remarkably, only 19% of these have identifiable homologues in the full genome of the patellogastropod Lottia scutum. The spatial expression profiles of mantle genes that belong to the secretome is restricted to discrete mantle zones, with each zone responsible for the fabrication of one of the structural layers of the shell. Patterned expression of a subset of genes along the length of the mantle is indicative of roles in shell ornamentation. For example, Has-sometsuke maps precisely to pigmentation patterns in the shell, providing the first case of a gene product to be involved in molluskan shell pigmentation. We also describe the expression of two novel genes involved in nacre (mother of pearl) deposition.The unexpected complexity and evolvability of this secretome and the modular design of the molluskan mantle enables diversification of shell strength and design, and as such must contribute to the variety of adaptive architectures and colors found in mollusk shells. The composition of this novel mantle-specific secretome suggests that there are significant molecular differences in the ways in which gastropods synthesize their shells.The ability to synthesize rigid, mineralized structures is an essential trait to the majority of metazoan taxa. Vertebrates, echinoderms, mollusks, arthropods, brachiopods, bryozoans, annelids, cnidarians and sponges, amongst others, construct a spectacular diversity of endo- and exo-skeletons as well as sensory and protective structures from a range of minerals [1]. The importance of this trait is highlighted by the observation that the so called 'Cambrian explosion' was accompanied by the diversification of biomineralization mechanisms [2-4], despite the