title: A rapidly evolving secretome builds and patterns a sea shell.
abstract: BACKGROUND
  Instructions to fabricate mineralized structures with distinct nanoscale architectures, such as seashells and coral and vertebrate skeletons, are encoded in the genomes of a wide variety of animals. In mollusks, the mantle is responsible for the extracellular production of the shell, directing the ordered biomineralization of CaCO3 and the deposition of architectural and color patterns. The evolutionary origins of the ability to synthesize calcified structures across various metazoan taxa remain obscure, with only a small number of protein families identified from molluskan shells. The recent sequencing of a wide range of metazoan genomes coupled with the analysis of gene expression in non-model animals has allowed us to investigate the evolution and process of biomineralization in gastropod mollusks.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: School of Integrative Biology, University of Queensland, Brisbane Qld 4072, Australia. d.jackson@sib.uq.edu.au
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Jackson
authors.forename: Carmel
authors.initials: C
authors.lastname: McDougall
authors.forename: Kathryn
authors.initials: K
authors.lastname: Green
authors.forename: Fiona
authors.initials: F
authors.lastname: Simpson
authors.forename: Gert
authors.initials: G
authors.lastname: Wörheide
authors.forename: Bernard M
authors.initials: BM
authors.lastname: Degnan
mesh_terms: D000825:Animal Structures / Q000254:growth & development* / Q000378:metabolism / Q000648:ultrastructure; D000818:Animals; D002119:Calcium Carbonate / Q000378:metabolism; D020224:Expressed Sequence Tags; D016326:Extracellular Matrix Proteins / Q000378:metabolism*; D049851:Gastropoda / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism / Q000648:ultrastructure; D018507:Gene Expression Regulation, Developmental; D017403:In Situ Hybridization; D046529:Microscopy, Electron, Transmission; D010858:Pigmentation / Q000235:genetics
chemical_list: D016326:Extracellular Matrix Proteins; D002119:Calcium Carbonate