Abstract
Mollusc shells are a result of the deposition of crystalline and amorphous calcite catalyzed by enzymes and shell matrix proteins (SMP). Developing a detailed understanding of bivalve mollusc biomineralization pathways is complicated not only by the multiplicity of shell forms and microstructures in this class, but also by the evolution of associated proteins by domain co-option and domain shuffling. In spite of this, a minimal biomineralization toolbox comprising proteins and protein domains critical for shell production across species has been identified. Using a matched pair design to reduce experimental noise from inter-individual variation, combined with damage-repair experiments and a database of biomineralization SMPs derived from published works, proteins were identified that are likely to be involved in shell calcification. Eighteen new, shared proteins likely to be involved in the processes related to the calcification of shells were identified by the analysis of genes expressed during repair in Crassostrea gigas, Mytilus edulis, and Pecten maximus. Genes involved in ion transport were also identified as potentially involved in calcification either via the maintenance of cell acid–base balance or transport of critical ions to the extrapallial space, the site of shell assembly. These data expand the number of candidate biomineralization proteins in bivalve molluscs for future functional studies and define a minimal functional protein domain set required to produce solid microstructures from soluble calcium carbonate. This is important for understanding molluscan shell evolution, the likely impacts of environmental change on biomineralization processes, materials science, and biomimicry research.
Highlights
Calcium carbonate is a major skeletal component in the natural world
Shell damage-repair experiments were conducted on three species of bivalve molluscs: the blue mussel (M. edulis), the Pacific oyster (C. gigas), and the great scallop (P. maximus)
Gene transcription profiles were analyzed from the mantle edge and the central mantle tissue sections from damaged and control valves of the same individual using a matched pair design as previously described in Huning et al (2016) and Yarra et al, with holes drilled in the centers of the shells above the central mantle zone in cohorts of wild-sampled, live bivalves
Summary
The chemical reactions to produce solid calcium carbonate for marine skeletons are relatively simple, the structure of the biologically synthesized end products varies enormously (Clark 2020). This is true within the Mollusca, which comprise over 70,000 extant species, most of which have biomineralized shells, exhibiting a vast array of shapes, sizes, and colors (Rosenberg 2014; Williams 2017). The exact mix of these proteins with calcium carbonate crystals significantly influences the particular shell microstructure (e.g., prism, nacre, foliae, and crossed-lamellar structures [reviewed in Clark et al 2020])
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