A Bivalve Biomineralization Toolbox

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.

Keywords: Crassostrea gigas; Mytilus edulis; Pecten maximus; biomineralization; damage-repair; transcriptomics.

Fig. 1.

The four pairwise comparisons carried out in the damage-repair experiment.

Fig. 2.

Expression profiles of contigs with strong sequence similarity to SMPs. (a) Pecten maximus, (b) Crassostrea gigas, and (c) Mytilus edulis.

Fig. 3.

Expression patterns of SMPs and domains found in the mantle transcriptomes. Key: CM/ME: SMPs highly expressed in the central mantle (CM) or mantle edge (ME); **: FDR of at least 0.05; UP/DW/V: SMPs up-regulated (UP), down-regulated (DW), or variable (V) during repair in the central mantle of the damaged valve; P and blue shading: Putative shell proteins with no sequence similarity to, but with similar functional domains to known SMPs; Pink shading: differentially expressed contigs with sequence similarity to both SMPs and hemocyte ESTs.

Fig. 4.

Domains identified in the proteins predicted from differentially expressed contigs following damage-repair in the central mantle. Domains in black were previously identified in shell proteomes (Arivalagan et al. 2017); those in red were common among more species than was detected in the shell proteome alone.

Fig. 5.

Expression profiles of contigs with strong sequence similarity to transmembrane transporters. (a) Pecten maximus, (b) Crassostrea gigas, and (c) Mytilus edulis.