Transcriptomic analysis of shell repair and biomineralization in the blue mussel, Mytilus edulis

Abstract

Background Biomineralization by molluscs involves regulated deposition of calcium carbonate crystals within a protein framework to produce complex biocomposite structures. Effective biomineralization is a key trait for aquaculture, and animal resilience under future climate change. While many enzymes and structural proteins have been identified from the shell and in mantle tissue, understanding biomieralization is impeded by a lack of fundamental knowledge of the genes and pathways involved. In adult bivalves, shells are secreted by the mantle tissue during growth, maintenance and repair, with the repair process, in particular, amenable to experimental dissection at the transcriptomic level in individual animals. Results Gene expression dynamics were explored in the adult blue mussel, Mytilus edulis, during experimentally induced shell repair, using the two valves of each animal as a matched treatment-control pair. Gene expression was assessed using high-resolution RNA-Seq against a de novo assembled database of functionally annotated transcripts. A large number of differentially expressed transcripts were identified in the repair process. Analysis focused on genes encoding proteins and domains identified in shell biology, using a new database of proteins and domains previously implicated in biomineralization in mussels and other molluscs. The genes implicated in repair included many otherwise novel transcripts that encoded proteins with domains found in other shell matrix proteins, as well as genes previously associated with primary shell formation in larvae. Genes with roles in intracellular signalling and maintenance of membrane resting potential were among the loci implicated in the repair process. While haemocytes have been proposed to be actively involved in repair, no evidence was found for this in the M. edulis data. Conclusions The shell repair experimental model and a newly developed shell protein domain database efficiently identified transcripts involved in M. edulis shell production. In particular, the matched pair analysis allowed factoring out of much of the inherent high level of variability between individual mussels. This snapshot of the damage repair process identified a large number of genes putatively involved in biomineralization from initial signalling, through calcium mobilization to shell construction, providing many novel transcripts for future in-depth functional analyses.

The Mantle Transcriptome of Chamelea gallina (Mollusca: Bivalvia) and Shell Biomineralization

“…The comprehensive de novo transcriptome assembly from the mantle of C. gallina specimens showed comparable quality with the recently produced transcriptomes from the same tissue of Mytilus edulis [31,32]. Moreover, BUSCO assessment evidenced a high level of completeness with the detection of 77.2% of complete Mollusca BUSCOs.…”

Mussels Repair Shell Damage despite Limitations Imposed by Ocean Acidification

“…While only a minor component of the shell matrix, SMPs play an important role in CaCO 3 nucleation, growth, and polymorph determination [89,90], and their expression has been shown to be induced in the central mantle following shell injury [47]. Further, transcriptional analysis of the mantle following shell damage within M. edulis has identified transcripts that encode proteins with domains found in shell matrix proteins (SMPs), immune response proteins, and other proteins involved in biomineralization [45].…”

“…One possible explanation for this variation could be that, while great lengths were taken to standardize the depth with which drill holes were generated, variation in shell thickness may have resulted in different degrees of tissue damage. Alternatively, nonlocalized repair could be the result of the nonspecific deposition of shell matrix proteins (SMPs) that act as nucleation sites during calcite and aragonite formation [45,47,91]. While it is likely that the transcriptional response of mantle tissue to shell damage will be greatest next to the shell injury [31], the factors that govern how SMPs are deposited onto damaged shell after extrusion into the extrapallial fluid remain unclear.…”

“…In response to shell damage, the mantle extrudes a mixture of polysaccharides and glycoproteins into the extrapallial fluid, forming an organic membrane that covers the interior of the shell breach through a process that has been well described within mollusks [42,43]. A suite of conserved [44] and species-specific [45] shell matrix proteins (SMPs) then determine which mineral polymorph forms on the membrane by facilitating the nucleation, growth, and termination of calcium carbonate crystals (for a review, see [46]). Organic sheet synthesis has been observed to occur 20 days after shell injury in Mytilus, followed by the deposition of calcite crystals (29 days) and aragonite tablets (36 days) [47].…”

A Chitin-Binding Protein Ultra-highly expressed in the Outer Fold of Mantle is Related to Shell Colour in Pacific Oyster Crassostrea gigas

“…Mantle plays an important role in shell biological mineralisation [38][39][40] and is mainly divided into central mantle and mantle edge. The central mantle is involved in the calcification and thickening of the shell, such as inhibiting CaCO3 crystal and regulating the growth of epithelial cells and Ca 2+ ; the mantle edge contains a large number of conchiolin proteins prompting the shell to grow outward and participating in shell periostraca formation 18,39,41,42 . Shell colour is also correlated with the colour of the mantle edge 13,22,43,44 contains a large number of genes and genetic locus related to shell colour 8,[45][46][47][48] .…”