Deciphering mollusc shell production: the roles of genetic mechanisms through to ecology, aquaculture and biomimetics

Abstract

Most molluscs possess shells, constructed from a vast array of microstructures and architectures. The fully formed shell is composed of calcite or aragonite. These CaCO3 crystals form complex biocomposites with proteins, which although typically less than 5% of total shell mass, play significant roles in determining shell microstructure. Despite much research effort, large knowledge gaps remain in how molluscs construct and maintain their shells, and how they produce such a great diversity of forms. Here we synthesize results on how shell shape, microstructure, composition and organic content vary among, and within, species in response to numerous biotic and abiotic factors. At the local level, temperature, food supply and predation cues significantly affect shell morphology, whilst salinity has a much stronger influence across latitudes. Moreover, we emphasize how advances in genomic technologies [e.g. restriction site‐associated DNA sequencing (RAD‐Seq) and epigenetics] allow detailed examinations of whether morphological changes result from phenotypic plasticity or genetic adaptation, or a combination of these. RAD‐Seq has already identified single nucleotide polymorphisms associated with temperature and aquaculture practices, whilst epigenetic processes have been shown significantly to modify shell construction to local conditions in, for example, Antarctica and New Zealand. We also synthesize results on the costs of shell construction and explore how these affect energetic trade‐offs in animal metabolism. The cellular costs are still debated, with CaCO3 precipitation estimates ranging from 1–2 J/mg to 17–55 J/mg depending on experimental and environmental conditions. However, organic components are more expensive (~29 J/mg) and recent data indicate transmembrane calcium ion transporters can involve considerable costs. This review emphasizes the role that molecular analyses have played in demonstrating multiple evolutionary origins of biomineralization genes. Although these are characterized by lineage‐specific proteins and unique combinations of co‐opted genes, a small set of protein domains have been identified as a conserved biomineralization tool box. We further highlight the use of sequence data sets in providing candidate genes for in situ localization and protein function studies. The former has elucidated gene expression modularity in mantle tissue, improving understanding of the diversity of shell morphology synthesis. RNA interference (RNAi) and clustered regularly interspersed short palindromic repeats ‐ CRISPR‐associated protein 9 (CRISPR‐Cas9) experiments have provided proof of concept for use in the functional investigation of mollusc gene sequences, showing for example that Pif (aragonite‐binding) protein plays a significant role in structured nacre crystal growth and that the Lsdia1 gene sets shell chirality in Lymnaea stagnalis. Much research has focused on the impacts of ocean acidification on molluscs. Initial studies were predominantly pessimistic for future molluscan biodiversity...

How the Pacific Oyster Responds to Ocean Acidification: Development and Application of a Meta-Analysis Based Adverse Outcome Pathway

“…In the future, the AOP framework could be used to consider mechanisms underlying varying responses that have been found, for example, between populations across large spatial areas (e.g., Parker et al, 2010;Ginger et al, 2013;Kraft et al, 2014;Falkenberg et al, 2019;Telesca et al, 2019;Clark et al, 2020), among distinct populations of the same species (e.g., Stapp et al, 2016), among closely related species (Vihtakari et al, 2016), and across different taxonomic groups (e.g., Kroeker et al, 2010). For example, recent studies considering a range of ectotherms from different taxonomic groups have suggested that Nab/Kb ATPase, a major cellular ATP consumer (Wieser and Krumschnabel, 2001), may be broadly susceptible to the effects of acidification (Pörtner et al, 2004;Lannig et al, 2010).…”

“…The effects of ocean acidification on calcifying mollusks have been suggested to result largely from impacts on the development of calcified structures, as well as their maintenance via enhanced erosion and dissolution due to the increased porosity of the shells' internal microstructure (e.g., Marshall et al, 2008;Nienhuis et al, 2010;Rodolfo-Metalpa et al, 2011;Fitzer et al, 2015Fitzer et al, , 2016Fitzer et al, , 2019Meng et al, 2018;Byrne and Fitzer, 2019;Leung et al, 2020a). Although calcification represents one of the physiological processes most susceptible to ocean acidification (Kroeker et al, 2010), studies have only relatively recently started to unravel the complex drivers of such effects (e.g., Meng et al, 2018;Clark et al, 2020;Leung et al, 2020b). Currently, the key mechanism impairing calcification under ocean acidification is considered to be the exacerbation of the energetic costs of maintaining internal pH homeostasis under the increased concentration of protons in the surrounding seawater (Barry et al, 2011;Stumpp et al, 2011;Waldbusser et al, 2015;Cyronak et al, 2016;Fitzer et al, 2016).…”

Mobilising molluscan models and genomes in biology

“…Molluscs are a powerful system in which to study the evolution and mechanics of biomineralization because of their high diversity and their highly complex, robust, and often patterned shells [11,12,95]. In this respect, bivalves in particular are also a subject of relatively intense study, both because of their important commercial applications and because a few bivalve species are highly invasive [54].…”

“…A key aim for the future must be to understand the regulatory networks that lie at the core of shell formation and whether there is a conserved genetic 'toolbox' [12]. Prior gene expression studies in a range of species have revealed several conserved genes expressed in discrete zones within and around the developing shell, hinting at a conserved network [refs in 11, 12].…”

A survey of miRNAs involved in biomineralization and shell repair in the freshwater gastropod Lymnaea stagnalis

“…While the inventories of molluscan shell-forming proteins continue to grow at an exponential rate [9,10], the ways in which these proteins are regulated and the underlying gene regulatory networks (GRNs) that have evolved to coordinate these processes [11] remain more obscure. Transcription factors and signaling molecules are arguably the primary elements of any GRN, and several have been directly implicated in coordinating the specification and differentiation of shell-forming cells in molluscs, and in directly regulating the biomineralization process [12][13][14][15][16][17].…”