The Molluscan Shell Secretome: Unlocking Calcium Pathways in a Changing World

How do molluscs build their shells? Despite hundreds of years of human fascination, the processes underpinning mollusc shell production are still considered a black box. We know molluscs can alter their shell thickness in response to environmental factors, but we do not have a mechanistic understanding of how the shell is produced and regulated. In this thesis I used a combination of methodologies - from traditional histology, to shell
damage-repair experiments and ‘omics technologies - to better understand the molecular mechanisms which control shell secretion in two species, the Antarctic clam Laternula elliptica and the temperate blunt-gaper clam Mya truncata. The integration of different methods was particularly useful for assigning putative biomineralisation functions to genes with no previous annotation. Each chapter of this thesis found reoccurring evidence for the involvement of vesicles in biomineralisation and for the duplication and subfunctionalisation of tyrosinase paralogues. Shell damage-repair experiments revealed biomineralisation in L. elliptica was variable, transcriptionally dynamic, significantly affected by age and inherently entwined with immune processes. The high amount of transcriptional variation across 78 individual animals was captured in a single mantle
regulatory gene network, which was used to predict the regulation of “classic”
biomineralisation genes, and identify novel biomineralisation genes. There were some general shared patterns in the molecular control of biomineralisation between the two species investigated in this thesis, but overall, the comparative work in this thesis, coupled to the growing body of literature on the evolution of molluscan biomineralisation, suggests that biomineralisation mechanisms are surprisingly divergent.