Characterisation and expression of the biomineralising gene Lustrin A during shell formation of the European abalone Haliotis tuberculata

The molluscan shell is a remarkable product of a highly biomineralisation process, and is composed of calcium carbonate most commonly in the form of calcite or aragonite. The exceptional mechanical properties of this biomaterial are imparted by the embedded organic matrix which is secreted by the underlying mantle tissue. While many shell-matrix proteins have already been identified within adult molluscan shell, their presence and role in the early developmental stages of larval shell formation are not well understood. In the European abalone Haliotis tuberculata, the shell first forms in the early trochophore larva and develops into a mineralised protoconch in the veliger. Following metamorphosis, the juvenile shell rapidly changes as it becomes flattened and develops a more complex crystallographic profile including an external granular layer and an internal nacreous layer. Among the matrix proteins involved in abalone shell formation, Lustrin A is thought to participate in the formation of the nacreous layer. Here we have identified a partial cDNA coding for the Lustrin A gene in H. tuberculata and have analysed its spatial and temporal expression during abalone development. RT-PCR experiments indicate that Lustrin A is first expressed in juvenile (post-metamorphosis) stages, suggesting that Lustrin A is a component of the juvenile shell, but not of the larval shell. We also detected Lustrin A mRNAs in non-nacre forming cells at the distal-most edge of the juvenile mantle as well as in the nacre-forming region of the mantle. Lustrin A was also expressed in 7-day-old post-larvae, prior to the formation of nacre. These results suggest that Lustrin A plays multiple roles in the shell-forming process and further highlight the dynamic ontogenic nature of molluscan shell formation.