Abstract
Nucleated pearls are produced by molluscs of the Pinctada genus through the biomineralisation activity of the pearl sac tissue within the recipient oyster. The pearl sac originates from graft tissue taken from the donor oyster mantle and its functioning is crucial in determining key factors that impact pearl quality surface characteristics. The specific role of related gene regulation during gem biogenesis was unknown, so we analysed the expression profiles of eight genes encoding nacreous (PIF, MSI60, PERL1) or prismatic (SHEM5, PRISM, ASP, SHEM9) shell matrix proteins or both (CALC1) in the pearl sac (N = 211) of Pinctada margaritifera during pearl biogenesis. The pearls and pearl sacs analysed were from a uniform experimental graft with sequential harvests at 3, 6 and 9 months post-grafting. Quality traits of the corresponding pearls were recorded: surface defects, surface deposits and overall quality grade. Results showed that (1) the first 3 months of culture seem crucial for pearl quality surface determination and (2) all the genes (SHEM5, PRISM, ASP, SHEM9) encoding proteins related to calcite layer formation were over-expressed in the pearl sacs that produced low pearl surface quality. Multivariate regression tree building clearly identified three genes implicated in pearl surface quality, SHEM9, ASP and PIF. SHEM9 and ASP were clearly implicated in low pearl quality, whereas PIF was implicated in high quality. Results could be used as biomarkers for genetic improvement of P. margaritifera pearl quality and constitute a novel perspective to understanding the molecular mechanism of pearl formation.