Pearls are highly-prized and structurally complex biominerals fabricated by a wide range of bivalve molluscs. Pearls that are composed of nacre (mother-of-pearl) are of commercial value and form the basis of a lucrative aquaculture industry. In cultured pearl formation a small section of the organ that secretes the shell (the mantle) is transplanted into the gonad of a receiving ‘host’ oyster, along with a spherical bead (the nucleus). The transplanted mantle cells proliferate and grow around the nucleus to form the pearl sac, which sec retes the pearl material around the nucleus.
The mantle controls shell (and pearl) formation by secreting an ‘organic matrix’ composed of proteins, polysaccharides, and lipids. This organic matrix is ultimately responsible for dictating shell architecture, including the formation of different pearl layers (e.g., nacreous and prismatic layers, Fig. 1). Studies in pearl oysters and other molluscs have determined that different regions of the mantle secrete different proteins, and that this correlates with the proteins isolated from different shell layers. These mantle regions are presumably specified during the development of the mantle by a regulatory gene network.
The aim of this study was to investigate regionalization of gene expression within the pearl oyster (Pinctada maxima) mantle to better understand the regulation of nacre formation . To achieve this we microdissected the mantle of four oysters and generated transcriptomes for each re gion individually, using a low-input RNAseq method (CEL-Seq2). We then performed differential gene expression analysis to identify genes that are associated with particular mantle regions, and used bioinformatic methods to predict the gene regulatory networks that drive differential gene expression. Our results clearly demonstrate the existence of a nacre-producing region within the mantle and suggest how this may be regulated. These results not only reveal core regulatory processes governing biomineralisation, but may also have application in the improvement of pearl quality.