Can the quality of pearls from the Japanese pearl oyster (Pinctada fucata) be explained by the gene expression patterns of the major shell matrix proteins in the pearl sac?

Abstract

For pearl culture, the pearl oyster is forced open and a nucleus is implanted into the gonad with a mantle graft. The outer mantle epithelial cells of the implanted mantle graft elongate and surrounding the nucleus a pearl sac is formed. Shell matrix proteins secreted by the pearl sac play an important role in the regulation of pearl formation. Recently, seven shell matrix proteins were identified from the pearl oyster Pinctada fucata. However, there is a paucity of information on the function of these proteins and their gene expression patterns. Our study aims to elucidate the relationship between pearl type, quality, and gene expression patterns of six shell matrix proteins (msi60, n16, nacrein, msi31, prismalin-14, and aspein) in the pearl sac based on real-time PCR analysis. After culturing for about 2 months, the pearl sac tissues were collected from 22 individuals: 12 with high quality (HP), nine with low quality (LP), and one with organic (ORG) pearl formation. The surface of each of the 12 HP pearls was composed only of a nacreous layer; in contrast, that of the nine LP pearls was composed of nacreous and prismatic layers. The six target gene expressions were detected in all individuals. However, delta threshold cycle (ΔC(T)) for msi31 was significantly higher in the HP than in the LP individuals (Mann-Whitney's U test, p=0.02). This means that the relative expression level of msi31, which constitutes the framework of the prismatic layer, was higher in the LP than in the HP individuals.