Expressions of Shell Matrix Protein Genes in the Pearl Sac and Its Correlation with Pearl Weight in the First 6 Months of Pearl Formation in Hyriopsis cumingii.

Matrix proteins regulate crystal nucleation, morphology, and polymorphism during pearl biomineralization and have significant correlations with pearl quality traits in nucleated pearls. However, there is little information about the connection between pearl quality traits and matrix proteins in non-nucleated pearls. In this study, we analyzed CaCO3 deposition during the first month of non-nucleated pearl formation and examined the expression patterns of ten shell matrix protein genes (Hcperlucin, hic31, silkmapin, hic22, hic74, hic52, HcTyr, HcCA3, hic24 and Hc-upsalin) in the pearl sac of Hyriopsis cumingii. During pearl formation, CaCO3 crystals were initially deposited in a disorderly manner during days 12 and 15 of pearl formation. On days 18 and 21, CaCO3 crystals gradually nucleated on an organic membrane, and the pattern of crystal deposition changed markedly. Between days 24 and 30, crystals similar to nacre tablets were deposited; they then grew and formed connections in a more orderly fashion, eventually forming the nacreous layer. We observed high expression levels of shell matrix proteins during the phases of disordered or ordered CaCO3 deposition, suggesting they were involved in non-nucleated pearl formation. Furthermore, the expressions of nine matrix proteins were significantly correlated with pearl weight during the first 6 months after grafting. The prismatic-layer matrix protein hic31 and nacreous-layer matrix protein hic22 showed negative correlations with pearl weight, but the other seven nacreous-layer matrix proteins had significantly positive correlations with pearl weight. These results show the involvement of matrix proteins in pearl formation and in determination of quality traits.