The temptin gene of the clade Lophotrochozoa is involved in formation of the prismatic layer during biomineralization in molluscs

The biomineralization mechanism of mollusc shell has been studied for a long time, but there is a lack of understanding about the relationship between the shell formation in vitro and the signaling system in vivo. In this study, we cloned a novel shell matrix protein gene (hc-temptin), which only be characterized as a water-borne protein pheromone of molluscs in previous studies, from the freshwater mussel Hyriopsis cumingii. By bioinformatics analysis we found that temptin was a gene unique to the clade Lophotrochozoa, and it exists in all mollusc taxa except Cephalopoda. The current data supported the premise that temptin was generated in the early emergence of molluscs and that it maintained a high mutation rate to evolve relative independently. The specificity of hc-temptin expression in the mantle tissue suggests its potential to participate in biomineralization. Its sequence contained typical Ca²⁺ binding sites. Our experiments involving the pearl formation process, damaged shell repair process, and RNAi experiment showed that hc-temptin was a shell matrix protein that plays an important role in formation of the prismatic layer. The results of this study provided new insights about the origin of the temptin gene and its role in molluscs.

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.