Characterization of the Zhikong scallop (Chlamys farreri) mantle transcriptome and identification of biomineralization-related genes.

Chlamys farreri is a significant species in aquaculture and fishery in East Asia. A deep understanding of its shell formation by studying the transcriptome of the mantle, a key organ in shell formation, could provide important guidance for its culture. Thus, we sequenced and analyzed the mantle transcriptome of C. farreri. The 77,975 unigenes were generated after Illumina sequencing and de novo assembly. The unigenes were annotated using authoritative databases (non-redundant (NR), COG, Gene Ontology (GO), and KEGG) to obtain functional information. BLASTX alignment was performed between unigenes and reported proteins related to biomineralization. The results identified 53 homologous genes representing 17 matrix proteins, most of which are involved in calcite formation, and 171 homologies with 26 proteins related to general processes of biomineralization. The discovery and unusually high expression of MSP-1 suggested its importance in scallops. Homologous unigenes with aragonite-formation-related matrix proteins were much fewer compared with those related to calcite formation. The results implied that, in C. farreri, the number and proportion of matrix proteins related to aragonite formation is much lower than those related to calcite formation, which was consistent with the proportions of aragonite and calcite in C. farreri shells. Thus, the formation of different polymorphs of calcium carbonate (calcite and aragonite) in molluskan shells is regulated by different groups of proteins. Moreover, 17 candidate unigenes, which are probably involved in biomineralization, were predicted by screening for gene products with secreted domains and tandem-arranged repeat units. Our results contribute to the understanding of biomineralization processes and the evolution of shell formation.