An exhaustive exploration of the genes involved in molluscan shell formation : a primer for a genome projec

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In this study, we aimed at elucidating the evolutionary processes and mechanisms of the shell by characterizing the (1) master genes shell formation, (2) proteins controlling shell (biomineralization), and (3) control precipitation genes for the shell left-right asymmetry, and obtained the following results.(1) master genes for shell formation Gene expression patterns of the transcription factor engrailed and the growth factor dpp in Lymnaea stagnalis indicated that engrailed is expressed around the shell gland just like in other molluscs, and dpp is expressed only in the right-hand side of the invaginated shell gland, suggesting that both genes are involved in shell formation and dpp could be further responsible for the shell left-right asymmetry.(2) biomineralization genes Phylogenetic analysis of the shell matrix protein Dermatopontin among eight species of pulmonate snails showed that the gene duplications that lead to the evolution of the shell matrix proteins occurred at least twice independently in pulmonates. Comparisons with the fossil record indicated that these duplications should have occurred after Mesozoic Era. In the pearl oyster Pinctada fucata, the primary structure of the unusually acidic shell matrix protein Aspein has been deduced. Analyses of expression patterns and in vitro functions demonstrated that Aspein is highly likely the factor controlling the selective precipitation of calcite shells in P. fucata.(3) control genes for the shell left right asymmetry We performed an exhaustive analysis using the HiCEP method for the mRNAs that show asymmetric expression patterns between the right-hand side and the left-hand side of the mantle epithelium in Lymnaea stagnalis, and found that a total of 11588 genes are expressed in the mantle and that dozens of genes show a left-right asymmetric gene expression pattern. Analysis is still on progress as to the structure and function for the 20 genes that indicated the highest extent of asymmetry.