Molluscs exhibit diverse shell colors. The molecular regulation of shell coloration is however not well understood. To investigate the connection of shell coloration with pigment synthesis, we analyzed the distribution of porphyrins, a widespread group of pigments in nature, in four Pacific oyster strains of different shell colors including black, orange, golden, and white. The porphyrin distribution was analyzed in oyster mantles and shells by fluorescence imaging and UV spectrophotometer. The results showed that red fluorescence emitted by porphyrins under the UV light was detected only on the nacre of the orange-shell strain and mantles of orange, black and white-shell strains. Extracts from newly deposit shell, nacre and mantle tissue from orange-shell specimens showed peaks in UV–vis spectra that are characteristic of porphyrins, but these were not observed for the other shell-color strains. In addition, genes of the haem synthetic pathway were isolated and characterized. Phylogenetic analysis of CgALAS, CgALAD, CgPBGD, CgUROS, and CgUROD provide further evidence for a conserved genetic pathway of haem synthesis during evolution. Differential expression of the haem genes expressed in mantle tissues support these findings and are consistent with porphyrins being produced by the orange strain only. Tissue in situ hybridization demonstrated the expression of these candidate genes at the outer fold of C. gigas mantles where shell is deposited. Our studies provide a better understanding of shell pigmentation in C. gigas and candidate genes for future mechanistic analysis of shell color formation in molluscs.

软体动物的外壳颜色多种多样。然而，壳着色的分子调控还不是很清楚。为了研究贝壳着色与色素合成的关系，我们分析了卟啉（自然界中广泛存在的一组色素）在四种不同贝壳颜色（包括黑色、橙色、金色和白色）的太平洋牡蛎品系中的分布。通过荧光成像和紫外分光光度计分析了卟啉在牡蛎地幔和贝壳中的分布。结果表明，卟啉在紫外光下发出的红色荧光仅在橙壳菌株的珍珠层和橙色、黑色和白色壳菌株的地幔上检测到。来自橙壳标本的新沉积壳、珍珠层和地幔组织的提取物在紫外可见光谱中显示出卟啉的特征峰，但在其他壳色菌株中没有观察到这些。此外，还分离并表征了血红素合成途径的基因。系统发育分析CgALAS、CgALAD、CgPBGD、CgUROS和CgUROD为进化过程中血红素合成的保守遗传途径提供了进一步的证据。在地幔组织中表达的血红素基因的差异表达支持这些发现，并且与仅由橙色菌株产生的卟啉一致。组织原位杂交证明了这些候选基因在壳沉积的C. gigas地幔外折叠处的表达。我们的研究为未来软体动物壳颜色形成的机制分析提供了对C. gigas壳色素沉着和候选基因的更好理解。