The expression of immune-related genes during the ontogenesis of scallop Chlamys farreri and their response to bacterial challenge

In the present study, the expression of some immune-related genes was examined as indicator to understand the development of immune defense system during the ontogenesis of scallop Chlamys farreri. The mRNA transcripts of pattern-recognition receptors (PRRs) including CfPGRP-S1, CfLGBP, CfLec-1 and CfLec-3 were observed at a low level or even undetected at early developmental stages from eggs to blastula, and then began to increase overwhelmingly in trochophore. For the genes of immune effector including CfLYZ, CfLBP/BPI, CfSOD and CfCAT, their mRNA transcripts were higher expressed in embryos, and increased significantly in D-hinged or early veliger larvae. The whole-mount immunofluorescence assay revealed two immunoreactive spots of CfPGRP-S1 were first observed in the mid-ventral region of prototroch in trochophore, and the immunopositive fluorescence of CfLGBP, CfLec-1 and CfLec-3 appeared at the same spots in early D-hinged larvae. Most of the PRRs were located in velum, mouth, esophagus and stomach region in early and mid-veliger larvae, and especially the strong immunopositive fluorescence of CfLec-3 was observed in velum. The immunoreactive areas of CfLYZ, CfLBP/BPI, CfSOD and CfCAT were observed in trochophore and early D-hinged larvae. After D-hinged larvae, they distributed in different tissues from the edge of velum, mouth, esophagus to the region around digestive gland. After bacterial challenge, the mRNA expression of CfLGBP, CfLec-1 and CfLec-3 did not change significantly in trochophore, while a down-regulation of CfPGRP-S1 was observed at 6 h (P < 0.05). The expression of CfPGRP-S1 and CfLGBP decreased or increased inversely in D-hinged and late veliger larvae respectively, whereas CfLec-1 and CfLec-3 increased significantly during 6–24 h after bacterial challenge in the two stages (P < 0.05). In contrast, the expressions of immune effectors in trochophore and late veliger larvae were significant up-regulated at 6 h, 12 h or 24 h after bacterial challenge (P < 0.05). However, in late D-hinged larvae, CfLYZ and CfSOD expressions were significantly down-regulated at 6 h, while CfLBP/BPI expression was up-regulated at 6 h and 24 h post challenge (P < 0.05). These results indicated that the immune defense system of scallop might appear firstly in the mid-ventral region of prototroch in trochophore, and developed maturely after late D-hinged larvae. The developing immune system in the D-hinged and late veliger larvae could respond to the immune stimulation in different manner.