The first evidence of positive selection in peptidoglycan recognition protein (PGRP) genes of Crassostrea gigas.

The oyster Crassostrea gigas is thought to have developed effective immunity to potentially harmful pathogens while under continuous exposure to marine microorganisms; however, the evolutionary mechanisms by which such immunity developed has not been understood. To understand the evolution of immunity, we characterized the family of peptidoglycan recognition proteins in the oyster (CgPGRPs). These proteins are crucial pattern recognition receptors for peptidoglycans (PGNs) and thereby, for activating the innate immune response of host. Herein, we identify seven new CgPGRP genes. Phylogenetic analysis of the seven new and five previously reported CgPRGP genes reveals that the CgPRGP gene family can be clustered into two groups, CgPRGPS and CgPRGPL. Moreover, the CgPRGPS group can be further divided into five subgroups. A codon-substitution model and three likelihood ratio tests (LRTs) suggest that seven sites in the CgPGRP family of genes have been subjected to strong positive selection (ω = 3.035-4.143), Three dimensional modeling revealed that these sites are found primarily at the periphery of coils and α-helices rather than in β-strands, perhaps allowing PGRP to adapt to, and recognize, variability of PGN structure. In conclusion, our studies provide the first evidence of positive Darwinian selection in the CgPGRP family, contributing to a better understanding of the adaptive mechanism of host-pathogens interaction in marine mollusks.