Highly diverse fibrinogen-related proteins in the Pacific oyster Crassostrea gigas

• More than 190 predicted FREPs were identified in Crassostrea gigas genome.
• There were high numbers of polymorphisms in the C. gigas FREP genes.
• There was lineage-specific duplication of the FREP genes in C. gigas.
• CgFREP mRNAs were highly expressed in gills, digestive glands, and hemocytes.

Fibrinogen-related proteins (FREPs) are a family of proteins with high sequence diversity, and they play crucial roles in invertebrate immune response. However, few studies have characterized this diversity at the whole-genome level. In the present study, approximately 190 predicted FREPs with more than 200 fibrinogen-like (FBG) domains were identified in the genome of the Pacific oyster (Crassostrea gigas), suggesting a historical expansion of this protein family. A sequence analysis showed high numbers of polymorphisms in C. gigas FREP (CgFREP) genes, which may contribute to the versatile immune function of FREPs. A phylogenetic analysis of molluscan FREP sequences indicated lineage-specific duplication of these genes in C. gigas. Additionally, several CgFREP mRNAs were highly expressed in the gills, digestive glands, and hemocytes. Taken together, these findings will help elucidate FREP immune function and facilitate studies of the functional validation of this gene family.