Multiple ferritin subunit genes of the Pacific oyster Crassostrea gigas and their distinct expression patterns during early development

• Both non-secretory and secretory ferritin subunit genes were observed in oyster.
• cgi-fer2 was the major house-keeping ferritin subunit gene in early development.
• cgi-fer2 was suggested to play important roles in ciliated tissues.
• Roles of cgi-fer1, 3 and 4 were emphasized in the digestive system of D-veligers.
• Roles of ferritin in shell formation might be different in bivalves and gastropods.

Multiple ferritin subunit genes are reported in mollusks, but they have not been systematically classified. Based on the recently published whole genome sequence, we screened out the four ferritin subunit genes (cgi-fer1–cgi-fer4) from the Pacific oyster Crassostrea gigas. The four genes were predicted to encode two non-secretory and two secretory peptides. Further phylogenetic analyses revealed two groups of non-secretory and secretory ferritin subunits in mollusks. This differs dramatically from the situation in mammals or insects, which contain only non-secretory or secretory ferritin subunits. These results emphasize the evolution of molluscan ferritin subunit genes. The expression patterns of the four genes during early development exhibited dramatic differences, indicating the functional diversity of these genes. Among them, cgi-fer2 was the only gene expressed prevalently and is thus suggested to be the major house-keeping ferritin subunit gene. The expression of the other three genes was tissue-specific beginning in the D-veliger stage. Based on their expression patterns, we inferred important functions of cgi-fer2 in ciliated tissues and of the other three genes in the digestive system. Moreover, our results indicated potentially different roles of ferritin subunit genes during larval shell formation in gastropods and bivalves, which may be helpful in understanding the molecular mechanisms that cause the different shells of gastropods and bivalves. In addition, we conducted a further semi-quantitative analysis of the four genes in four major developmental stages and five adult tissues. The results also revealed dramatically different expression patterns of the genes, which brought additional functional indications. This work may promote studies on molluscan ferritins and shed light on the evolution of ferritin subunit genes among different animal groups.