Cg-TGF-β, a TGF-β/activin homologue in the Pacific Oyster Crassostrea gigas, is involved in immunity against Gram-negative microbial infection

Transforming growth factor-β (TGF-β) members represent a widespread protein superfamily in the animal kingdom, but few members have been characterised in lophotrochozoans, a major clade of invertebrates. Here, we report the identification of Crassostrea gigas-TGF-β (Cg-TGF-β), a homologue of vertebrate TGF-β and activin, from the bivalve mollusc C. gigas. Phylogenetic analysis suggests an early ancestral origin of this subgroup of TGF-β superfamily member. Investigation of the spatio–temporal expression of Cg-TGF-β gene by real-time quantitative RT-PCR showed a ubiquitous pattern in all adult tissues. These findings imply that Cg-TGF-β has multiple functions as described for its vertebrate counterparts. Moreover, Cg-TGF-β was upregulated in haemocytes during infection by a Gram-negative bacterium, suggesting that it could act as a cytokine involved in immunity in molluscs.