Evolutionary insights on a novel mussel-specific foot protein-3 gene family

Silky byssus threads enable a number of marine and freshwater bivalve mollusks to attach themselves to hard substrates. Byssus production is an energy-costly process, which accompany the switch from planktonic to sessile life. Pointing the attention to a small foot protein (fp-3α) first identified in Perna viridis and abundantly secreted during the bissogenesis, we report the presence of a fp-3α gene family in species of the Mytilus complex, byssogenic bivalve mollusks mostly inhabiting marine waters. In the genome of Mytilus galloprovincialis we identified twelve fp-3α genes showing differences in exon-intron organization and suggesting that, as in the case of arthropod and mollusk defensins, exon shuffling could have played an important role in the evolution of this gene family. Also, the different tissue expression patterns of these mussel genes support their functional diversification. All predicted fp-3α proteins curiously possess a Csαβ three-dimensional motif based on 10 highly conserved cysteines and exhibit structural similarity to invertebrate defensins. The role of these small cysteine-rich proteins in supporting the byssus-mediated mussel adhesion or their action as host defence peptides remains to be established with further study.