Diversification of Ig superfamily genes in an invertebrate.

The freshwater snail Biomphalaria glabrata possesses a diverse family of fibrinogen-related proteins (FREPs), hemolymph polypeptides that consist of one or two amino-terminal immunoglobulin superfamily (IgSF) domains and a carboxyl-terminal fibrinogen domain. Here, we show that the IgSF1 domain of the FREP3 subfamily is diversified at the genomic level at higher rates than those recorded for control genes. All sequence variants are derived from a small set of nine source sequences by point mutation and recombinatorial processes. Diverse FREP3 transcripts are also produced. We hypothesize a mechanism present in snails that is capable of diversifying molecules involved in internal defense.