Gene identifications in three bivalve genomes further gain insights into evolutionary mechanism of Family I84 protease inhibitors

Abstract The family I84 serine protease inhibitors are believed to be mollusk specific immune effectors involved in host defense. However, there have few reports systematically describing its distribution and evolution origin. In this study, we performed genome-wide identification of family I84 protease inhibitor genes were analyzed in three bivalves Crassostrea gigas, C. virginica and Tegillarca granosa. A total of 66 Family I84 members were identified, including 22 in C. gigas, 28 in C. virginicaand 16 in T. granosa. The Family I84 genes were unevenly distributed on four, five and five chromosomes in C. gigas, C. virginica and T. granosa, respectively, and some genes were tandem duplicated. Most Family I84 genes contained 3 exons, while 1 gene (TgSi11) contained 2 exons and 12 genes contained 4 exons. The physiological and biochemical parameters of the Family I84 were similar, while TgSi11 and CgSi17 were failed to predict for signal peptides. Four conserved motifs were detected in 66 mature proteins. The phylogenetic results showed that the genes of the two oysters clustered with each other, members in some species are closely clustered, suggesting that they may be expanded by tandem duplication. Expression profile of both C. gigas and T. granosa in development stages, different tissues and co-expression diversified. Gene identifications in three bivalve genomes further indicate molecular and evolutionary diversities of Family I84 protease inhibitors, these results provide comprehensive understanding of the function of such unique protease inhibitor.