Genome-wide Identification and Expression Profiling Indicate the Segon Gene Family Diversity and their Possible Role in Shell Formation in Oysters.

Bivalves dominate global marine aquaculture, with shell biomineralization mediated by hemocytes and specialized proteins. Segon, the second-most abundant plasma protein in oysters, contains high calcium content and is hypothesized to participate in shell formation via calcium transportation. However, its gene family distribution and functional dynamics remain unexplored. This study aimed to systematically identify segon gene family, analyze their structural and evolutionary features, and elucidate expression patterns for developmental stages and shell damage response. Twelve segon genes were identified exclusively in family Ostreidae, with no gene found in other studied mollusks. All Segon proteins were predicted to be acidic (pI 4.98 ~ 6.07), hydrophilic, and localized to the extracellular space with a high Asp/Glu content indicating predicted calcium-binding capacity. Analysis of aligned segon sequences revealed that in Crassostrea virginica, segon and dominin did not form a fusion gene, whereas in all other species, segon sequences encompassed the dominin sequence. In silico predicted tertiary structures revealed conserved β-sheets and α-helices. segon transcript abundance was nearly undetectable in early larvae but sharply increased in spat-stage across oyster species, coinciding with the transition from aragonite to calcite shell polymorphs. The segon gene family is unique to Ostreidae and likely evolved through gene fusion events. Its spatiotemporal expression and structural features suggest that Segon possibly plays a vital role in calcium transport, offering novel targets for understanding oyster biomineralization and aquaculture enhancement.