Evolutionary diversification and Vibrio-responsive regulation of NF-κB signaling in bivalves: Evidence from Mimachlamys nobilis and comparative datasets.

The nuclear factor kappa B (NF-κB) pathway is a central regulator of innate immunity in animals, yet its evolutionary diversification in bivalve molluscs remains incompletely understood. Here, we analyzed NF-κB/Rel genes across 29 representative bivalve species from 11 families. Our analyses revealed substantial variation in observed annotated copy number and a conserved separation between Rel-type and NF-κB-type proteins, both of which possess the Rel homology domain (RHD), whereas ankyrin (ANK) repeats were predominantly associated with NF-κB-type members. Notably, McoRel_1 clustered within the NF-κB-type lineage but lacked detectable ANK-associated motifs in its predicted 355-aa protein model, while complementary ANK-containing sequence information was identified in an immediately adjacent 891-aa model, consistent with possible local annotation fragmentation. In Mimachlamys nobilis, RNA-seq of 27 independent libraries from nine tissues of six individuals and qRT-PCR after Vibrio parahaemolyticus challenge revealed tissue- and time-dependent transcriptional patterns of NF-κB/IKK-related genes. Together, these results provide an integrated evolutionary and transcriptional framework for understanding NF-κB/Rel diversification in bivalves and highlight lineage-associated structural variation and Vibrio-responsive genes as priorities for future functional investigation.