The evolutionary history of the Methyltransf_FA domain and its role in JH signal in silkworm, Bombyx mori

The Methyltransf_farnesoic acid (MtFA) domain is a characteristic domain of farnesoic acid O-methyltransferase (FAMeT). FAMeT serves as a key enzyme in crustaceans, catalyzing the rate-limiting step in the biosynthesis of methyl farnesoate (MF). Although homologs of FAMeT have been widely identified in insects, their role in the juvenile hormone (JH) signaling pathway remains unclear. Moreover, the origin and evolutionary history of the MtFA domain remain poorly understood. Interestingly, insect FAMeT contains the MtFA domain and the DUF3421 domain of unknown function. In this study, we found that the MtFA domain was widely distributed across diverse taxa, including protists, vertebrates, and angiosperms. MtFA domain–containing genes likely originated from the common ancestor of prokaryotes and eukaryotes but were lost in archaea. The clustering of MtFA sequences was generally consistent with the classification of MtFA fusion proteins, indicating that domain variation might have been related to protein type. Overexpression of BmFAMeT-2 (which encodes a protein containing 2 MtFA domains and 1 DUF3421 domain) or BmFAMeT-1 (which encodes a protein containing 1 MtFA domain and 1 DUF3421 domain) led to the upregulation of BmKr-h1 expression. In contrast, overexpression of BGIBMGA006319 (encoding a protein containing only 1 DUF3421 domain) led to the downregulation of BmKr-h1 expression. Knockdown of BGIBMGA006319 led to an increase in JH titers, whereas knockdown of BmFAMeT-1 or BmFAMeT-2 resulted in a decrease, with the effect being more pronounced for BmFAMeT-2 than for BmFAMeT-1. These findings suggest that the MtFA domain plays a promoting role in JH signaling, while the DUF3421 domain plays an inhibitory role in JH signaling in silkworms.