High-level expression of the Geranylgeranyl diphosphate synthase gene in the frontal gland of soldiers in Reticulitermes speratus (Isoptera: Rhinotermitidae).

Defensive strategies of termite soldiers are roughly classified as either mechanical, using mandibles and/or the whole head, or chemical, using frontal gland secretion. Soldiers of the genus Nasutitermes (Termitidae, Nasutitermitinae), which is one of the most derived termite genera, use only chemical defenses, and diterpene defensive secretions were suggested to be synthesized through geranylgeranyl diphosphate (GGPP). On the other hand, soldiers of the genus Reticulitermes (Rhinotermitidae, Heterotermitinae) mainly use mechanical defenses, but also use supplementary chemical defenses involving frontal gland secretions, including diterpene alcohol. In this study, to confirm whether the GGPP is used for diterpene synthesis in a representative of an earlier-branching termite lineage, the GGPP synthase gene (RsGGPPS) was identified in the rhinotermitid Reticulitermes speratus (Kolbe). The relative expression level of RsGGPPS in soldiers was three-fold higher than in workers. Furthermore, RsGGPPS gene expression was detected in epithelial class 1 gland cells around the frontal-gland reservoir. Although GGPP is used for various essential cellular roles in animals, RsGGPPS is suggested to be used not only for these essential roles but also for diterpene synthesis in order to produce defensive secretions. Chemical structures of the diterpene identified from Reticulitermes and Nasutitermes are extremely different from each other, and the two genera are phylogenetically distant from each other. Thus, these two lineages may have independently acquired the abilities of diterpene synthesis from GGPP.