First evaluation of the Vairimorpha (Nosema) ceranae genome’s DNA replication genes as targets for RNA interference-mediated suppression of the honey bee Apis mellifera nosemosis

In recent decades, infection of the European honey bee Apis mellifera with the highly virulent microsporidium Vairimorpha (Nosema) ceranae has become globally prevalent. It causes serious losses in beekeeping worldwide and requires new approaches to control bee nosemosis. Since this intracellular parasite has retained components of the RNA interference pathway, double-stranded RNA (dsRNA) treatment of insects may be effective in controlling V. ceranae infection. Inhibition of microsporidia growth in bees fed with 1.8 or even 0.04 μg dsRNA per ml of sugar syrup has been reported in the literature. Considering the crucial role of the genome’s DNA replication machinery for any cell, we synthetized in vitro dsRNA fragments of four V. ceranae genes encoding two subunits (delta and epsilon) of DNA polymerase, helicase and topoisomerase II and fed them to the infected bees with a relatively low dose of 1 µg per ml of sugar syrup. Surprisingly, PCR and qPCR analyses of V. ceranae growth in the midgut of dsRNA-treated insects at 7- and 12-days post-infection revealed neither inhibition of microsporidia growth nor downregulation of target genes. It is worth noting that we collected worker bees of different ages directly from a hive, to simulate the conditions during colony treatment in apiaries. At the same time, newly emerged insects reared in the laboratory were used in the successful experiments mentioned above. This suggests that bee housing conditions as well as gut content may affect the efficiency of RNA interference and requires further increase in dsRNA doses or their mixing with nanoparticle carriers.