The efficacy of RNA interference against the microsporidia Vairimorpha ceranae in honeybees Apis mellifera. Can it be enhanced to the level of antiviral and anti-mite activity shown for the Remebee-I and Vadescana dsRNA?

Abstract

Microsporidia Vairimorpha ceranae are virulent pathogens of the European honeybee Apis mellifera. About ten studies have confirmed that RNA interference (RNAi)-based silencing of the parasite’s genes inhibits microsporidia growth. To further evaluate the RNAi effectiveness against V. ceranae and methods for enhancing it, we synthesized in vitro double-stranded RNA (dsRNA) fragments of five genes encoding microsporidian DNA replication enzymes, their chimeric variants composed of predicted small interfering RNAs (siRNAs), and the fragments of four host genes. Fragments of the parasite’s genes encoding the PTP3 and SWP8 proteins, effective against V. ceranae at low concentrations, were used as positive controls. Ten-day feeding of individually infected and individually maintained young bees with dsRNA was followed by spore counting. This demonstrated (1) increased mortality of individually housed bees, (2) more effective infection suppression at lower dsRNA concentrations, and (3) the effectiveness of siRNA-containing chimeras and a gene fragment encoding a putative host apoptosis inhibitor in reducing spore load. Unfortunately, we were unable to enhance the RNAi efficiency by mixing the V. ceranae PTP3 fragment with polyethyleneimine and chitosan polymers, and V. ceranae growth was also unaffected when hive-derived bees of different ages were fed with E. coliexpressed dsRNA, effective in young insects. Analysis of the RNAi application to control bee pathogens indicates the need to further improve the efficiency of V. ceranae inhibition by searching for new target genes, optimizing the fragment concentration in syrup, and using the most effective dsRNA or siRNA, nanoparticles and liposomes.