The western honey bee (Apis mellifera) is exceedingly important to human populations, because the bee is a critical pollinator of flowering plants, including so much of our food. Yet, despite close scrutiny, this fragile bee recently appears to be under serious global threat from a range of environmental stressors. Chief among these, are pests, pathogens and toxic chemicals — remedies to which are either ineffective, short-term, expensive, or impractical. Authors [see attached article & editorial] present a solution which these GEITP pages believe is a Major Breakthrough: authors have genetically modified honey bee gut bacteria, in order to induce a host RNA interference (RNAi)–based defense that is reported to be effective, long-term, potentially inexpensive, and easy to apply. This important approach may not only provide a solution to many of the honey bee’s problems; it also offers a new functional genomic toolkit with which to dissect the molecular intricacies of insects and their societies.
High honey bee colony mortality has recently been attributed largely to synergistic interactions between parasitic mites (Varroa destructor) and RNA viruses. Honey bees possess the molecular machinery for RNAi (this is an antiviral immune system in which double-stranded RNA (dsRNA) triggers degradation of other RNAs with similar sequences). The process of RNAi can be induced by feeding or injecting dsRNA, and this has been used to “knock-down” the expression of bee genes and to impair replication of RNA viruses. The dsRNA administered to bees — is transmitted to their eukaryotic parasites (recall that eukaryotes have chromosome pairs, not a single chromosome like bacteria, prokaryotes) and can induce a parasite RNAi response.
However, use of dsRNA for sustained manipulation of bee gene expression, or control of bee pests, has proven difficult. Even administration of dsRNA to individual bees yields only a sporadic and transient gene knockdown, and dsRNA can have off-target effects (leading to unintended consequences). There are even greater obstacles to using dsRNA to defend entire bee hives located in the field against pathogens, because dsRNA is expensive to produce and it degrades rapidly in the environment.
Authors circumvented these problems by successfully engineering Snodgrassella alvi wkB2 (a symbiotic bacterium normally living in the bee intestine; recall that ‘symbiotic’ denotes ‘living together in harmony’, i.e. a mutually beneficial relationship), to continuously produce dsRNA — which, in turn, manipulates host gene expression and protects bees against pathogens and parasites. Authors [see attached article] show that genetically-engineered S. alvi can stably recolonize in bees and produce dsRNA, which then activates RNAi, and represses host gene expression; the result is altered bee physiology, behavior, and growth. Authors used this approach to improve bee survival after a viral challenge. Authors demonstrated that engineered S. alvi can kill parasitic Varroa mites by triggering the mite RNAi response. This symbiont-mediated RNAi approach is a new tool for studying bee functional genomics and potentially for safeguarding bee health. 😊
Science 14 Feb 2020; 367: 573-576 & editorial pp 504-506