A transmissible RNA pathway in honey bees
Preprint posted on 9 August 2018 https://www.biorxiv.org/content/early/2018/08/09/299800
Article now published in Cell Reports at http://dx.doi.org/10.1016/j.celrep.2019.04.073
The proof is in the (jelly) pudding: bees horizontally transmit ingested biologically active RNA to peers and future generations by secreting it into worker and royal jellies in a remarkable process of social immunity and signalling.Nicola Stevenson
RNA interference, the process by which small double stranded RNA (dsRNA) molecules inhibit homologous protein coding mRNAs, was first recognised as a key mechanism of gene silencing in C. elegans in 19981. Such was its importance that the authors Fire and Mello were awarded the Nobel Prize in physiology or medicine in 2006. Since then, besides being harnessed as an important experimental tool, RNAi has largely been studied in the context of gene expression within an organism, such as during intercellular communication, or in host-virus interactions. But is this the whole story? Recently studies have begun to emerge suggesting biologically active dsRNA can be transferred between different individuals of the same or different species. For example, vertical transfer of microRNA can occur between a mother and her infant via breast milk2 and consumption of plants expressing transgenic dsRNA can have an impact on invertebrate herbivore survival3,4.
In a previous study by these authors5, it was demonstrated that feeding honey bees with dsRNA targeting Israeli Acute Paralysis Virus conferred resistance to disease on the inoculated hive and improved honey flow. Most interestingly, this resistance persisted for at least 3-4 months – longer than the expected lifespan of the inoculated adults – suggesting resistance was passed on to the next generation. In the current preprint, the authors pursue this hypothesis and demonstrate that ingested dsRNA survives the honey bee gut and is taken up into the hemolymph. It is then secreted into the royal and worker jellies used to feed the upcoming generation of larvae, thus spreading pathogen resistance and other signals through the hive.
- dsRNA eaten by a honey bee is taken up by the gut and reaches the hemolymph (circulation) where it is protected from degradation by an unidentified protein complex.
- In mini hives fed with GFP-RNA the GFP-RNA can later be found in royal and worker jelly.
- If a queen bee from a control hive is transferred to a hive that has been fed dsRNA, the dsRNA is detectable in the larvae she produces after transfer, suggesting horizontal transfer from the inoculated workers to the larvae.
- If a queen bee fed with dsRNA is transferred to a control colony, the larvae produced after transfer do not carry the dsRNA, suggesting it cannot be transmitted vertically from the queen to her offspring.
- Bees fed with dsRNA targeting vitellogenin and transferred to a control hive induce vitellogenin knockdown in the new emerging workers suggesting the horizontally transferred RNA is biologically active.
- RNAseq analysis of royal and worker jellies revealed extensive RNA profiles which were different between the two, possibly affecting queen vs worker development. Only small amounts of this RNA was derived from bees – the rest was of plant, bacterial, viral and fungal origin.
- The viral RNA sequences detected were from both sense- and antisense-strands. As the antisense strand is the viral replicative form made inside cells this RNA must have come from infected bees not just from environmental capsids.
Significance and future questions
The highly integrated social utopia of a beehive never ceases to amaze and the discovery of a mechanism that could confer social immunity on a population across generations is quite remarkable. I chose this paper because of the novelty of such a discovery and the implied potential to harness this system to help rescue bee populations currently in decline and subject to disease. As the authors point out, bees have relatively few genes associated with immunity and it was presumed behavioural defences were the most important factors. The current study would suggest that in fact molecular defences are employed by bees, but in less conventional ways which require further study.
The key remaining questions for me include the nature of the protein complexes protecting the dsRNA inside the bee after ingestion, as well as the protection mechanism inside the jelly. It is also unclear to me from this study how many generations the dsRNA can penetrate. Do the bees replicate and amplify the dsRNA in any way to ensure sufficient quantities appear in the jelly to have an affect? If so can the larvae eating the jelly containing the dsRNA then secrete it into the jelly once they are adult nurse bees ready for the next generation?
- Fire, A., SiQun X., Montgomery, M.K., Kostas, S.A., Driver, S.E. and Mello, C. Potent and specific genetic interference by double-stranded RNA in Caenorhabditis elegans. 1998. Nature 391:806-811
- Kosaka, N., Izumi, H., Sekine, K. and Ochiya, T. microRNA as a new immune-regulatory agent in breast milk. 2010. Silence 1:7
- Mao, Y.-B., Cai, W.J., Wang, J.W., Hong, G.-J., Tao, X.-Y., Wang, L.-J., Huang, Y.-P., and Chen, X.-Y. Silencing a cotton bollworm P450 monooxygenase gene by plant-mediated RNAi impairs larval tolerance of gossypol. 2007. Nat. Biotechnol 25:1307–1313
- Zhang, J., Khan, S.A., Hasse, C., Ruf, S., Heckel, D.G., and Bock, R. Pest control. Full crop protection from an insect pest by expression of long doublestranded RNAs in plastids. 2015. Science 347: 991–994.
- Maori, E., Paldi, N., Shafir, S., Kalev, H., Tsur, E., Glick, E., and Sela, I. IAPV, a bee-affecting virus associated with Colony Collapse Disorder can be silenced by dsRNA ingestion. 2009. Insect Mol. Biol. 18: 55–60.
Posted on: 17 August 2018Read preprint