Israeli acute paralysis virus associated paralysis symptoms, viral tissue distribution and Dicer-2 induction in bumblebee workers (Bombus terrestris)

Abstract

Although it is known that Israeli acute paralysis virus (IAPV) can cause bee mortality, the symptoms of paralysis and the distribution of the virus in different body tissues and their potential to respond with an increase of the siRNA antiviral immune system have not been studied. In this project we worked with Bombus terrestris, which is one of the most numerous bumblebee species in Europe and an important pollinator for wild flowers and many crops in agriculture. Besides the classic symptoms of paralysis and trembling prior to death, we report a new IAPVrelated symptom, crippled/immobilized forelegs. Reverse-transcriptase quantitative PCR showed that IAPV accumulates in different body tissues (midgut, fat body, brain and ovary). The highest levels of IAPV were observed in the fat body. With fluorescence in situ hybridization (FISH) we detected IAPV in the Kenyon cells of mushroom bodies and neuropils from both antennal and optic lobes of the brain in IAPV-infected workers. Finally, we observed an induction of Dicer-2, a core gene of the RNAi antiviral immune response, in the IAPV-infected tissues of B. terrestris workers. According to our results, tissue tropism and the induction strength of Dicer-2 could not be correlated with virus-related paralysis symptoms. INTRODUCTIONHoneybees and bumblebees are the two most important bee species used in modern apiculture and agriculture. They host many viruses. When looking at the honeybee, there have been around 23 viruses reported and many of these viruses only persist as covert and asymptomatic infections which show no detectable impact at both the individual and colony level (McMenamin & Genersch, 2015). However, under certain conditions, for example the presence of the ectoparasitic honeybee mite Varroa destructor, the covert infections of some viruses turn into overt ones with observable symptoms (Martin et al., 2012;McMahon et al., 2015;Singh et al., 2010). The symptoms of overt infections vary from virus to virus, including deformed wings, discoloration, hair losses, bloated abdomen, paralysis, decreased locomotion, impaired cognition and both brood and adult mortality (Lanzi et al., 2006;McMenamin & Genersch, 2015;Singh et al., 2010). Some symptoms can be easily recognized by the behaviour and appearance of the infected bees, while others can only be confirmed through meticulous and continuous observations or by molecular techniques.In bumblebees (Bombus spp.) a subset of honeybee-infecting viruses has been reported, such as deformed wing virus (DWV), Israeli acute paralysis virus (IAPV), acute bee paralysis virus (ABPV), Kashmir bee virus (KBV), slow bee paralysis virus (SBPV), sacbrood virus (SBV), black queen cell virus (BQCV) and Lake Sinai virus (LSV) (Genersch et al., 2006;Levitt et al., 2013;Parmentier et al., 2016;Peng et al., 2011). However, only DWV, KBV and IAPV have been reported to cause clinical symptoms (Genersch et al., 2006;Meeus et al., 2014). DWV caused crippled wings in bumblebee (Bombus terrestris and Bombus pascuorum) workers even without ...

Infections of virulent and avirulent viruses differentially influenced the expression of dicer-1, ago-1, and microRNAs in Bombus terrestris

“…However, the answer to whether these results are associated with incomplete silencing of these key components from the RNAi pathway demands further study. In addition, it has been noted that various bumblebee tissues show different susceptibilities to viral infection22, but yet the virus-bumblebee interactions at the level of tissues rather than whole body or body parts awaits further studies. Recent studies showed that RNAi and JAK-STAT showed a cross-talk activity during viral infections2324.…”

RNAi in Insects: A Revolution in Fundamental Research and Pest Control Applications

“…Could the exposure to RNAi-based products lead to effects in non-target organisms at the level of the immune system? It has been described that exposure to non-specific dsRNAs leads to a stimulation of the RNAi pathway and different immune pathways in bumblebees, for example [ 42 , 43 ]. However, whether this has any consequences in terms of fitness cost or whether it changes the capacity to fight off viral or other infections has not been investigated so far.…”

IAPV-Induced Paralytic Symptoms Associated with Tachypnea via Impaired Tracheal System Function

“…IAPV infection induces a broad spectrum of clinical signs, from asymptomatic infection to a series of apparent depressive symptoms, including paralysis and body trembling, indicating that the virus most likely infects the neuron system [ 14 ]. However, recent studies showed that nerve tissues did not harbor the highest viral loads [ 10 ].…”

“…However, recent studies showed that nerve tissues did not harbor the highest viral loads [ 10 ]. Transcriptomic analysis was applied on bees with IAPV infection and it was found that the preferred tropism was the gut, followed by nerves [ 10 ], while this was contradicted by a recent observation that the fat body harbored the highest level of IAPV, followed by the ovary, brain and midgut [ 14 ]. Thus, the relationship between paralysis symptoms and tropism has not been fully determined, hindering our understanding of pathogenesis of IAPV infection and transmission routes.…”