Facultative symbionts did not influence APV transmission
We found that vertical transmission rates of APV were about 35% and not
impacted by H. defensa (Table 2A). This rate is similar with
prior reports for APV (Lu et al. 2020) and other aphid viruses
(Laubscher and Von Wechmar 1992, van Munster et al. 2003). It was also
previously reported that feeding by aphids resulted in the horizontal
transfer of APV to plants, with the virus persisting up to 7 days
without replication in plant tissues (Lu et al. 2020). Here, we
confirmed lateral transmission of APV through food plants by showing
that that APV-free aphids readily acquired the virus through phloem
feeding on plants previously fed on by APV+ aphids (Table 2B). We
further showed that APV acquisition rates were not affected by the
presence of H. defensa or R. insecticola . Thus, despite
reducing APV abundance and improving tolerance to infection as described
above, R. insecticola did not impede APV acquisition. We also
found that parasitoids can transfer APV via oviposition, a previously
undescribed route of transmission for pathogenic viruses. Rates of
wasp-mediated transfer of APV were low and only occurred when
oviposition occurred immediately following contact with a virus-infected
aphid. Parasitoids have been shown to move H. defensahorizontally among black bean aphids (Gehrer and Vorburger 2012)
suggesting microbial transfer via the contaminated ovipositors of wasps
may be an underappreciated route of microbe exchange among multicellular
eukaryotes.
Together these results show moderate vertical transmission and
horizontal transfer through food plants are the major routes of APV
spread. That APV can promote aphid feeding and colonization by
modulating plant defenses (Lu et al. 2020) suggests this virus may
employ tactics that facilitate its spread as seen for plant viruses
vectored by sap-feeding insects (Roossinck 2015).