APV titers are lower in R. insecticola harboring aphids but not in those with H. defensa:
Given that R. insecticola ameliorated APV infection costs andH. defensa increased them, we hypothesized that the former reduces virus infection load, while the latter does not. We also investigated whether APV influenced facultative symbionts titers. Focusing first on the facultative symbionts, we compared their relative abundance in aphids that were persistently infected with APV to aphids that were not from day 2 when nymphs were second instars to day 16 when they were mature adults. R. insecticola progressively increased in abundance in both APV-infected and non-infected aphids although relative abundances were significantly higher in the latter until day 16 when they were similar (Fig. 3A). H. defensa with APSE-2 also progressively increased in abundance with aphid age, while H. defensa with APSE-3 exhibited little change in abundance until day 16 (Fig. 3B, C). However, no differences in these trajectories were found between APV-infected and non-infected aphids. We also measured relative abundances of APSE-2 and APSE3, which only modestly increased with aphid age and also exhibited almost no differences between APV-infected and non-infected aphids (Fig. 3D, E). We thus concluded that APV infection overall had modest effects on the relative abundance of R. insecticola an no effect on H. defensa abundance.
We then compared APV abundance in persistently infected aphids that either hosted these facultative symbionts or were symbiont-free. For the LSR aphid genotype, APV titer progressively increased with aphid age but titers were significantly lower at days 8 and 16 aphids in aphids withR. insecticola (LSR1.Ri) compared to controls lacking the facultative symbiont (LSR1-AB)(Fig. 4A). For the ND18 and 5D-AB lines, APV titers more rapidly increased than was observed in the LSR1-AB line, but exhibited no differences between aphids that hosted H. defensa/ APSE-3 or H. defensa/ APSE-2 versus no symbionts (Fig. 4B, C). Aphid genotype had little impact on APV infection trajectories (Fig. 4).