R. insecticola greatly reduces fitness costs associated with APV infection, but H. defensa exacerbates costs
Persistent infection by APV was previously reported to reduce pea aphid fitness (Van den Heuvel et al. 1997, Lu et al. 2020). However, these studies did not control for aphid genotype or the presence of facultative symbionts, which occur in most pea aphids and are known to confer protection to specialized natural enemies (Russell et al. 2013, Oliver et al. 2014). Here, we generalize prior findings by showing that persistent APV infections reduced aphid fecundity and survival across multiple pea aphid genotypes lacking facultative symbionts (Table 3). In aphids without facultative symbionts, we also found that APV titer exhibited similar trajectories over aphid lifespan, which is consistent with the infection costs we observed (Fig. 4).
When we examined our experimental lines with and without two common and closely related protective facultative symbionts H. defensa andR. insecticola, we found that the fitness of aphids with persistent APV infections varied dramatically depending on which symbiont was present. In aphids carrying R. insecticola , costs to persistent infection with APV were largely eliminated (Fig 2A-B, Table 3). Not only were fitness estimates similar between R. insecticola carrying aphids with and without APV, but aphids with both APV and R. insecticola produced statistically similar numbers of offspring compared to the control line (no APV or symbiont). APV abundance was also lower in aphids with R. insecticola versus those without this symbiont, although significantly so only in older aphids (Fig. 4A). Taken together, these results indicate that R. insecticola provides substantial protection against infection with APV.
To our knowledge, R. insecticola represents only the second heritable symbiont known to confer protection against viral pathogens. Some strains of the ubiquitous Wolbachia symbiont confer protection against specialized RNA viruses in natural hosts (Hedges et al. 2008, Teixeira et al. 2008, Pimentel et al. 2021). ThoughWolbachia’s pathogen blocking mechanisms remain poorly understood, and may vary between natural and novel associations, hypotheses include immune priming, resource competition, or modification of the host cell environment (Terradas and McGraw 2017, Lindsey et al. 2018). Associations between anti-viral Wolbachia strains introduced into important insect vectors are actively being researched and applied in real-world efforts to mitigate human disease such as dengue (Nazni et al. 2019, O’Neill et al. 2019). Hence, having a second heritable symbiont with anti-viral properties in a system with unparalleled in vivo experimental protocols and developingin vitro ones (Brandt et al. 2017, Patel et al. 2019) provides excellent opportunities to develop an additional model of anti-viral symbiosis. One caveat to our study is that we only examined a single strain of R. insecticola in one aphid background. However, this is by far the most common of only two strains recovered from recent surveys of N. American pea aphids on alfalfa (Peng et al. 2022). And given prior findings that R. insecticola improves pea aphid fitness in the presence of specialized fungal pathogens (Parker et al. 2013), and when coinfecting aphids alongside costly strains of the facultative symbionts H. defensa and Spiroplasma(Mathé-Hubert et al. 2019, Weldon et al. 2020), this appears to be a common phenotype associated with R. insecticola .
In contrast to R. insecticola , APV infection costs were significantly exacerbated in aphid lines carrying H. defensa(Table 3C & D) and H. defensa did not influence APV abundance (Fig. 2D-I & Fig. 4). While symbionts that protect hosts receive the most interest, those that enhance pathogen infection are nonetheless important for natural symbiont maintenance and disease dynamics (Graham et al. 2012, Amuzu et al. 2018).