More Is Not Always Better: Coinfections with Defensive Symbionts Generate Highly Variable Outcomes

Abstract

Animal-associated microbes are highly variable, contributing to a diverse set of symbiont-mediated phenotypes. Given that host and symbiont genotypes, and their interactions, can impact symbiont-based phenotypes across environments, there is potential for extensive variation in fitness outcomes. Pea aphids, Acyrthosiphon pisum, host a diverse assemblage of heritable facultative symbionts (HFS) with characterized roles in host defense. Protective phenotypes have been largely studied as single infections, but pea aphids often carry multiple HFS species, and particular combinations may be enriched or depleted compared to expectations based on chance. Here, we examined the consequences of single infection versus coinfection with two common HFS exhibiting variable enrichment, the antiparasitoid Hamiltonella defensa and the antipathogen Regiella insecticola, across three host genotypes and environments. As expected, single infections with either H. defensa or R. insecticola raised defenses against their respective targets. Single infections with protective H. defensa lowered aphid fitness in the absence of enemy challenge, while R. insecticola was comparatively benign. However, as a coinfection, R. insecticola ameliorated H. defensa infection costs. Coinfected aphids continued to receive antiparasitoid protection from H. defensa, but protection was weakened by R. insecticola in two clones. Notably, H. defensa eliminated survival benefits conferred after pathogen exposure by coinfecting R. insecticola. Since pathogen sporulation was suppressed by R. insecticola in coinfected aphids, the poor performance likely stemmed from H. defensa-imposed costs rather than weakened defenses. Our results reveal a complex set of coinfection outcomes which may partially explain natural infection patterns and suggest that symbiont-based phenotypes may not be easily predicted based solely on infection status. IMPORTANCE The hyperdiverse arthropods often harbor maternally transmitted bacteria that protect against natural enemies. In many species, low-diversity communities of heritable symbionts are common, providing opportunities for cooperation and conflict among symbionts, which can impact the defensive services rendered. Using the pea aphid, a model for defensive symbiosis, we show that coinfections with two common defensive symbionts, the antipathogen Regiella and the antiparasite Hamiltonella, produce outcomes that are highly variable compared to single infections, which consistently protect against designated enemies. Compared to single infections, coinfections often reduced defensive services during enemy challenge yet improved aphid fitness in the absence of enemies. Thus, infection with multiple symbionts does not necessarily create generalist aphids with “Swiss army knife” defenses against numerous enemies. Instead, particular combinations of symbionts may be favored for a variety of reasons, including their abilities to lessen the costs of other defensive symbionts when enemies are not present.

Does getting defensive get you anywhere?—Seasonal balancing selection, temperature, and parasitoids shape real‐world, protective endosymbiont dynamics in the pea aphid

“…ervi correlations in 2012 would be, in hindsight, unsurprising. In light of our positive correlation in 2011, this raises important questions about temporal variability in the efficacy of Hamiltonella ‐encoded defence, and how it is shaped by host and parasitoid genotype, symbiont strain variation, and coinfection with other symbionts (Dion et al, 2011; Weldon et al, 2020). It also suggests the need for a more comprehensive understanding of post‐parasitism fitness effects, with fecundity estimates being necessary to complement more prevalent survival measures in fully understanding Hamiltonella' s defensive benefits (Martinez et al, 2018).…”

Impact of Facultative Bacteria on the Metabolic Function of an Obligate Insect-Bacterial Symbiosis

“…On the one hand, the substantial among-genotype differences in the aphid metabolome identified in this study suggests that further insights can be gleaned from analysis of intraspecific variation in these interactions. This avenue would be a productive extension of recent research on variation in aphid- Buchnera interactions ( 50 , 52 ) and idiosyncratic effects of facultative symbionts on host phenotype ( 59 – 61 ). On the other hand, the broad principle of multiway interactions may be general to multiple taxa localized to the hemolymph and cells of insects ( 68 ), including other facultative symbionts of aphids and bacteria with a broad distribution in arthropods, e.g., Wolbachia , Spiroplasma .…”

“…Possible contributory factors were that we used naturally occurring aphid- Hamiltonella combinations and a susceptible plant cultivar for insect culture. In published studies, the effect of Hamiltonella on aphid performance varies with aphid and bacterial genotype ( 59 – 61 ) and can be particularly deleterious for aphids reared on partially resistant plants ( 36 , 62 ).…”

Paraburkholderia Symbionts Display Variable Infection Patterns That Are Not Predictive of Amoeba Host Outcomes

“…The ability to establish a high infection load without compromising host fitness in this system is puzzling as even in clearly mutualistic symbioses high symbiont loads are associated with contextual costs for their hosts. For instance, increased densities of defensive symbionts in aphid hosts correlates to strong declines in host fitness [33][34][35]. In plant-mycorrhizae symbiosis, symbiont abundance plays a dynamic role in host fitness for different traits [36][37][38].…”