Variation in density, immune gene suppression, and coinfection outcomes among strains of the aphid endosymbiont Regiella insecticola

Abstract

Many insects harbor heritable microbes that influence host phenotypes. Symbiont strains establish at different densities within hosts. This variation is important evolutionarily because within-host density has been linked to the costs and benefits of the symbiosis for both partners. Studying the factors shaping within-host density is important to our broader understanding of host-microbe coevolution. Here we focused on different strains of Regiella insecticola, a facultative symbiont of aphids. We first showed that strains of Regiella establish in pea aphids at drastically different densities. We then found that variation in density is correlated with the expression levels of two key insect immune system genes (phenoloxidase and hemocytin), with the suppression of immune gene expression correlating with higher Regiella density. We then performed an experiment where we established co-infections of a higher- and a lower-density Regiella strain, and we showed that the higher-density strain is better able to persist in co-infections than the lower-density strain. Together, our results point to a potential mechanism that contributes to strain-level variation in symbiont density in this system, and our data suggest that symbiont fitness may be increased by establishing at higher density within hosts. Our work highlights the importance of within-host dynamics shaping symbiont evolution.

Aphid facultative symbionts confer no protection against the fungal entomopathogen Batkoa apiculata

“…In fact, we found that aphids harboring one strain of R. insecticola (.CF7) appear to be more susceptible to B. apiculata infection. This strain comes from a specific clade of Regiella (referred to as 'clade 2' in the literature [24]) that we have found establishes at extremely high densities within hosts [26] by suppressing aphid innate immune genes like Phenoloxidase [17,29]. One possibility is that reduced expression of Phenoloxidase, which is an important component of aphid immunity to fungal pathogens [30,31], makes aphids more susceptible to fungal pathogens like B. apiculata in the absence of symbiont mediated protection.…”

“…We collected fungal entomopathogens from field aphids in Knoxville, TN, in 2022 and used molecular techniques to determine the species of each fungal isolate. We then established a panel of aphids that each had the same host genotype with a single infection of a facultative symbiont including strains of Regiella and Spiroplasma that we have found to be protective against P. neoaphidis [15][16][17] and Serratia symbiotica, which confers no protection against P. neoaphidis. We found that aphid facultative symbionts do not protect against B. apiculata, and we discuss these results within the context of aphid-symbiont-pathogen coevolution.…”

Untangling an insect’s virome from its endogenous viral elements

“…We screened the line for the seven species of facultative symbionts found in aphids using established PCR protocols [ 41 , 42 ]. For this screen, we extracted DNA using ‘Bender buffer’ and ethanol precipitation as in previous studies [ 43 , 44 ]. We then used PCR with species-specific primers [ 42 , 45 ] to screen for Hamiltonella defensa, Fukatsuia symbiotica (previously referred to as X-type), Regiella insecticola , Rickettsia sp., Ricketsiella sp., Serratia symbiotica , and Spiroplasma sp.…”