When different symbionts are simultaneously present within the same host, interactions between them will take place, which might affect the dynamics of the microbial population. The interaction of the common endosymbiont Wolbachia with other symbionts has received increasing attention. An asymmetrical interaction has been found between Wolbachia and Spiroplasma (Goto et al., 2006). Our aim in this study was to test whether interactions between Wolbachia and another popular symbiont, Arsenophonus, in the same host could affect the titre of the symbionts. We established 5 double-infected lines from different natural populations of N. lugens, and they were stable co-infections.
Previous studies have shown that the brown planthopper population from Laos was extensively infected by Wolbachia or Arsenophonus and the two bacteria may be exclusive in each host individual (Qu et al., 2013). We found that Arsenophonus and Wolbachia could coexist in the same individual of brown planthopper in all the tested populations from China and differences among populations might result from differences in population resources.
The double-infection incidence of Wolbachia and Arsenophonus in brown planthopper varied with the geographical populations in China. In the ZJ population, the double-infection incidence was the highest, with half of the individuals simultaneously harbouring Wolbachia and Arsenophonus, whereas in the NJ and MS populations, less than 3% were infected with the two symbionts. The variance in double infection has been found in small brown planthopper, with a significantly higher co-infection incidence of Wolbachia and Serratia observed in the buprofezin-resistant strain compared with that of the buprofezin-susceptible strain (Li et al., 2018).
Interactions between coexisting symbionts may affect infection densities because the symbionts may compete for available resources and space in the host body or they may share the resources and habitats by regulating their own exploitation to avoid damaging the whole symbiotic system (Koga et al., 2003; Hughes et al., 2004; Sakurai et al., 2005; Choisy et al., 2010). In pea aphids, the density of the primary symbiont Buchnera aphidicola is depressed when the insect is co-infected with Serratia symbiotica (Koga, R. et al. (2003) or Rickettsia (Sakurai, M. et al., 2005). An antagonistic interaction between Hamiltonella and Cardinium has also been found in Bemisia tabaci, and the density of Cardinium increased across time and led to a decrease of Hamiltonella density (Zhao et al., 2018). Asymmetrical interactions have been found between the reproductive parasites Spiroplasma and Wolbachia in Drosophila melanogaster in which the population of Wolbachia organisms was affected by Spiroplasma while the population of Spiroplasma was not affected by Wolbachia (Goto et al., 2006). Other than the interaction between different species of symbionts, interactions are also observed between different strains of the same symbiont. When multiple Wolbachia strains were observed in the same host, the density of each strain was specifically regulated (Ikeda, T. et al. 2003; Kondo, 2005), which limited the segregation of symbionts through inefficient transmission by maintaining a sufficiently high density of each symbiont (Engelstadter et al., 2007).
In our study, we found that in brown planthopper, co-infection with Wolbachia did not negatively affect eh Arsenophonus population and did not lead to lower net bacterial densities. In addition, the relative ratio of Wolbachia and Arsenophonus quantity in the double-infected lines of N. lugens varied with the geographic population. In the double-infected lines from the NN, ZJ, NT and MS populations, Wolbachia was the dominant symbiont, whereas in the double-infected line from the NJ population, Arsenophonus was the dominant symbiont and had a significantly higher density than that of Wolbachia. The difference in Arsenophonus density among lines might be related to the period of maintenance in the lab because the NJ population has been maintained for more than 14 years before investigation, which is at least 7 years longer than the other populations. This longer period of maintenance may possibly benefit the accumulation of Arsenophonus.
Wolbachia can provide protection against environmental stress, including RNA viruses and insecticides (Hedges et al., 2008; Walker et al., 2011, Li et al., 2018; Liu et al., 2019), and this genus also confers certain fitness benefits to their hosts (Weeks et al., 2007; Moriyama et al., 2015); however, Wolbachia can also have deleterious effects on the life history of their hosts (Hale and Hoffmann, 1990; Fleury et al. 2000). Arsenophonus was also found to provide protection against environmental stress, such as protection against the entomopathogenic fungi Metarhizium anisopliae (Zhu et al., 2017), although it also induced negative effects on their hosts, such as decreasing the chemical insecticide (imidachloprid) resistance of rice brown planthopper (Pang et al., 2018). Co-infection of Wolbachia and Arsenophonus is stable in brown planthopper, which raises the question of how these genera evolve and the effect that they have on the phenotype of their host.