Cellular Localization of Two Rickettsia Symbionts in the Digestive System and within the Ovaries of the Mirid Bug, Macrolophous pygmaeus

SIMPLE SUMMARY: Like most insects, those that feed on both prey and plant materials harbor symbiotic bacteria in their body. Yet the involvement of bacteria in the feeding habits of these omnivorous consumers has yet to be investigated. In the present study, we took the first step toward testing the hypothesis that bacterial symbionts are involved in the feeding habits of the omnivorous bug Macrolophus pygmaeus. We (I) characterized the microbiome (the assembly of bacteria and fungi) of M. pygmaeus, and (II) determined the identity and location of the most dominant bacteria species within the host body. We found that M. pygmaeus microbiome is dominated by two Rickettsia species, R. belli and R. limoniae. These bacteria are found in high numbers in the digestive system of the bug, each exhibiting a unique distribution pattern, and for the most part, do not share the same cells in the gut. These results strongly suggest that the host bug may gain some nutritional benefits by hosting the two dominant symbiotic bacteria in its gut. ABSTRACT: Bacterial symbionts in arthropods are common, vary in their effects, and can dramatically influence the outcome of biological control efforts. Macrolophus pygmaeus (Heteroptera: Miridae), a key component of biological control programs, is mainly predaceous but may also display phytophagy. M. pygmaeus hosts symbiotic Wolbachia, which induce cytoplasmic incompatibility, and two Rickettsia species, R. bellii and R. limoniae, which are found in all individuals tested. To test possible involvement of the two Rickettsia species in the feeding habits of M. pygmaeus, we first showed that the microbiome of the insect is dominated by these three symbionts, and later described the distribution pattern of the two Rickettsia species in its digestive system. Although both Rickettsia species were located in certain gut bacteriocyes, in caeca and in Malpighian tubules of both sexes, each species has a unique cellular occupancy pattern and specific distribution along digestive system compartments. Infrequently, both species were found in a cell. In females, both Rickettsia species were detected in the germarium, the apical end of the ovarioles within the ovaries, but not in oocytes. Although the cause for these Rickettsia distribution patterns is yet unknown, it is likely linked to host nutrition while feeding on prey or plants.