Interfaces between microbes and membranes of host epithelial cells in hemipteran midguts

A strong priority effect in the assembly of a specialized insect-microbe symbiosis

Specialized host-microbe symbioses are ecological communities, whose composition is shaped by various processes. Microbial community assembly in these symbioses is determined in part by interactions between taxa that colonize ecological niches available within habitat patches. The outcomes of these interactions, and by extension the trajectory of community assembly, can display priority effects-dependency on the order in which taxa first occupy these niches. The underlying mechanisms of these phenomena vary from system to system and are often not well resolved. Here, we characterize priority effects in colonization of the squash bug (Anasa tristis) by bacterial symbionts from the genus Caballeronia, using pairs of strains that are known to strongly compete during host colonization, as well as strains that are isogenic and thus functionally identical. By introducing symbiont strains into individual bugs in a sequential manner, we show that within-host populations established by the first colonist are extremely resistant to invasion, regardless of strain identity and competitive interactions. By knocking down the population of an initial colonist with antibiotics, we further show that colonization success by the second symbiont is still diminished even when space in the symbiotic organ is available and ostensibly accessible for colonization. We speculate that resident symbionts exclude subsequent infections by manipulating the host environment, partially but not exclusively by eliciting tissue remodeling of the symbiont organ.

IMPORTANCE

Host-associated microbial communities underpin critical ecosystem processes and human health, and their ability to do so is determined in turn by the various processes that shape their composition. While selection deterministically acts on competing genotypes and species during community assembly, the manner by which selection determines the trajectory of community assembly can differ depending on the sequence by which taxa are established within that community. We document this phenomenon, known as a priority effect, during experimental colonization of a North American insect pest, the squash bug Anasa tristis, by its betaproteobacterial symbionts in the genus Caballeronia. Our study demonstrates how stark, strain-level variation can emerge in specialized host-microbe symbioses simply through differences in the order by which strains colonize the host. Understanding the mechanistic drivers of community structure in host-associated microbiomes can highlight both pitfalls and opportunities for the engineering of these communities and their constituent taxa for societal benefit.

Rickettsia association with two Macrolophus (Heteroptera: Miridae) species: A comparative study of phylogenies and within-host localization patterns

Many arthropods host bacterial symbionts, some of which are known to influence host nutrition and diet breadth. Omnivorous bugs of the genus Macrolophus (Heteroptera: Miridae) are mainly predatory, but may also feed on plants. The species M. pygmaeus and M. melanotoma (=M. caliginosus) are key natural enemies of various economically important agricultural pests, and are known to harbor two Rickettsia species, R. bellii and R. limoniae. To test for possible involvement of symbiotic bacteria in the nutritional ecology of these biocontrol agents, the abundance, phylogeny, and distribution patterns of the two Rickettsia species in M. pygmaeus and M. melanotoma were studied. Both of the Rickettsia species were found in 100 and 84% of all tested individuals of M. pygmaeus and M. melanotoma, respectively. Phylogenetic analysis showed that a co-evolutionary process between Macrolophus species and their Rickettsia is infrequent. Localization of R. bellii and R. limoniae has been detected in both female and male of M. pygmaeus and M. melanotoma. FISH analysis of female gonads revealed the presence of both Rickettsia species in the germarium of both bug species. Each of the two Rickettsia species displayed a unique distribution pattern along the digestive system of the bugs, mostly occupying separate epithelial cells, unknown caeca-like organs, the Malpighian tubules and the salivary glands. This pattern differed between the two Macrolophus species: in M. pygmaeus, R. limoniae was distributed more broadly along the host digestive system and R. bellii was located primarily in the foregut and midgut. In contrast, in M. melanotoma, R. bellii was more broadly distributed along the digestive system than the clustered R. limoniae. Taken together, these results suggest that Rickettsia may have a role in the nutritional ecology of their plant-and prey-consuming hosts.

The molecular interplay of the establishment of an infection - gene expression of Diaphorina citri gut and Candidatus Liberibacter asiaticus

BACKGROUND

Candidatus Liberibacter asiaticus (CLas) is one the causative agents of greening disease in citrus, an unccurable, devastating disease of citrus worldwide. CLas is vectored by Diaphorina citri, and the understanding of the molecular interplay between vector and pathogen will provide additional basis for the development and implementation of successful management strategies. We focused in the molecular interplay occurring in the gut of the vector, a major barrier for CLas invasion and colonization.

RESULTS

We investigated the differential expression of vector and CLas genes by analyzing a de novo reference metatranscriptome of the gut of adult psyllids fed of CLas-infected and healthy citrus plants for 1-2, 3-4 and 5-6 days. CLas regulates the immune response of the vector affecting the production of reactive species of oxygen and nitrogen, and the production of antimicrobial peptides. Moreover, CLas overexpressed peroxiredoxin, probably in a protective manner. The major transcript involved in immune expression was related to melanization, a CLIP-domain serine protease we believe participates in the wounding of epithelial cells damaged during infection, which is supported by the down-regulation of pangolin. We also detected that CLas modulates the gut peristalsis of psyllids through the down-regulation of titin, reducing the elimination of CLas with faeces. The up-regulation of the neuromodulator arylalkylamine N-acetyltransferase implies CLas also interferes with the double brain-gut communication circuitry of the vector. CLas colonizes the gut by expressing two Type IVb pilin flp genes and several chaperones that can also function as adhesins. We hypothesized biofilm formation occurs by the expression of the cold shock protein of CLas.

CONCLUSIONS

The thorough detailed analysis of the transcritome of Ca. L. asiaticus and of D. citri at different time points of their interaction in the gut tissues of the host led to the identification of several host genes targeted for regulation by L. asiaticus, but also bacterial genes coding for potential effector proteins. The identified targets and effector proteins are potential targets for the development of new management strategies directed to interfere with the successful utilization of the psyllid vector by this pathogen.

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.

Morphology and composition of the midgut bacterial community of Scaptocoris castanea Perty, 1830 (Hemiptera: Cydnidae)

The burrower bug Scaptocoris castanea is an important soybean and pasture pest in Brazil, with an underground habit feeding directly on the sap of the roots. Underground habit hinders control and knowledge of the biology and physiology of this pest. This study describes the anatomy, histology, ultrastructure and symbionts of the midgut of S. castanea. The midgut of S. castanea is anatomically divided into five regions (ventricles). Ventricles 1-3 are similar between males and females, with cells specialized in digestion and absorption of nutrients, water transport and homeostasis. Ventricle 4 has squamous epithelium forming crypts and harboring bacteria in the lumen. Ventricle 5 of males is small with cells containing apical microvilli and broad basal folds with many openings for hemocoel, while in females, this region of the midgut is well developed and colonized by intracellular bacteria, characterizing bacteriocytes. The main bacteria are Gammaproteobacteria. The results show sexual dimorphism in ventricle 5 of the midgut of S. castanea, with formation of bacteriocytes in the females, while the other regions are involved in digestive processes in both sexes.