Bacteriocyte dynamics during development of a holometabolous insect, the carpenter ant Camponotus floridanus

Effects of B vitamin deletion in chemically defined diets on brood development in Camponotus vicinus (Hymenoptera: Formicidae)

The potential contributions of B vitamins by a yeast associate to the nutrition of the carpenter ant Camponotus vicinus Mayr was examined as part of an effort to develop a chemically defined diet. This diet was used to test the effects of individual B vitamin and other nutrient deletions on larval development. The chemically defined diet contained amino acids, vitamins, minerals, and other growth factors in a liquid sucrose matrix. C. vicinus worker colonies with third- and fourth-instar larvae were fed a complete artificial diet or that diet with a component deleted for a 12-wk period. There was a significant effect of diet on larval growth and number of adult worker ants produced in the overall nutrient deletion test, but ant development was often better on incomplete diets with one B vitamin deleted compared with the complete holidic basal diet. Thiamine deletion resulted in significantly higher brood weights compared with the complete diet. Diets of sugar water plus all B vitamins, sugar water only, or a diet minus all B vitamins and cholesterol were associated with significantly lower brood weights. Significantly more adult worker ants were produced by worker colonies fed diets minus cholesterol, choline, thiamine, or riboflavin compared with the complete basal diet. The results suggest that the diet, while suitable for rearing, could benefit from further study to better define component levels. The potential relationship of C. vicinus with yeast associates is discussed in relation to further studies.

The molecular basis of bacterial-insect symbiosis

Insects provide experimentally tractable and cost-effective model systems to investigate the molecular basis of animal-bacterial interactions. Recent research is revealing the central role of the insect innate immune system, especially anti-microbial peptides and reactive oxygen species, in regulating the abundance and composition of the microbiota in various insects, including Drosophila and the mosquitoes Aedes and Anopheles. Interactions between the immune system and microbiota are, however, bidirectional with evidence that members of the resident microbiota can promote immune function, conferring resistance to pathogens and parasites by both activation of immune effectors and production of toxins. Antagonistic and mutualistic interactions among bacteria have also been implicated as determinants of the microbiota composition, including exclusion of pathogens, but the molecular mechanisms are largely unknown. Some bacteria are crucial for insect nutrition, through provisioning of specific nutrients (e.g., B vitamins, essential amino acids) and modulation of the insect nutritional sensing and signaling pathways (e.g., insulin signaling) that regulate nutrient allocation, especially to lipid and other energy reserves. A key challenge for future research is to identify the molecular interaction between specific bacterial effectors and animal receptors, as well as to determine how these interactions translate into microbiota-dependent signaling, metabolism, and immune function in the host.

Brood ball-mediated transmission of microbiome members in the dung beetle, Onthophagus taurus (Coleoptera: Scarabaeidae)

Insects feeding on plant sap, blood, and other nutritionally incomplete diets are typically associated with mutualistic bacteria that supplement missing nutrients. Herbivorous mammal dung contains more than 86% cellulose and lacks amino acids essential for insect development and reproduction. Yet one of the most ecologically necessary and evolutionarily successful groups of beetles, the dung beetles (Scarabaeinae) feeds primarily, or exclusively, on dung. These associations suggest that dung beetles may benefit from mutualistic bacteria that provide nutrients missing from dung. The nesting behaviors of the female parent and the feeding behaviors of the larvae suggest that a microbiome could be vertically transmitted from the parental female to her offspring through the brood ball. Using sterile rearing and a combination of molecular and culture-based techniques, we examine transmission of the microbiome in the bull-headed dung beetle, Onthophagus taurus. Beetles were reared on autoclaved dung and the microbiome was characterized across development. A ~1425 bp region of the 16S rRNA identified Pseudomonadaceae, Enterobacteriaceae, and Comamonadaceae as the most common bacterial families across all life stages and populations, including cultured isolates from the 3^rd instar digestive system. Finer level phylotyping analyses based on lepA and gyrB amplicons of cultured isolates placed the isolates closest to Enterobacter cloacae, Providencia stuartii, Pusillimonas sp., Pedobacter heparinus, and Lysinibacillus sphaericus. Scanning electron micrographs of brood balls constructed from sterile dung reveals secretions and microbes only in the chamber the female prepares for the egg. The use of autoclaved dung for rearing, the presence of microbes in the brood ball and offspring, and identical 16S rRNA sequences in both parent and offspring suggests that the O. taurus female parent transmits specific microbiome members to her offspring through the brood chamber. The transmission of the dung beetle microbiome highlights the maintenance and likely importance of this newly-characterized bacterial community.

Gene expression analysis of the endosymbiont-bearing midgut tissue during ontogeny of the carpenter ant Camponotus floridanus

Insects have frequently evolved mutualistic relationships with extracellular and/or intracellular bacterial endosymbionts. Infection with endosymbionts seems to affect several cellular functions of the host such as immune pathways, oxidative stress regulation and autophagy. Our current knowledge about specific host factors leading to endosymbiont tolerance and/or control is still scarce and is based on very few associations between insect hosts and bacteria only. Camponotus floridanus ants harbour the obligate intracellular bacterium Blochmannia floridanus within specialized midgut cells called bacteriocytes. The number of Blochmannia endosymbionts within the midgut tissue increases strongly during host development and reaches a maximum at the late pupal stage, where the entire midgut is transformed into a symbiotic organ. After eclosion of workers the number of Blochmannia strongly decreases again. We chose 15 candidate genes from C. floridanus likely to be involved in host-symbiont interactions based on their significant homology to previously investigated symbiosis-relevant genes from other insects. We determined the expression of these genes in the endosymbiont-bearing midgut tissue in comparison to the residual body tissue at different developmental stages of C. floridanus in order to reveal changes in gene expression correlating with changes in endosymbiont number per host. Strikingly, two pattern recognition receptors (amidase PGRP-LB and PGRP-SC2) were highly expressed in the midgut tissue at the pupal stage, potentially down-modulating the IMD pathway to enable endosymbiont tolerance. Moreover, we investigated the immune gene expression in response to bacterial challenge at the pupal stage. Results showed that the midgut tissue differs in expression pattern in contrast to the residual body. Our results support a key role for amidase PGRPs, especially PGRP-LB, in regulation of the immune response towards endosymbionts in C. floridanus and suggest an involvement of the lysosomal system in control of Blochmannia endosymbionts.

Some like it hot: evolution and ecology of novel endosymbionts in bat flies of cave-roosting bats (hippoboscoidea, nycterophiliinae)

We investigated previously unknown associations between bacterial endosymbionts and bat flies of the subfamily Nycterophiliinae (Diptera, Streblidae). Molecular analyses revealed a novel clade of Gammaproteobacteria in Nycterophilia bat flies. This clade was not closely related to Arsenophonus-like microbes found in its sister genus Phalconomus and other bat flies. High population infection rates in Nycterophilia across a wide geographic area, the presence of the symbionts in pupae, the general codivergence between hosts and symbionts, and high AT composition bias in symbiont genes together suggest that this host-symbiont association is obligate in nature and ancient in origin. Some Nycterophilia samples (14.8%) also contained Wolbachia supergroup F (Alphaproteobacteria), suggesting a facultative symbiosis. Likelihood-based ancestral character mapping revealed that, initially, obligate symbionts exhibited association with host-specific Nycterophilia bat flies that use a broad temperature range of cave environments for pupal development. As this mutualism evolved, the temperature range of bat flies narrowed to an exclusive use of hot caves, which was followed by a secondary broadening of the bat flies' host associations. These results suggest that the symbiosis has influenced the environmental tolerance of parasite life history stages. Furthermore, the contingent change to an expanded host range of Nycterophilia bat flies upon narrowing the ecological niche of their developmental stages suggests that altered environmental tolerance across life history stages may be a crucial factor in shaping parasite-host relationships.

The gut bacteria associated with Camponotus japonicus Mayr with culture-dependent and DGGE methods

The bacterial composition and distribution in the different gut regions of Camponotus japonicus were investigated using both culture-dependent method and culture-independent method of polymerase chain reaction and denaturing gradient gel electrophoresis (PCR-DGGE). Five different bacterial strains were isolated using culture-dependent method, and they all belong to the phylum Firmicutes, including three genera of bacteria Bacillus, Paenibacillus, and Enterococcus. Bacillus cereus and Enterococcus mundtii were found in the midgut; Paenibacillus sp. was isolated from the hindgut; and the other two Bacillus spp. were isolated from the crop. Twelve distinct DGGE bands were found using PCR-DGGE method, and their sequences blasting analysis shows that they are members of the Proteobacteria and the Firmicutes, respectively, including three genera (Pseudomonas, Candidatus Blochmannia, Fructobacillus) and one uncultured bacterium, in which Pseudomonas was the most dominant bacteria group in all the three gut regions. According to the DGGE profile, the three gut regions had very similar gut communities, and all the DGGE bands were presented in the midgut and hindgut, while just two bands representing Blochmannia were not present in the crop. The results of our study indicate that the gut of C. japonicus harbors several other bacteria besides the obligate endosymbionts Blochmannia, and more work should be carried on to verify if they are common in the guts of other Camponotus ants.

Endosymbiont Tolerance and Control within Insect Hosts

Bacterial endosymbioses are very common in insects and can range from obligate to facultative as well as from mutualistic to pathogenic associations. Several recent studies provide new insight into how endosymbionts manage to establish chronic infections of their hosts without being eliminated by the host immune system. Endosymbiont tolerance may be achieved either by specific bacterial adaptations or by host measurements shielding bacteria from innate defense mechanisms. Nevertheless, insect hosts also need to sustain control mechanisms to prevent endosymbionts from unregulated proliferation. Emerging evidence indicates that in some cases the mutual adaptations of the two organisms may have led to the integration of the endosymbionts as a part of the host immune system. In fact, endosymbionts may provide protective traits against pathogens and predators and may even be required for the proper development of the host immune system during host ontogeny. This review gives an overview of current knowledge of molecular mechanisms ensuring maintenance of chronic infections with mutualistic endosymbionts and the impact of endosymbionts on host immune competence.

LNA probes substantially improve the detection of bacterial endosymbionts in whole mount of insects by fluorescent in-situ hybridization

Background

Detection of unculturable bacteria and their localization in the host, by fluorescent in-situ hybridization (FISH), is a powerful technique in the study of host-bacteria interaction. FISH probes are designed to target the 16 s rRNA region of the bacteria to be detected. LNA probes have recently been used in FISH studies and proven to be more efficient. To date no report has employed LNA probes for FISH detection of bacterial endosymbiont in the whole mount tissues. Further, though speculated, bacteriocytes have not been reported from males of Bemisia tabaci.

Results

In this study, we compared the efficiency in detecting bacteria by fluorescent DNA oligonucleotides versus modified probes containing Locked Nucleic Acid (LNA) substitution in their structure. We used the insect Bemisia tabaci as the experimental material since it carried simultaneous infection by two bacteria: one a primary endosymbiont, Portiera (and present in more numbers) while the other a secondary endosymbiont Arsenophonus (and present in less numbers). Thus a variation in the abundance of bacteria was expected. While detecting both the bacteria, we found a significant increase in the signal whenever LNA probes were used. However, the difference was more pronounced in detecting the secondary endosymbiont, wherein DNA probes gave weak signals when compared to LNA probes. Also, signal to noise ratio for LNA probes was higher than DNA probes. We found that LNA considerably improved sensitivity of FISH, as compared to the commonly used DNA oligonucleotide probe.

Conclusion

By employing LNA probes we could detect endosymbiotic bacteria in males, which have never been reported previously. We were able to detect bacteriocytes containing Portiera and Arsenophonus in the males of B. tabaci. Thus, employing LNA probes at optimized conditions will help to significantly improve detection of bacteria at the lowest concentration and may give a comprehensible depiction about their specific distribution within samples.

Understanding the role of host hemocytes in a squid/vibrio symbiosis using transcriptomics and proteomics

The symbiosis between the squid, Euprymna scolopes, and the bacterium, Vibrio fischeri, serves as a model for understanding interactions between beneficial bacteria and animal hosts. The establishment and maintenance of the association is highly specific and depends on the selection of V. fischeri and exclusion of non-symbiotic bacteria from the environment. Current evidence suggests that the host's cellular innate immune system, in the form of macrophage-like hemocytes, helps to mediate host tolerance of V. fischeri. To begin to understand the role of hemocytes in this association, we analyzed these cells by high-throughput 454 transcriptomic and liquid chromatography/tandem mass spectrometry (LC-MS/MS) proteomic analyses. 454 high-throughput sequencing produced 650, 686 reads totaling 279.9 Mb while LC-MS/MS analyses of circulating hemocytes putatively identified 702 unique proteins. Several receptors involved with the recognition of microbial-associated molecular patterns were identified. Among these was a complete open reading frame to a putative peptidoglycan recognition protein (EsPGRP5) with conserved residues for amidase activity. Assembly of the hemocyte transcriptome showed EsPGRP5 had high coverage, suggesting it is among the 5% most abundant transcripts in circulating hemocytes. Other transcripts and proteins identified included members of the conserved NF-κB signaling pathway, putative members of the complement pathway, the carbohydrate binding protein galectin, and cephalotoxin. Quantitative Real-Time PCR of complement-like genes, cephalotoxin, EsPGRP5, and a nitric oxide synthase showed differential expression in circulating hemocytes from adult squid with colonized light organs compared to those isolated from hosts where the symbionts were removed. These data suggest that the presence of the symbiont influences gene expression of the cellular innate immune system of E. scolopes.

Tissue distribution and transmission routes for the tsetse fly endosymbionts

The tsetse fly Glossina is the vector of the protozoan Trypanosoma brucei spp., which causes Human and Animal African Trypanosomiasis in sub-Saharan African countries. To supplement their unbalanced vertebrate bloodmeal diet, flies permanently harbor the obligate bacterium Wigglesworthia glossinidia, which resides in bacteriocytes in the midgut bacteriome organ as well as in milk gland organ. Tsetse flies also harbor the secondary facultative endosymbionts (S-symbiont) Sodalis glossinidius that infects various tissues and Wolbachia that infects germ cells. Tsetse flies display viviparous reproductive biology where a single embryo hatches and completes its entire larval development in utero and receives nourishments in the form of milk secreted by mother's accessory glands (milk glands). To analyze the precise tissue distribution of the three endosymbiotic bacteria and to infer the way by which each symbiotic partner is transmitted from parent to progeny, we conducted a Fluorescence In situ Hybridization (FISH) study to survey bacterial spatial distribution across the fly tissues. We show that bacteriocytes are mono-infected with Wigglesworthia, while both Wigglesworthia and Sodalis are present in the milk gland lumen. Sodalis was further seen in the uterus, spermathecae, fat body, milk and intracellular in the milk gland cells. Contrary to Wigglesworthia and Sodalis, Wolbachia were the only bacteria infecting oocytes, trophocytes, and embryos at early embryonic stages. Furthermore, Wolbachia were not seen in the milk gland and in the fat body. This work further highlights the diversity of symbiont interactions in multipartner associations and supports two maternal routes of symbiont inheritance in the tsetse fly: Wolbachia through oocytes, and, Wigglesworthia and Sodalis by means of milk gland bacterial infection at early post-embryonic stages.

Bacteria associated with gut lumen of Camponotus japonicus Mayr

Camponotus ants harbor the obligate intracellular endosymbiont Blochmannia in their midgut bacteriocytes, but little is known about intestinal bacteria living in the gut lumen. In this paper we reported the results of a survey of the intestinal microflora of Camponotus japonicus Mayr based on small-subunit rRNA genes (16S rRNAs) polymerase chain reaction (PCR)-restriction fragment-length polymorphism analysis of worker guts. From 107 clones, 11 different restriction fragment-length polymorphism profiles were identified, and sequences blasting analysis found these represent four types of bacteria. Most (91.6%) of the clones were "Candidatus Blochmannia", the obligate endosymbionts of Camponotus ants, and 6.5% of the clones were "Candidatus Serratia symbiotica", a secondary endosymbiont of aphids; the remaining 2% clones were Fructobacillus fructosus and uncultured Burkholderiales bacterium, respectively. These results show that the diversity of gut bacteria in C. japonicus was low. "Candidatus Serratia symbiotica" was identified from Camponotus ants for the first time, an interesting result because Blochmannia's closest bacterial relative is also in the genus Serratia. This discovery supports the scenario that consumption of aphid honeydew or tissue provides an initial step in the evolution of an advanced symbiosis, and suggests that Camponotus ant could acquire other secondary endosymbionts from Hemiptera host through their diet. In addition, Burkholderiales bacterium also was identified from the gut of C. japonicus for the first time, and whether it is a nitrogen-recycling endosymbiont in Camponotus ants needs to be investigated further.

Immune response of the ant Camponotus floridanus against pathogens and its obligate mutualistic endosymbiont

Numerous insect species harbor mutualistic endosymbionts that play a role in nutrient cycling or confer other fitness benefits to their hosts. Insect hosts face the problem of having to maintain such mutualistic bacteria while staging an immune response towards pathogens upon infection. In addition, hosts may regulate the number of endosymbionts present in their tissues via the innate immune system. Camponotus floridanus ants harbor the obligate endosymbiont Blochmannia floridanus in specialized midgut cells and ovaries. We identified genes transcriptionally induced in response to septic injury by suppression subtractive hybridization (SSH). Among these were genes involved in pathogen recognition (e.g. GNBP), signal transduction (e.g. MAPK-kinase), antimicrobial activity (e.g. defensin and hymenoptaecin), or general stress response (e.g. heat shock protein). A quantitative analysis of immune-gene expression revealed different expression kinetics of individual factors and also characteristic expression profiles after injection of gram-negative and gram-positive bacteria. Likewise, B. floridanus injected into the hemocoel elicited a comparable immune response of its host C. floridanus. Thus, the host immune system may contribute to controlling the endosymbiont population.