Summary
1Many insects derive nutritional advantage from persistent associations with microorganisms that variously synthesize essential nutrients or digest and detoxify ingested food. These persistent relationships are symbioses.
2There is strong experimental evidence that symbiotic microorganisms provide plant sap-feeding insects with essential amino acids and contribute to the digestion of cellulose in some wood-feeding insects, including lower termites. Basic nutritional information is, however, lacking for many associations, including the relative roles of microbial and intrinsic sources of cellulose degradation in many insects and B-vitamin provisioning by microorganisms in blood-feeding insects.
3Some nutritional interactions between insects and their symbiotic microorganisms vary among conspecifics and closely related species. This variation can, in principle, contribute to nutritional explanations for variation in the abundance and distribution of insects. For example, the plant utilization traits of phloem-feeding aphids and stinkbugs have been demonstrated to depend on the identity of microbial partners. Evidence that associations can evolve rapidly comes from the demonstration that the impact of the bacterium Wolbachia on natural populations of its insect host can change from deleterious to beneficial within two decades.
4Developing genomic tools, especially massively parallel sequencing and metagenomic analyses, offer the opportunity to explore the metabolic capabilities of symbiotic microorganisms and their insect hosts, from which defined hypotheses of nutritional function can be constructed. Nutritional ecology provides the appropriate framework to test these hypotheses in the relevant ecological context.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2435.2008.01442.x

The microbial dimension in insect nutritional ecology

Many insects that rely on a single food source throughout their developmental cycle harbor beneficial microbes that provide nutrients absent from their restricted diet. Tsetse flies, the vectors of African trypanosomes, feed exclusively on blood and rely on one such intracellular microbe for nutritional provisioning and fecundity. As a result of co-evolution with hosts over millions of years, these mutualists have lost the ability to survive outside the sheltered environment of their host insect cells. We present the complete annotated genome of Wigglesworthia glossinidia brevipalpis, which is composed of one chromosome of 697,724 base pairs (bp) and one small plasmid, called pWig1, of 5,200 bp. Genes involved in the biosynthesis of vitamin metabolites, apparently essential for host nutrition and fecundity, have been retained. Unexpectedly, this obligate's genome bears hallmarks of both parasitic and free-living microbes, and the gene encoding the important regulatory protein DnaA is absent.

Genome sequence of the endocellular obligate symbiont of tsetse flies, Wigglesworthia glossinidia.

1. Non-pathogenic microorganisms, known as mycetocyte symbionts, are located in specialized 'mycetocyte' cells of many insects that feed on nutritionally unbalanced or poor diets. The insects include cockroaches, Cimicidae and Lygaeidae (Heteroptera), the Homoptera, Anoplura, the Diptera Pupiparia, some formicine ants and many beetles. 2. Most mycetocyte symbionts are prokaryotes and a great diversity of forms has been described. None has been cultured in vitro and their taxonomic position is obscure. Yeasts have been reported in Cerambycidae and Anobiidae (Coleoptera) and a few planthoppers. They are culturable and those in anobiids have been assigned to the genus Torulopsis. 3. The mycetocyte cells may be associated with the gut, lie free in the abdominal haemocoel or be embedded in the fat body of the insect. The mycetocytes are large polyploid cells which rarely divide and the symbionts are restricted to their cytoplasm. 4. The mycetocyte symbionts are transmitted maternally from one insect generation to the next. In many beetles (Anobiidae, Cerambycidae, Chrysomelidae and cleonine Curculionidae), the microoganisms are smeared onto the eggs and consumed by the hatching larvae. In other insects, they are transferred from mycetocytes to oocytes in the ovary, a process known as transovarial transmission. The details of transmission in the different insect groups vary with the age of the mother (adult, larva or embryo) at which symbiont transfer to the ovary is initiated; whether isolated symbionts or intact mycetocytes are transferred; and the site of entry of symbionts to the egg (anterior, posterior or apolar). 5. Within an individual insect, the biomass of symbionts varies in a regular fashion with age, weight and sex of the insect. Suppression of symbiont growth rate and lysis of 'excess' microorganisms may contribute to the regulation of symbionts (including freshly-isolated preparations of unculturable forms) are used to investigate interactions between the partners. However, some methods to obtain aposymbiotic insects (e.g. antibiotics and lysozyme) deleteriously affect certain insects and aposymbionts may differ from the symbiont-containing stocks from which they were derived. 7. The mycetocyte symbionts have been proposed to synthesize various nutrients required by the insect. The symbionts of beetles and haematophagous insects may provide B vitamins and those in cockroaches and the Homoptera essential amino acids. The role of symbionts in the sterol nutrition of insects is equivocal. 8. Mycetocyte symbionts may have evolved from gut symbionts or guest microorganisms. The association is monophyletic in cockroaches but polyphyletic in many groups, including the sucking lice, beetles and scale insects.(ABSTRACT TRUNCATED AT 400 WORDS)

Mycetocyte symbiosis in insects.

E ndosymbiosis has been proposed as an evolutionary innovation that underlies the success and diversification of some groups of organisms by enabling them to exploit new ecological niches (e.g., Margulis and Fester 1991, Maynard Smith and Szathmary 1995). Three kinds of information are needed to evaluate this proposal. First, it is necessary to know something of the role played by symbionts in the lives of hosts: Are their effects ones that allow use of novel ecological niches? Second, information is needed on the evolutionary history of endosymbioses: How old are they and, in particular, do endosymbiotic infections predate diversification of major host groups? Finally, the evolutionary consequences for hosts of acquiring new capabilities in the form of symbiotic associations must be evaluated: Are these novel traits stable? Do they impose constraints as well as new capabilities on hosts? Recent studies have begun to shed light on these questions for bacteriocyte-associated endosymbioses in insects. Bacteriocytes (or mycetocytes) are cells of animal hosts that are positioned at characteristic locations in the body and appear to be specialized for housing bacteria (Figure 1; Buchner 1965, Dasch et al.

Bacteriocyte-Associated Endosymbionts of Insects

Background: The insect gut harbors a variety of microorganisms that probably exceed the number of cells in insects themselves These microorganisms can live and multiply in the insect, contributing to digestion, nutrition, and development of their host Recent studies have shown that midgut bacteria appear to strengthen the mosquito’s immune system and indirectly enhance protection from invading pathogens Nevertheless, the physiological significance of these bacteria for mosquitoes has not been established to date In this study, oral administration of antibiotics was employed in order to examine the contribution of gut bacteria to blood digestion and fecundity in Aedes aegypti Results: The antibiotics carbenicillin, tetracycline, spectinomycin, gentamycin and kanamycin, were individually offered to female mosquitoes Treatment of female mosquitoes with antibiotics affected the lysis of red blood cells (RBCs), retarded the digestion of blood proteins and reduced egg production In addition, antibiotics did not affect the survival of mosquitoes Mosquito fertility was restored in the second gonotrophic cycle after suspension of the antibiotic treatment, showing that the negative effects of antibiotics in blood digestion and egg production in the first gonotrophic cycle were reversible Conclusions: The reduction of bacteria affected RBC lysis, subsequently retarded protein digestion, deprived mosquito from essential nutrients and, finally, oocyte maturation was affected, resulting in the production of fewer viable eggs These results indicate that Ae aegypti and its midgut bacteria work in synergism to digest a blood meal Our findings open new possibilities to investigate Ae aegypti-associated bacteria as targets for mosquito control strategies Background Insects host many microorganisms that colonize and grow inside their tissues, mainly in the digestive system These microbes are involved in various physiological functions, including food digestion, nutrition, nitrogen fixation and reproduction Particularly, the role of midgut-associated bacteria in digestion of food has been demonstrated in several insect species [1] These indigenous bacteria are essential sources of carbohydrases improving digestion efficiency of plant- derived polymers such as lignin, hemicellulose and cellulose, xylan

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Contribution of midgut bacteria to blood digestion and egg production in aedes aegypti (diptera: culicidae) (L.).

Tsetse flies fed on blood containing oxytetracycline, sulphaquinoxaline or lysozyme do not reproduce. It could be proved that primarily the symbionts in flies are damaged, which secondarily leads to sterility.

Sterility in tsetse flies (Glossina morsitans Westwood) caused by loss of symbionts

By electrophoresis it became obvious that larvae ofHypoderma bovis are able to take up complete proteins from their host's serum directly into the haemolymphe, fatbody and ovarium. This could be proved by experiments with radioactive albumin.

G. Nogge

Papers

Sterility in tsetse flies (Glossina morsitans Westwood) caused by loss of symbionts

Resorption von Serumproteinen in die Haemolymphe bei Larven vonHypoderma bovis (De Geer) (Diptera, Hypodermatidae)