Bacteriome

A bacteriome (historically also termed a mycetome when earlier microscopists mistook the intracellular bacteria or yeast-like mutualists for fungi) is a specialized, developmentally dedicated organ within the body cavity of many insects that houses obligate, vertically transmitted primary intracellular endosymbionts (Entomology; Buchner, 1965; Baumann, 2005). Constructed from giant, endopolyploid host cells known as bacteriocytes (or mycetocytes) surrounded by a thin epithelial sheath and richly supplied by oxygen-delivering tracheoles and circulating hemolymph, the bacteriome functions as a unified metabolic chimera between the hexapod host and its sequestered bacteria. Across more than 10–15% of all described insect species, reliance on a bacteriome has enabled lineages to radiate onto nutritionally unbalanced, single-source diets that would otherwise be lethal to animals: phloem-sap and xylem-sap feeding Hemiptera (aphids, psyllids, whiteflies, cicadas, spittlebugs, and planthoppers), strictly hematophagous (vertebrate blood-feeding) disease vectors and ectoparasites (tsetse flies, sucking lice, bedbugs, and ticks), and grain- or wood-feeding Coleoptera (weevils, grain beetles, and carpenter ants; Moran et al., 2008; Douglas, 2015).

Giant Polyploid Bacteriocytes and Developmental Specification

First systematically cataloged across hundreds of insect families by the German zoologist Paul Buchner (1886–1978), bacteriomes adopt lineage-specific anatomical configurations—forming a bilobed organ in the abdominal hemocoel above the ovaries in aphids (Acyrthosiphon pisum), a U-shaped cellular zone integrated into the anterior midgut epithelium in tsetse flies (Glossina morsitans), or paired lateral abdominal clusters adjacent to the fat body and oenocytes in bedbugs (Cimex lectularius) and cockroaches (Buchner, 1965; Moran et al., 2008):

• Homeotic Specification (Ultrabithorax, abdominal-A, and Distal-less): In the pea aphid Acyrthosiphon pisum, embryonic differentiation of bacteriocytes is directed by sequential expression of the host transcription factors Distal-less (Dll), engrailed, and the Hox genes Ultrabithorax (Ubx) and abdominal-A (abd-A) before the maternal bacteria even enter the embryo (Braendle et al., 2003).
• High Endopolyploidy: As the insect grows through successive nymphal or larval instars, bacteriocytes cease mitotic cytokinesis and undergo repeated cycles of DNA endoreplication, attaining ploidy levels of 64C to 256C and cell diameters of 80 to >150 μm. This amplification of host nuclear gene templates supports abundant transcription of amino-acid transporters, metabolic enzymes that complete truncated bacterial pathways, and symbiosome membrane proteins.
• The Symbiosome Membrane Interface: Within the bacteriocyte cytoplasm, thousands of spherical or pleomorphic bacterial cells (e.g., ~50–100 cells of Buchnera aphidicola per neonatal bacteriocyte, expanding to >5,000 per mature adult bacteriocyte) are individually enclosed inside a host-derived vacuolar lipid bilayer termed the symbiosome membrane. This triple-membrane interface (bacterial inner membrane, bacterial outer membrane with reduced lipopolysaccharide, and host symbiosome membrane) mediates bidirectional metabolite exchange via upregulated host solute carriers (such as the glutamine transporter ApGLNT1).

Because animals lack the metabolic pathways to synthesize the nine or ten proteinogenic essential amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, and arginine) as well as B-complex vitamins (biotin, folate, riboflavin, thiamine, pyridoxine, pantothenate), insects feeding on phloem sap, xylem sap, or vertebrate blood partner with specific bacteriome endosymbionts whose genomes retain the biosynthetic pathways missing from the host's diet (Baumann, 2005; Moran et al., 2008; Akman et al., 2002):

Shared Enzymatic Pathways Across the Symbiosome Boundary

Genomic and transcriptomic analyses of the pea aphid–Buchnera bacteriome revealed that essential amino acid synthesis is not performed by the bacterium alone, but is metabolically partitioned between the bacterial genome and the host nuclear genome (Wilson et al., 2010):

• In the synthesis of the branched-chain amino acids isoleucine, leucine, and valine, Buchnera aphidicola encodes all enzymes of the pathway except the terminal transaminase (branched-chain amino acid transaminase, BCAT / ilvE), which has been lost from the Buchnera genome.
• Instead, the host aphid bacteriocyte strongly upregulates its own nuclear-encoded cytosolic BCAT, importing the bacterial keto-acid intermediates (2-oxoisovalerate, 2-oxomethylvalerate, and 4-methyl-2-oxopentanoate) across the symbiosome membrane and transaminating them with glutamate inside the host cytoplasm. This metabolic interdependence enables the host insect to throttle bacterial amino acid output by controlling substrate supply via the glutamine transporter ApGLNT1.

Because obligate bacteriome endosymbionts have been strictly confined inside host cells for 50 to 270 million years without horizontal gene transfer, homologous recombination, or bacteriophage infection, and undergo severe population bottlenecks during maternal transovarial transmission from the mother's bacteriome into developing oocytes or viviparous larvae, their genomes experience Muller's ratchet (irreversible accumulation of slightly deleterious mutations by genetic drift; Moran, 1996; McCutcheon & Moran, 2012):

1. AT-Rich Mutational Bias and Pseudogene Elimination: Loss of DNA mismatch and excision repair genes (mutS, mutL, recA, uvrABC) drives extreme adenine + thymine bias (70% to 86% A+T content) and strips away non-coding DNA, outer-membrane lipopolysaccharide synthesis genes, cell-division regulators (ftsZ is lost in Carsonella and Sulcia), and flagellar/catabolic operons.
2. GroEL Chaperonin Overexpression: To buffer the destabilizing effects of amino acid substitutions in their remaining essential enzymes, bacteriome symbionts constitutively express the heat-shock chaperonin GroEL (Hsp60) at 10% to 20% of total bacterial cell protein.
3. Cellular Organelle-Like Minimum Genomes: In sap-feeding leafhoppers, mealybugs (Planococcus citri, where the β-proteobacterium Tremblaya princeps with a 139 kb genome harbors the γ-proteobacterium Moranella endobia nested inside its own cytoplasm), and spittlebugs, bacteriome symbiont genomes shrink to 112–160 kb—smaller than many plastid and mitochondrial genomes.

To prevent the insect innate immune system from destroying its own obligate mutualists while maintaining systemic antimicrobial defense in the open hemolymph, bacteriocytes establish an immunologically privileged sanctuary (Anselme et al., 2008; Maire et al., 2019):

• In the cereal weevil Sitophilus, bacteriocytes constitutively express a specialized secreted amidase peptidoglycan recognition protein (PGRP-LB) that cleaves bacterial tracheal cytotoxin (monomeric diaminopimelic-acid peptidoglycan) before it can leak out of the bacteriome and trigger systemic Imd-pathway NF-κB Relish inflammation.
• Simultaneously, the weevil bacteriocyte secretes a single bacteriocyte-specific cationic antimicrobial peptide, Coleoptericin A (ColA), which selectively targets the intracellular symbiont Sodalis pierantonius to inhibit bacterial cell division (cytokinesis) without lysing the cell—transforming the bacteria into giant, metabolically hyperactive filaments locked safely inside the bacteriome.

1. Buchner, P. (1965). Endosymbiosis of Animals with Plant Microorganisms. Interscience Publishers / John Wiley & Sons. https://doi.org/10.1126/science.151.3711.678
2. Akman, L., Yamashita, A., Watanabe, H., Oshima, K., Shiba, T., Hattori, M., & Aksoy, S. (2002). Genome sequence of the endocellular obligate symbiont of tsetse flies, Wigglesworthia glossinidia. Nature Genetics, 32(3), 402–407. https://doi.org/10.1038/ng986
3. Baumann, P. (2005). Biology of bacteriocyte-associated endosymbionts of plant sap-sucking insects. Annual Review of Microbiology, 59, 155–189. https://doi.org/10.1146/annurev.micro.59.030804.121041
4. Moran, N. A., McCutcheon, J. P., & Nakabachi, A. (2008). Genomics and evolution of heritable bacterial symbionts. Annual Review of Genetics, 42, 165–190. https://doi.org/10.1146/annurev.genet.41.110306.130119
5. McCutcheon, J. P., & Moran, N. A. (2012). Extreme genome reduction in symbiotic bacteria. Nature Reviews Microbiology, 10(1), 13–26. https://doi.org/10.1038/nrmicro2670