Multimodal dynamic response of the Buchnera aphidicola pLeu plasmid to variations in leucine demand of its host, the pea aphid Acyrthosiphon pisum

Paleocene origin of a streamlined digestive symbiosis in leaf beetles

Timing the acquisition of a beneficial microbe relative to the evolutionary history of its host can shed light on the adaptive impact of a partnership. Here, we investigated the onset and molecular evolution of an obligate symbiosis between Cassidinae leaf beetles and Candidatus Stammera capleta, a γ-proteobacterium. Residing extracellularly within foregut symbiotic organs, Stammera upgrades the digestive physiology of its host by supplementing plant cell wall-degrading enzymes. We observe that Stammera is a shared symbiont across tortoise and hispine beetles that collectively comprise the Cassidinae subfamily, despite differences in their folivorous habits. In contrast to its transcriptional profile during vertical transmission, Stammera elevates the expression of genes encoding digestive enzymes while in the foregut symbiotic organs, matching the nutritional requirements of its host. Despite the widespread distribution of Stammera across Cassidinae beetles, symbiont acquisition during the Paleocene (∼62 mya) did not coincide with the origin of the subfamily. Early diverging lineages lack the symbiont and the specialized organs that house it. Reconstructing the ancestral state of host-beneficial factors revealed that Stammera encoded three digestive enzymes at the onset of symbiosis, including polygalacturonase-a pectinase that is universally shared. Although non-symbiotic cassidines encode polygalacturonase endogenously, their repertoire of plant cell wall-degrading enzymes is more limited compared with symbiotic beetles supplemented with digestive enzymes from Stammera. Highlighting the potential impact of a symbiotic condition and an upgraded metabolic potential, Stammera-harboring beetles exploit a greater variety of plants and are more speciose compared with non-symbiotic members of the Cassidinae.

Host-specific co-evolution likely driven by diet in Buchnera aphidicola

BACKGROUND

Russian wheat aphid (Diuraphis noxia Kurd.) is a severe pest to wheat, and even though resistance varieties are available to curb this pest, they are becoming obsolete with the development of new virulent aphid populations. Unlike many other aphids, D noxia only harbours a single endosymbiont, Buchnera aphidicola. Considering the importance of Buchnera, this study aimed to elucidate commonalities and dissimilarities between various hosts, to better understand its distinctiveness within its symbiotic relationship with D. noxia. To do so, the genome of the D. noxia's Buchnera was assembled and compared to those of other aphid species that feed on diverse host species.

RESULTS

The overall importance of several features such as gene length and percentage GC content was found to be critical for the maintenance of Buchnera genes when compared to their closest free-living relative, Escherichia coli. Buchnera protein coding genes were found to have percentage GC contents that tended towards a mean of ~ 26% which had strong correlation to their identity to their E. coli homologs. Several SNPs were identified between different aphid populations and multiple isolates of Buchnera were confirmed in single aphids.

CONCLUSIONS

Establishing the strong correlation of percentage GC content of protein coding genes and gene identity will allow for identifying which genes will be lost in the continually shrinking Buchnera genome. This is also the first report of a parthenogenically reproducing aphid that hosts multiple Buchnera strains in a single aphid, raising questions regarding the benefits of maintaining multiple strains. We also found preliminary evidence for post-transcriptional regulation of Buchnera genes in the form of polyadenylation.

Coordination of host and endosymbiont gene expression governs endosymbiont growth and elimination in the cereal weevil Sitophilus spp

BACKGROUND

Insects living in nutritionally poor environments often establish long-term relationships with intracellular bacteria that supplement their diets and improve their adaptive and invasive powers. Even though these symbiotic associations have been extensively studied on physiological, ecological, and evolutionary levels, few studies have focused on the molecular dialogue between host and endosymbionts to identify genes and pathways involved in endosymbiosis control and dynamics throughout host development.

RESULTS

We simultaneously analyzed host and endosymbiont gene expression during the life cycle of the cereal weevil Sitophilus oryzae, from larval stages to adults, with a particular emphasis on emerging adults where the endosymbiont Sodalis pierantonius experiences a contrasted growth-climax-elimination dynamics. We unraveled a constant arms race in which different biological functions are intertwined and coregulated across both partners. These include immunity, metabolism, metal control, apoptosis, and bacterial stress response.

CONCLUSIONS

The study of these tightly regulated functions, which are at the center of symbiotic regulations, provides evidence on how hosts and bacteria finely tune their gene expression and respond to different physiological challenges constrained by insect development in a nutritionally limited ecological niche. Video Abstract.

Bacteriocyte plasticity in pea aphids facing amino acid stress or starvation during development

An important contributing factor to the evolutionary success of insects is nutritional association with microbial symbionts, which provide the host insects with nutrients lacking in their unbalanced diets. These symbionts are often compartmentalized in specialized cells of the host, the bacteriocytes. Even though bacteriocytes were first described more than a century ago, few studies have explored their dynamics throughout the insect life cycle and in response to environmental stressors. Here, we use the Buchnera aphidicola/pea aphid symbiotic system to study how bacteriocytes are regulated in response to nutritional stress throughout aphid development. Using artificial diets, we analyzed the effects of depletion or excess of phenylalanine or leucine, two amino acids essential for aphid growth and whose biosynthetic pathways are shared between the host and the symbiont. Bacteriocytes responded dynamically to those treatments, while other tissues showed no obvious morphological change. Amino acid depletion resulted in an increase in bacteriocyte numbers, with the extent of the increase depending on the amino acid, while excess either caused a decrease (for leucine) or an increase (for phenylalanine). Only a limited impact on survival and fecundity was observed, suggesting that the adjustment in bacteriocyte (and symbiont) numbers is sufficient to withstand these nutritional challenges. We also studied the impact of more extreme conditions by exposing aphids to a 24 h starvation period at the beginning of nymphal development. This led to a dramatic drop in aphid survival and fecundity and a significant developmental delay. Again, bacteriocytes responded dynamically, with a considerable decrease in number and size, correlated with a decrease in the number of symbionts, which were prematurely degraded by the lysosomal system. This study shows how bacteriocyte dynamics is integrated in the physiology of insects and highlights the high plasticity of these cells.

SymbiQuant: A Machine Learning Object Detection Tool for Polyploid Independent Estimates of Endosymbiont Population Size

Quantifying the size of endosymbiont populations is challenging because endosymbionts are typically difficult or impossible to culture and commonly polyploid. Current approaches to estimating endosymbiont population sizes include quantitative PCR (qPCR) targeting endosymbiont genomic DNA and flow-cytometry. While qPCR captures genome copy number data, it does not capture the number of bacterial cells in polyploid endosymbiont populations. In contrast, flow cytometry can capture accurate estimates of whole host-level endosymbiont population size, but it is not readily able to capture data at the level of endosymbiotic host cells. To complement these existing approaches for estimating endosymbiont population size, we designed and implemented an object detection/segmentation tool for counting the number of endosymbiont cells in micrographs of host tissues. The tool, called SymbiQuant, which makes use of recent advances in deep neural networks includes a graphic user interface that allows for human curation of tool output. We trained SymbiQuant for use in the model aphid/Buchnera endosymbiosis and studied Buchnera population dynamics and phenotype over aphid postembryonic development. We show that SymbiQuant returns accurate counts of endosymbionts, and readily captures Buchnera phenotype. By replacing our training data with data composed of annotated microscopy images from other models of endosymbiosis, SymbiQuant has the potential for broad application. Our tool, which is available on GitHub, adds to the repertoire of methods researchers can use to study endosymbiosis at the organismal, genome, and now endosymbiotic host tissue or cell levels.

Changes in Aphid Host Plant Diet Influence the Small-RNA Expression Profiles of Its Obligate Nutritional Symbiont, Buchnera

Plants are a difficult food resource to use, and herbivorous insects have evolved a variety of mechanisms that allow them to fully exploit this poor nutritional resource. One such mechanism is the maintenance of bacterial symbionts that aid in host plant feeding and development. The majority of these intracellular symbionts have highly eroded genomes that lack many key regulatory genes; consequently, it is unclear if these symbionts can respond to changes in the insect's diet to facilitate host plant use. There is emerging evidence that symbionts with highly eroded genomes express small RNAs (sRNAs), some of which potentially regulate gene expression. In this study, we sought to determine if the reduced genome of the nutritional symbiont (Buchnera) in the pea aphid responds to changes in the aphid's host plant diet. Using transcriptome sequencing (RNA-seq), Buchnera sRNA expression profiles were characterized within two Buchnera life stages when pea aphids fed on either alfalfa or fava bean. Overall, this study demonstrates that Buchnera sRNA expression changes not only with life stage but also with changes in aphid host plant diet. Of the 321 sRNAs characterized in this study, 47% were previously identified and 22% showed evidence of conservation in two or more Buchnera taxa. Functionally, 13 differentially expressed sRNAs were predicted to target genes related to pathways involved in essential amino acid biosynthesis. Overall, results from this study reveal that host plant diet influences the expression of conserved and lineage-specific sRNAs in Buchnera and that these sRNAs display distinct host plant-specific expression profiles among biological replicates.IMPORTANCE In general, the genomes of intracellular bacterial symbionts are reduced compared to those of free-living relatives and lack many key regulatory genes. Many of these reduced genomes belong to obligate mutualists of insects that feed on a diet that is deficient in essential nutrients, such as essential amino acids. It is unclear if these symbionts respond with their host to changes in insect diet, because of their reduced regulatory capacity. Emerging evidence suggests that these symbionts express small RNAs (sRNAs) that regulate gene expression at the posttranscriptional level. Therefore, in this study, we sought to determine if the reduced genome of the nutritional symbiont Buchnera in the pea aphid responds to changes in the aphid's host plant diet. This study demonstrates for the first time that Buchnera sRNAs, some conserved in two or more Buchnera lineages, are differentially expressed when aphids feed on different plant species and potentially target genes within essential amino acid biosynthesis pathways.

Effects of Endosymbiont Disruption on the Nutritional Dynamics of the Pea Aphid Acyrthosiphon pisum

Pea aphid (Acyrthosiphon pisum) is a worldwide pest that feeds exclusively on the phloem sap of numerous host plants. It harbours a well-known primary endosymbiont Buchneraaphidicola that helps to overcome the nutritional deficiency of a plant-based diet. However, how the Buchnera contributes to the nutritional and energy metabolism of its aphid host is unclear to date. In the current study, the function of Buchnera in relation to nutritional synthesis of pea aphid was investigated by disrupting the primary endosymbiont with an antibiotic rifampicin. Our findings revealed that the disruption of Buchnera led to infertility and higher loss in body mass of aphid hosts. Body length and width were also decreased significantly compared to healthy aphids. The detection of nutrition indicated that the quantity of proteins, soluble sugars, and glycogen in aposymbiotic pea aphids increased slowly with the growth of the aphid host. In comparison, the quantities of all the nutritional factors were significantly lower than those of symbiotic pea aphids, while the quantity of total lipid and neutral fat in aposymbiotic pea aphids were distinctly higher than those of symbiotic ones. Thus, we concluded that the significant reduction of the total amount of proteins, soluble sugars, and glycogen and the significant increase of neutral fats in aposymbiotic pea aphids were due to the disruption of Buchnera, which confirmed that the function of Buchnera is irreplaceable in the pea aphid.

Bacteriocyte Reprogramming to Cope With Nutritional Stress in a Phloem Sap Feeding Hemipteran, the Pea Aphid Acyrthosiphon pisum

Nutritional symbioses play a central role in the ability of insects to thrive on unbalanced diets and in ensuring their evolutionary success. A genomic model for nutritional symbiosis comprises the hemipteran Acyrthosiphon pisum, and the gamma-3-proteobacterium, Buchnera aphidicola, with genomes encoding highly integrated metabolic pathways. A. pisum feeds exclusively on plant phloem sap, a nutritionally unbalanced diet highly variable in composition, thus raising the question of how this symbiotic system responds to nutritional stress. We addressed this by combining transcriptomic, phenotypic and life history trait analyses to determine the organismal impact of deprivation of tyrosine and phenylalanine. These two aromatic amino acids are essential for aphid development, are synthesized in a metabolic pathway for which the aphid host and the endosymbiont are interdependent, and their concentration can be highly variable in plant phloem sap. We found that this nutritional challenge does not have major phenotypic effects on the pea aphid, except for a limited weight reduction and a 2-day delay in onset of nymph laying. Transcriptomic analyses through aphid development showed a prominent response in bacteriocytes (the core symbiotic tissue which houses the symbionts), but not in gut, thus highlighting the role of bacteriocytes as major modulators of this homeostasis. This response does not involve a direct regulation of tyrosine and phenylalanine biosynthetic pathway and transporter genes. Instead, we observed an extensive transcriptional reprogramming of the bacteriocyte with a rapid down-regulation of genes encoding sugar transporters and genes required for sugar metabolism. Consistently, we observed continued overexpression of the A. pisum homolog of RRAD, a small GTPase implicated in repressing aerobic glycolysis. In addition, we found increased transcription of genes involved in proliferation, cell size control and signaling. We experimentally confirmed the significance of these gene expression changes detecting an increase in bacteriocyte number and cell size in vivo under tyrosine and phenylalanine depletion. Our results support a central role of bacteriocytes in the aphid response to amino acid deprivation: their transcriptional and cellular responses fine-tune host physiology providing the host insect with an effective way to cope with the challenges posed by the variability in composition of phloem sap.

Novel Insights into Insect-Microbe Interactions-Role of Epigenomics and Small RNAs

It has become increasingly clear that microbes form close associations with the vast majority of animal species, especially insects. In fact, an array of diverse microbes is known to form shared metabolic pathways with their insect hosts. A growing area of research in insect-microbe interactions, notably for hemipteran insects and their mutualistic symbionts, is to elucidate the regulation of this inter-domain metabolism. This review examines two new emerging mechanisms of gene regulation and their importance in host-microbe interactions. Specifically, we highlight how the incipient areas of research on regulatory "dark matter" such as epigenomics and small RNAs, can play a pivotal role in the evolution of both insect and microbe gene regulation. We then propose specific models of how these dynamic forms of gene regulation can influence insect-symbiont-plant interactions. Future studies in this area of research will give us a systematic understanding of how these symbiotic microbes and animals reciprocally respond to and regulate their shared metabolic processes.

Direct flow cytometry measurements reveal a fine-tuning of symbiotic cell dynamics according to the host developmental needs in aphid symbiosis

Endosymbiotic associations constitute a driving force in the ecological and evolutionary diversification of metazoan organisms. Little is known about whether and how symbiotic cells are coordinated according to host physiology. Here, we use the nutritional symbiosis between the insect pest, Acyrthosiphon pisum, and its obligate symbiont, Buchnera aphidicola, as a model system. We have developed a novel approach for unculturable bacteria, based on flow cytometry, and used this method to estimate the absolute numbers of symbionts at key stages of aphid life. The endosymbiont population increases exponentially throughout nymphal development, showing a growing rate which has never been characterized by indirect molecular techniques. Using histology and imaging techniques, we have shown that the endosymbiont-bearing cells (bacteriocytes) increase significantly in number and size during the nymphal development, and clustering in the insect abdomen. Once adulthood is reached and the laying period has begun, the dynamics of symbiont and host cells is reversed: the number of endosymbionts decreases progressively and the bacteriocyte structure degenerates during insect aging. In summary, these results show a coordination of the cellular dynamics between bacteriocytes and primary symbionts and reveal a fine-tuning of aphid symbiotic cells to the nutritional demand imposed by the host physiology throughout development.

Coevolution of the Organization and Structure of Prokaryotic Genomes

The cytoplasm of prokaryotes contains many molecular machines interacting directly with the chromosome. These vital interactions depend on the chromosome structure, as a molecule, and on the genome organization, as a unit of genetic information. Strong selection for the organization of the genetic elements implicated in these interactions drives replicon ploidy, gene distribution, operon conservation, and the formation of replication-associated traits. The genomes of prokaryotes are also very plastic with high rates of horizontal gene transfer and gene loss. The evolutionary conflicts between plasticity and organization lead to the formation of regions with high genetic diversity whose impact on chromosome structure is poorly understood. Prokaryotic genomes are remarkable documents of natural history because they carry the imprint of all of these selective and mutational forces. Their study allows a better understanding of molecular mechanisms, their impact on microbial evolution, and how they can be tinkered in synthetic biology.

Matching the supply of bacterial nutrients to the nutritional demand of the animal host

Various animals derive nutrients from symbiotic microorganisms with much-reduced genomes, but it is unknown whether, and how, the supply of these nutrients is regulated. Here, we demonstrate that the production of essential amino acids (EAAs) by the bacterium Buchnera aphidicola in the pea aphid Acyrthosiphon pisum is elevated when aphids are reared on diets from which that EAA are omitted, demonstrating that Buchnera scale EAA production to host demand. Quantitative proteomics of bacteriocytes (host cells bearing Buchnera) revealed that these metabolic changes are not accompanied by significant change in Buchnera or host proteins, suggesting that EAA production is regulated post-translationally. Bacteriocytes in aphids reared on diet lacking the EAA methionine had elevated concentrations of both methionine and the precursor cystathionine, indicating that methionine production is promoted by precursor supply and is not subject to feedback inhibition by methionine. Furthermore, methionine production by isolated Buchnera increased with increasing cystathionine concentration. We propose that Buchnera metabolism is poised for EAA production at certain maximal rates, and the realized release rate is determined by precursor supply from the host. The incidence of host regulation of symbiont nutritional function via supply of key nutritional inputs in other symbioses remains to be investigated.

Widespread expression of conserved small RNAs in small symbiont genomes

Genome architecture of a microbe markedly changes when it transitions from a free-living lifestyle to an obligate symbiotic association within eukaryotic cells. These symbiont genomes experience numerous rearrangements and massive gene loss, which is expected to radically alter gene regulatory networks compared with those of free-living relatives. As such, it remains unclear whether and how these small symbiont genomes regulate gene expression. Here, using a label-free mass-spec quantification approach we found that differential protein regulation occurs in Buchnera, a model symbiont with a reduced genome, when it transitions between two distinct life stages. However, differential mRNA expression could not be detected between Buchnera life stages, despite the presence of a small number of putative transcriptional regulators. Instead a comparative analysis of small RNA expression profiles among five divergent Buchnera lineages, spanning a variety of Buchnera life stages, reveals 140 novel intergenic and antisense small RNAs and 517 untranslated regions that were significantly expressed, some of which have been conserved for ∼65 million years. In addition, the majority of these small RNAs exhibit both sequence covariation and thermodynamic stability, indicators of a potential structural RNA role. Together, these data suggest that gene regulation at the post-transcriptional level may be important in Buchnera. This is the first study to empirically identify Buchnera small RNAs, and we propose that these novel small RNAs may facilitate post-transcriptional regulation through translational inhibition/activation, and/or transcript stability. Ultimately, post-transcriptional regulation may shape metabolic complementation between Buchnera and its aphid host, thus impacting the animal's ecology and evolution.

Collection and chemical composition of phloem sap from Citrus sinensis L. Osbeck (sweet orange)

Through utilizing the nutrient-rich phloem sap, sap feeding insects such as psyllids, leafhoppers, and aphids can transmit many phloem-restricted pathogens. On the other hand, multiplication of phloem-limited, uncultivated bacteria such as Candidatus Liberibacter asiaticus (CLas) inside the phloem of citrus indicates that the sap contains all the essential nutrients needed for the pathogen growth. The phloem sap composition of many plants has been studied; however, to our knowledge, there is no available data about citrus phloem sap. In this study, we identified and quantified the chemical components of phloem sap from pineapple sweet orange. Two approaches (EDTA enhanced exudation and centrifugation) were used to collect phloem sap. The collected sap was derivatized with methyl chloroformate (MCF), N-methyl-N- [tert-butyl dimethylsilyl]-trifluroacetamide (MTBSTFA), or trimethylsilyl (TMS) and analyzed with GC-MS revealing 20 amino acids and 8 sugars. Proline, the most abundant amino acid, composed more than 60% of the total amino acids. Tryptophan, tyrosine, leucine, isoleucine, and valine, which are considered essential for phloem sap-sucking insects, were also detected. Sucrose, glucose, fructose, and inositol were the most predominant sugars. In addition, seven organic acids including succinic, fumaric, malic, maleic, threonic, citric, and quinic were detected. All compounds detected in the EDTA-enhanced exudate were also detected in the pure phloem sap using centrifugation. The centrifugation technique allowed estimating the concentration of metabolites. This information expands our knowledge about the nutrition requirement for citrus phloem-limited bacterial pathogen and their vectors, and can help define suitable artificial media to culture them.

Tyrosine pathway regulation is host-mediated in the pea aphid symbiosis during late embryonic and early larval development

BACKGROUND

Nutritional symbioses play a central role in insects' adaptation to specialized diets and in their evolutionary success. The obligatory symbiosis between the pea aphid, Acyrthosiphon pisum, and the bacterium, Buchnera aphidicola, is no exception as it enables this important agricultural pest insect to develop on a diet exclusively based on plant phloem sap. The symbiotic bacteria provide the host with essential amino acids lacking in its diet but necessary for the rapid embryonic growth seen in the parthenogenetic viviparous reproduction of aphids. The aphid furnishes, in exchange, non-essential amino acids and other important metabolites. Understanding the regulations acting on this integrated metabolic system during the development of this insect is essential in elucidating aphid biology.

RESULTS

We used a microarray-based approach to analyse gene expression in the late embryonic and the early larval stages of the pea aphid, characterizing, for the first time, the transcriptional profiles in these developmental phases. Our analyses allowed us to identify key genes in the phenylalanine, tyrosine and dopamine pathways and we identified ACYPI004243, one of the four genes encoding for the aspartate transaminase (E.C. 2.6.1.1), as specifically regulated during development. Indeed, the tyrosine biosynthetic pathway is crucial for the symbiotic metabolism as it is shared between the two partners, all the precursors being produced by B. aphidicola. Our microarray data are supported by HPLC amino acid analyses demonstrating an accumulation of tyrosine at the same developmental stages, with an up-regulation of the tyrosine biosynthetic genes. Tyrosine is also essential for the synthesis of cuticular proteins and it is an important precursor for cuticle maturation: together with the up-regulation of tyrosine biosynthesis, we observed an up-regulation of cuticular genes expression. We were also able to identify some amino acid transporter genes which are essential for the switch over to the late embryonic stages in pea aphid development.

CONCLUSIONS

Our data show that, in the development of A. pisum, a specific host gene set regulates the biosynthetic pathways of amino acids, demonstrating how the regulation of gene expression enables an insect to control the production of metabolites crucial for its own development and symbiotic metabolism.

Genomic analysis of the regulatory elements and links with intrinsic DNA structural properties in the shrunken genome of Buchnera

Background

Buchnera aphidicola is an obligate symbiotic bacterium, associated with most of the aphididae, whose genome has drastically shrunk during intracellular evolution. Gene regulation in Buchnera has been a matter of controversy in recent years as the combination of genomic information with the experimental results has been contradictory, refuting or arguing in favour of a functional and responsive transcription regulation in Buchnera.

The goal of this study was to describe the gene transcription regulation capabilities of Buchnera based on the inventory of cis- and trans-regulators encoded in the genomes of five strains from different aphids (Acyrthosiphon pisum, Schizaphis graminum, Baizongia pistacea, Cinara cedri and Cinara tujafilina), as well as on the characterisation of some intrinsic structural properties of the DNA molecule in these bacteria.

Results

Interaction graph analysis shows that gene neighbourhoods are conserved between E. coli and Buchnera in structures called transcriptons, interactons and metabolons, indicating that selective pressures have acted on the evolution of transcriptional, protein-protein interaction and metabolic networks in Buchnera. The transcriptional regulatory network in Buchnera is composed of a few general DNA-topological regulators (Nucleoid Associated Proteins and topoisomerases), with the quasi-absence of any specific ones (except for multifunctional enzymes with a known gene expression regulatory role in Escherichia coli, such as AlaS, PepA and BolA, and the uncharacterized hypothetical regulators YchA and YrbA). The relative positioning of regulatory genes along the chromosome of Buchnera seems to have conserved its ancestral state, despite the genome erosion. Sigma-70 promoters with canonical thermodynamic sequence profiles were detected upstream of about 94% of the CDS of Buchnera in the different aphids. Based on Stress-Induced Duplex Destabilization (SIDD) measurements, unstable σ⁷⁰ promoters were found specifically associated with the regulator and transporter genes.

Conclusions

This genomic analysis provides supporting evidence of a selection of functional regulatory structures and it has enabled us to propose hypotheses concerning possible links between these regulatory elements and the DNA-topology (i.e., supercoiling, curvature, flexibility and base-pair stability) in the regulation of gene expression in the shrunken genome of Buchnera.

A genomic reappraisal of symbiotic function in the aphid/Buchnera symbiosis: reduced transporter sets and variable membrane organisations

Buchnera aphidicola is an obligate symbiotic bacterium that sustains the physiology of aphids by complementing their exclusive phloem sap diet. In this study, we reappraised the transport function of different Buchnera strains, from the aphids Acyrthosiphon pisum, Schizaphis graminum, Baizongia pistaciae and Cinara cedri, using the re-annotation of their transmembrane proteins coupled with an exploration of their metabolic networks. Although metabolic analyses revealed high interdependencies between the host and the bacteria, we demonstrate here that transport in Buchnera is assured by low transporter diversity, when compared to free-living bacteria, being mostly based on a few general transporters, some of which probably have lost their substrate specificity. Moreover, in the four strains studied, an astonishing lack of inner-membrane importers was observed. In Buchnera, the transport function has been shaped by the distinct selective constraints occurring in the Aphididae lineages. Buchnera from A. pisum and S. graminum have a three-membraned system and similar sets of transporters corresponding to most compound classes. Transmission electronic microscopic observations and confocal microscopic analysis of intracellular pH fields revealed that Buchnera does not show any of the typical structures and properties observed in integrated organelles. Buchnera from B. pistaciae seem to possess a unique double membrane system and has, accordingly, lost all of its outer-membrane integral proteins. Lastly, Buchnera from C. cedri revealed an extremely poor repertoire of transporters, with almost no ATP-driven active transport left, despite the clear persistence of the ancestral three-membraned system.