Host cell allometry and regulation of the symbiosis between pea aphids, Acyrthosiphon pisum, and bacteria, Buchnera

Ploidy dynamics in aphid host cells harboring bacterial symbionts

Aphids have evolved bacteriocytes or symbiotic host cells that harbor the obligate mutualistic bacterium Buchnera aphidicola. Because of the large cell size (approximately 100 μm in diameter) of bacteriocytes and their pivotal role in nutritional symbiosis, researchers have considered that these cells are highly polyploid and assumed that bacteriocyte polyploidy may be essential for the symbiotic relationship between the aphid and the bacterium. However, little is known about the ploidy levels and dynamics of aphid bacteriocytes. Here, we quantitatively analyzed the ploidy levels in the bacteriocytes of the pea-aphid Acyrthosiphon pisum. Image-based fluorometry revealed the hyper polyploidy of the bacteriocytes ranging from 16- to 256-ploidy throughout the lifecycle. Bacteriocytes of adult parthenogenetic viviparous females were ranged between 64 and 128C DNA levels, while those of sexual morphs (oviparous females and males) were comprised of 64C, and 32–64C cells, respectively. During post-embryonic development of viviparous females, the ploidy level of bacteriocytes increased substantially, from 16 to 32C at birth to 128–256C in actively reproducing adults. These results suggest that the ploidy levels are dynamically regulated among phenotypes and during development. Our comprehensive and quantitative data provides a foundation for future studies to understand the functional roles and biological significance of the polyploidy of insect bacteriocytes.

Intraspecific variation in symbiont density in an insect-microbe symbiosis

Many insects host vertically transmitted microbes, which can confer benefits to their hosts but are costly to maintain and regulate. A key feature of these symbioses is variation: for example, symbiont density can vary among host and symbiont genotypes. However, the evolutionary forces maintaining this variation remain unclear. We studied variation in symbiont density using the pea aphid (Acyrthosiphon pisum) and the bacterium Regiella insecticola, a symbiont that can protect its host against fungal pathogens. We found that relative symbiont density varies both between two Regiella phylogenetic clades and among aphid "biotypes." Higher density symbiont infections are correlated with stronger survival costs, but variation in density has little effect on the protection Regiella provides against fungi. Instead, we found that in some aphid genotypes, a dramatic decline in symbiont density precedes the loss of a symbiont infection. Together, our data suggest that the optimal density of a symbiont infection is likely different from the perspective of aphid and microbial fitness. Regiella might prevent loss by maintaining high within-host densities, but hosts do not appear to benefit from higher symbiont numbers and may be advantaged by losing costly symbionts in certain environments. The standing variation in symbiont density observed in natural populations could therefore be maintained by antagonistic coevolutionary interactions between hosts and their symbiotic microbes.

What can a weevil teach a fly, and reciprocally? Interaction of host immune systems with endosymbionts in Glossina and Sitophilus

The tsetse fly (Glossina genus) is the main vector of African trypanosomes, which are protozoan parasites that cause human and animal African trypanosomiases in Sub-Saharan Africa. In the frame of the IAEA/FAO program ‘Enhancing Vector Refractoriness to Trypanosome Infection’, in addition to the tsetse, the cereal weevil Sitophilus has been introduced as a comparative system with regards to immune interactions with endosymbionts. The cereal weevil is an agricultural pest that destroys a significant proportion of cereal stocks worldwide. Tsetse flies are associated with three symbiotic bacteria, the multifunctional obligate Wigglesworthia glossinidia, the facultative commensal Sodalis glossinidius and the parasitic Wolbachia. Cereal weevils house an obligatory nutritional symbiosis with the bacterium Sodalis pierantonius, and occasionally Wolbachia. Studying insect host-symbiont interactions is highly relevant both for understanding the evolution of symbiosis and for envisioning novel pest control strategies. In both insects, the long co-evolution between host and endosymbiont has led to a stringent integration of the host-bacteria partnership. These associations were facilitated by the development of specialized host traits, including symbiont-housing cells called bacteriocytes and specific immune features that enable both tolerance and control of the bacteria. In this review, we compare the tsetse and weevil model systems and compile the latest research findings regarding their biological and ecological similarities, how the immune system controls endosymbiont load and location, and how host-symbiont interactions impact developmental features including cuticle synthesis and immune system maturation. We focus mainly on the interactions between the obligate symbionts and their host’s immune systems, a central theme in both model systems. Finally, we highlight how parallel studies on cereal weevils and tsetse flies led to mutual discoveries and stimulated research on each model, creating a pivotal example of scientific improvement through comparison between relatively distant models.

Intraspecific genetic variation in hosts affects regulation of obligate heritable symbionts

Symbiotic relationships promote biological diversification by unlocking new ecological niches. Over evolutionary time, hosts and symbionts often enter intimate and permanent relationships, which must be maintained and regulated for both lineages to persist. Many insect species harbor obligate, heritable symbiotic bacteria that provision essential nutrients and enable hosts to exploit niches that would otherwise be unavailable. Hosts must regulate symbiont population sizes, but optimal regulation may be affected by the need to respond to the ongoing evolution of symbionts, which experience high levels of genetic drift and potential selection for selfish traits. We address the extent of intraspecific variation in the regulation of a mutually obligate symbiosis, between the pea aphid (Acyrthosiphon pisum) and its maternally transmitted symbiont, Buchnera aphidicola Using experimental crosses to identify effects of host genotypes, we measured symbiont titer, as the ratio of genomic copy numbers of symbiont and host, as well as developmental time and fecundity of hosts. We find a large (>10-fold) range in symbiont titer among genetically distinct aphid lines harboring the same Buchnera haplotype. Aphid clones also vary in fitness, measured as developmental time and fecundity, and genetically based variation in titer is correlated with host fitness, with higher titers corresponding to lower reproductive rates of hosts. Our work shows that obligate symbiosis is not static but instead is subject to short-term evolutionary dynamics, potentially reflecting coevolutionary interactions between host and symbiont.

Host Plant Determines the Population Size of an Obligate Symbiont (Buchnera aphidicola) in Aphids

Buchnera aphidicolais an obligate endosymbiont that provides aphids with several essential nutrients. Though much is known about aphid-Buchnera interactions, the effect of the host plant on Buchnera population size remains unclear. Here we used quantitative PCR (qPCR) techniques to explore the effects of the host plant on Buchnera densities in the cotton-melon aphid, Aphis gossypii Buchneratiters were significantly higher in populations that had been reared on cucumber for over 10 years than in populations maintained on cotton for a similar length of time. Aphids collected in the wild from hibiscus and zucchini harbored more Buchnera symbionts than those collected from cucumber and cotton. The effect of aphid genotype on the population size of Buchnera depended on the host plant upon which they fed. When aphids from populations maintained on cucumber or cotton were transferred to novel host plants, host survival and Buchnera population size fluctuated markedly for the first two generations before becoming relatively stable in the third and later generations. Host plant extracts from cucumber, pumpkin, zucchini, and cowpea added to artificial diets led to a significant increase in Buchnera titers in the aphids from the population reared on cotton, while plant extracts from cotton and zucchini led to a decrease in Buchnera titers in the aphids reared on cucumber. Gossypol, a secondary metabolite from cotton, suppressed Buchnera populations in populations from both cotton and cucumber, while cucurbitacin from cucurbit plants led to higher densities. Together, the results suggest that host plants influence Buchnera population processes and that this may provide phenotypic plasticity in host plant use for clonal aphids.

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.

Bacteria-bacteria interactions within the microbiota of the ancestral metazoan Hydra contribute to fungal resistance

Epithelial surfaces of most animals are colonized by diverse microbial communities. Although it is generally agreed that commensal bacteria can serve beneficial functions, the processes involved are poorly understood. Here we report that in the basal metazoan Hydra, ectodermal epithelial cells are covered with a multilayered glycocalyx that provides a habitat for a distinctive microbial community. Removing this epithelial microbiota results in lethal infection by the filamentous fungus Fusarium sp. Restoring the complex microbiota in gnotobiotic polyps prevents pathogen infection. Although mono-associations with distinct members of the microbiota fail to provide full protection, additive and synergistic interactions of commensal bacteria are contributing to full fungal resistance. Our results highlight the importance of resident microbiota diversity as a protective factor against pathogen infections. Besides revealing insights into the in vivo function of commensal microbes in Hydra, our findings indicate that interactions among commensal bacteria are essential to inhibit pathogen infection.

Effects of parasitism on aphid nutritional and protective symbioses

Insects often carry heritable symbionts that negotiate interactions with food plants or natural enemies. All pea aphids, Acyrthosiphon pisum, require infection with the nutritional symbiont Buchnera, and many are also infected with Hamiltonella, which protects against the parasitoid Aphidius ervi. Hamiltonella-based protection requires bacteriophages called APSEs with protection levels varying by strain and associated APSE. Endoparasitoids, including A. ervi, may benefit from protecting the nutritional symbiosis and suppressing the protective one, while the aphid and its heritable symbionts have aligned interests when attacked by the wasp. We investigated the effects of parasitism on the abundance of aphid nutritional and protective symbionts. First, we determined strength of protection associated with multiple symbiont strains and aphid genotypes as these likely impact symbiont responses. Unexpectedly, some A. pisum genotypes cured of facultative symbionts were resistant to parasitism and resistant aphid lines carried Hamiltonella strains that conferred no additional protection. Susceptible aphid clones carried protective strains. qPCR estimates show that parasitism significantly influenced both Buchnera and Hamiltonella titres, with multiple factors contributing to variation. In susceptible lines, parasitism led to increases in Buchnera near the time of larval wasp emergence consistent with parasite manipulation, but effects were variable in resistant lines. Parasitism also resulted in increases in APSE and subsequent decreases in Hamiltonella, and we discuss how this response may relate to the protective phenotype. In summary, we show that parasitism alters the within-host ecology of both nutritional and protective symbioses with effects likely significant for all players in this antagonistic interaction.

Effect of Host Genotype on Symbiont Titer in the Aphid-Buchnera Symbiosis

Obligate nutritional symbioses require balance between the energetic needs of the host and the symbiont. The resident symbiont population size within a host may have major impacts on host fitness, as both host and symbiont consume and supply metabolites in a shared metabolite pool. Given the massive genome degradation that is a hallmark of bacterial endosymbionts of insects, it is unclear at what level these populations are regulated, and how regulation varies among hosts within natural populations. We measured the titer of the endosymbiont Buchnera aphidicola from different clones of the pea aphid, Acyrthosiphon pisum, and found significant variation in titer, measured as Buchnera genomes per aphid genome, among aphid clones. Additionally, we found that titer can change with the age of the host, and that the number of bacteriocytes within an aphid is one factor likely controlling Buchnera titer. Buchnera titer measurements in clones from a sexual cross indicate that the symbiont genotype is not responsible for variation in titer and that this phenotype is likely non-heritable across sexual reproduction. Symbiont titer is more variable among lab-produced F₁ aphid clones than among field-collected ones, suggesting that intermediate titer is favored in natural populations. Potentially, a low heritability of titer during the sexual phase may generate clones with extreme and maladaptive titers each season.

New clues about the evolutionary history of metabolic losses in bacterial endosymbionts, provided by the genome of Buchnera aphidicola from the aphid Cinara tujafilina

The symbiotic association between aphids (Homoptera) and Buchnera aphidicola (Gammaproteobacteria) started about 100 to 200 million years ago. As a consequence of this relationship, the bacterial genome has undergone a prominent size reduction. The downsize genome process starts when the bacterium enters the host and will probably end with its extinction and replacement by another healthier bacterium or with the establishment of metabolic complementation between two or more bacteria. Nowadays, several complete genomes of Buchnera aphidicola from four different aphid species (Acyrthosiphon pisum, Schizaphis graminum, Baizongia pistacea, and Cinara cedri) have been fully sequenced. C. cedri belongs to the subfamily Lachninae and harbors two coprimary bacteria that fulfill the metabolic needs of the whole consortium: B. aphidicola with the smallest genome reported so far and "Candidatus Serratia symbiotica." In addition, Cinara tujafilina, another member of the subfamily Lachninae, closely related to C. cedri, also harbors "Ca. Serratia symbiotica" but with a different phylogenetic status than the one from C. cedri. In this study, we present the complete genome sequence of B. aphidicola from C. tujafilina and the phylogenetic analysis and comparative genomics with the other Buchnera genomes. Furthermore, the gene repertoire of the last common ancestor has been inferred, and the evolutionary history of the metabolic losses that occurred in the different lineages has been analyzed. Although stochastic gene loss plays a role in the genome reduction process, it is also clear that metabolism, as a functional constraint, is also a powerful evolutionary force in insect endosymbionts.

Genomic revelations of a mutualism: the pea aphid and its obligate bacterial symbiont

The symbiosis of the pea aphid Acyrthosphion pisum with the bacterium Buchnera aphidicola APS represents the best-studied insect obligate symbiosis. Here we present a refined picture of this symbiosis by linking pre-genomic observations to new genomic data that includes the complete genomes of the eukaryotic and prokaryotic symbiotic partners. In doing so, we address four issues central to understanding the patterns and processes operating at the A. pisum/Buchnera APS interface. These four issues include: (1) lateral gene transfer, (2) host immunity, (3) symbiotic metabolism, and (4) regulation.

Tangible benefits of the aphid Acyrthosiphon pisum genome sequencing for aphid proteomics: Enhancements in protein identification and data validation for homology-based proteomics

Homology-driven proteomics promises to reveal functional biology in insects with sparse genome sequence information. A proteomics study comparing plant virus transmission competent and refractive genotypes of the aphid Schizaphis graminum isolated numerous candidate proteins involved in virus transmission, but limited genome sequence information hampered their identification. The complete genome of the pea aphid, Acyrthosiphon pisum, released in 2008, enabled us to double the number of protein identifications beyond what was possible using available EST libraries and other insect sequences. This was concomitant with a dramatic increase of the number of MS and MS/MS peptide spectra matching the genome-derived protein sequence. LC-MS/MS proved to be the most robust method of peptide detection. Cross-matching spectral data to multiple EST sequences and error tolerant searching to identify amino acid substitutions enhanced the percent coverage of the Schizaphis graminum proteins. 2-D electrophoresis provided the protein pI and MW which enabled the refinement of the candidate protein selection and provided a measure of protein abundance when coupled to the spectral data. Thus, the homology-based proteomics pipeline for insects should include efforts to maximize the number of peptide matches to the protein to increase certainty in protein identification and relative protein abundance.

Role of xylem consumption on osmoregulation in Macrosiphum euphorbiae (Thomas)

Aphids are phloem feeders that occasionally ingest xylem sap. The duration of xylem consumption by Macrosiphum euphorbiae (Hemiptera: Aphididae) was positively correlated with the level of dehydration of alate aphids of different ages after a period of starvation, supporting the hypothesis that aphids ingest xylem sap to replenish their water balance. However, the duration of xylem sap ingestion but not phloem sap consumption varied in unstarved alate adults of different ages. Furthermore, both alate and apterous aphids ingested xylem sap at the end of their life, when aphids were not dehydrated but when fecundity started to decrease. Fecundity was negatively correlated with the proportion of time spent ingesting xylem sap, and that over the entire reproductive life of alate and apterous aphids. The lower proportion of xylem ingested by apterous than by alate aphids during the first few days of adult life may be related to a higher symbiont density in apterous morphs. As previous studies have demonstrated a relationship between sucrose assimilation, which is directly influenced by fecundity and symbiont density, and osmoregulation, we suggest that xylem consumption may play a role in the osmoregulation of haemolymph of aphids.

Tritrophic interactions among Macrosiphum euphorbiae aphids, their host plants and endosymbionts: investigation by a proteomic approach

The Mi-1.2 gene in tomato confers resistance against certain clones of the potato aphid (Macrosiphum euphorbiae). This study used 2D-DIGE coupled with protein identification by MALDI-TOF-MS to compare the proteome patterns of avirulent and semivirulent potato aphids and their bacterial endosymbionts on resistant (Mi-1.2+) and susceptible (Mi-1.2-) tomato lines. Avirulent aphids had low survival on resistant plants, whereas the semivirulent clone could colonize these plants. Eighty-two protein spots showed significant quantitative differences among the four treatment groups, and of these, 48 could be assigned putative identities. Numerous structural proteins and enzymes associated with primary metabolism were more abundant in the semivirulent than in the avirulent aphid clone. Several proteins were also up-regulated in semivirulent aphids when they were transferred from susceptible to resistant plants. Nearly 25% of the differentially regulated proteins originated from aphid endosymbionts and not the aphid itself. Six were assigned to the primary endosymbiont Buchnera aphidicola, and 5 appeared to be derived from a Rickettsia-like secondary symbiont. These results indicate that symbiont expression patterns differ between aphid clones with differing levels of virulence, and are influenced by the aphids' host plant. Potentially, symbionts may contribute to differential adaptation of aphids to host plant resistance.

The striking case of tryptophan provision in the cedar aphid Cinara cedri

Buchnera aphidicola BCc has lost its symbiotic role as the tryptophan supplier to the aphid Cinara cedri. We report the presence of a plasmid in this endosymbiont that contains the trpEG genes. The remaining genes for the pathway (trpDCBA) are located on the chromosome of the secondary endosymbiont "Candidatus Serratia symbiotica." Thus, we propose that a symbiotic consortium is necessary to provide tryptophan.

Bivariate line-fitting methods for allometry

Fitting a line to a bivariate dataset can be a deceptively complex problem, and there has been much debate on this issue in the literature. In this review, we describe for the practitioner the essential features of line-fitting methods for estimating the relationship between two variables: what methods are commonly used, which method should be used when, and how to make inferences from these lines to answer common research questions. A particularly important point for line-fitting in allometry is that usually, two sources of error are present (which we call measurement and equation error), and these have quite different implications for choice of line-fitting method. As a consequence, the approach in this review and the methods presented have subtle but important differences from previous reviews in the biology literature. Linear regression, major axis and standardised major axis are alternative methods that can be appropriate when there is no measurement error. When there is measurement error, this often needs to be estimated and used to adjust the variance terms in formulae for line-fitting. We also review line-fitting methods for phylogenetic analyses. Methods of inference are described for the line-fitting techniques discussed in this paper. The types of inference considered here are testing if the slope or elevation equals a given value, constructing confidence intervals for the slope or elevation, comparing several slopes or elevations, and testing for shift along the axis amongst several groups. In some cases several methods have been proposed in the literature. These are discussed and compared. In other cases there is little or no previous guidance available in the literature. Simulations were conducted to check whether the methods of inference proposed have the intended coverage probability or Type I error. We identified the methods of inference that perform well and recommend the techniques that should be adopted in future work.

Transmission of symbiotic bacteria Buchnera to parthenogenetic embryos in the aphid Acyrthosiphon pisum (Hemiptera: Aphidoidea)

All phloem-feeding aphids have an absolute requirement for their primary bacterial symbionts Buchnera sp. The bacteria are transmitted vertically to either embryos in the viviparous morph or to eggs in the oviparous morph, with the implication that the symbiont population regularly passes through a genetic 'bottleneck', i.e. only a small proportion of the maternal symbiont population is transmitted to offspring. In this paper, we visualise this process in viviparous aphids using a specific immunolabelling technique for Buchnera. The images show a stream of bacteria originating from a single mycetocyte and entering the embryo, possibly via a membranous conduit, and individual bacterial cells free in the haemocoel of the aphid. Staining within the embryo blastoderm suggests over expression of antigen, perhaps indicative of rapid bacterial division immediately following infection.

Population dynamics of male-killing and non-male-killing spiroplasmas in Drosophila melanogaster

The endosymbiotic bacteria Spiroplasma spp. are vertically transmitted through female hosts and are known to cause selective death of male offspring in insects. One strain of spiroplasma, NSRO, causes male killing in Drosophila species, and a non-male-killing variant of NSRO, designated NSRO-A, has been isolated. It is not known why NSRO-A does not kill males. In an attempt to understand the mechanism of male killing, we investigated the population dynamics of NSRO and NSRO-A throughout the developmental course of the laboratory host Drosophila melanogaster by using a quantitative PCR technique. In the early development of the host insect, the titers of NSRO were significantly higher than those of NSRO-A at the first- and second-instar stages, whereas at the egg, third-instar, and pupal stages, the titers of the two spiroplasmas were almost the same. Upon adult emergence, the titers of the two spiroplasmas were similar, around 2 x 10(8) dnaA copy equivalents. However, throughout host aging, the two spiroplasmas showed strikingly different population growth patterns. The titers of NSRO increased exponentially for 3 weeks, attained a peak value of around 4 x 10(9) dnaA copy equivalents per insect, and then decreased. In contrast, the titers of NSRO-A were almost constant throughout the adult portion of the life cycle. In adult females, consequently, the titer of NSRO was significantly higher than the titer of NSRO-A except for a short period just after emergence. Although infection of adult females with NSRO resulted in almost 100% male killing, production of some male offspring was observed within 4 days after emergence when the titers of NSRO were as low as those of NSRO-A. Based on these results, we proposed a threshold density hypothesis for the expression of male killing caused by the spiroplasma. The extents of the bottleneck in the vertical transmission through host generations were estimated to be 5 x 10(-5) for NSRO and 3 x 10(-4) for NSRO-A.

The impact of host plant on the abundance and function of symbiotic bacteria in an aphid

The black-bean aphid Aphis fabae bears populations of coccoid symbiotic bacteria Buchnera spp. at 2.0–3.2×107cellsmg−1aphidmass and rod-shaped secondary symbionts of uncertain taxonomic affiliation at 0.1–0.6×107cellsmg−1aphidmass. Buchnera provides essential amino acids, supplementing the poor supply in the aphid diet of plant phloem sap. Comparison of the performance of A. fabae containing and experimentally deprived of their bacteria showed that the bacteria caused increased larval mass of aphids reared on Chenopodium album and Papaver dubium plants, but not when reared on Lamium purpureum. In the aphids reared on L. purpureum, the density of the bacteria, especially the secondary symbionts, was significantly elevated, and bacterial-mediated production of the essential amino acid threonine was reduced, even though the essential amino acid content of phloem exudates from L. purpureum had a low threonine content. It is proposed that the shortfall in threonine, possibly compounded by the high density of secondary symbionts, may contribute to the poor performance of the aphids on L. purpureum. This study offers the first evidence to suggest plant-mediated interference with the nutritional function of symbiotic bacteria in any phytophagous insect.

Low genetic diversity among pea aphid (Acyrthosiphon pisum) biotypes of different plant affiliation

Genetic diversity in the pea aphid Acyrthosiphon pisum was investigated by a restriction fragment length polymorphism (RFLP) analysis of three maternally inherited genomes (mitochondrial DNA and plasmids of the symbiotic bacteria Buchnera). Twenty-nine parthenogenetic clones of three A. pisum biotypes, defined by their capacity to use the legume crops pea, alfalfa and red clover, respectively, were analysed, and a total of 67 restriction sites was scored. No restriction site variation in the mitochondrial genome was obtained, but length variation at two regions (the A + T-rich region and ND3-ND5 region) was noted. One aphid clone bore a variant HindIII restriction site in the Buchnera leucine plasmid (pAPEleu), and two clones were heteroplasmic for a 0.76-kb deletion in the Buchnera tryptophan plasmid (pAPEtrp). Based on arthropod nucleotide substitution rates, it is proposed that the crop-feeding biotypes of A. pisum may have diversified within the last 100 000 years and possibly much more recently, since the advent of agriculture.