Aphid endosymbiont facilitates virus transmission by modulating the volatile profile of host plants

BACKGROUND

Most plant viruses rely on vectors for their transmission and spread. One of the outstanding biological questions concerning the vector-pathogen-symbiont multi-trophic interactions is the potential involvement of vector symbionts in the virus transmission process. Here, we used a multi-factorial system containing a non-persistent plant virus, cucumber mosaic virus (CMV), its primary vector, green peach aphid, Myzus persicae, and the obligate endosymbiont, Buchnera aphidicola to explore this uncharted territory.

RESULTS

Based on our preliminary research, we hypothesized that aphid endosymbiont B. aphidicola can facilitate CMV transmission by modulating plant volatile profiles. Gene expression analyses demonstrated that CMV infection reduced B. aphidicola abundance in M. persicae, in which lower abundance of B. aphidicola was associated with a preference shift in aphids from infected to healthy plants. Volatile profile analyses confirmed that feeding by aphids with lower B. aphidicola titers reduced the production of attractants, while increased the emission of deterrents. As a result, M. persicae changed their feeding preference from infected to healthy plants.

CONCLUSIONS

We conclude that CMV infection reduces the B. aphidicola abundance in M. persicae. When viruliferous aphids feed on host plants, dynamic changes in obligate symbionts lead to a shift in plant volatiles from attraction to avoidance, thereby switching insect vector's feeding preference from infected to healthy plants.

Coinfection of the secondary symbionts, Hamiltonella defensa and Arsenophonus sp. contribute to the performance of the major aphid pest, Aphis gossypii (Hemiptera: Aphididae)

Bacterial endosymbionts play important roles in ecological traits of aphids. In this study, we characterize the bacterial endosymbionts of A. gossypii collected in Karaj, Iran and their role in the performance of the aphid. Our results indicated that beside Buchnera aphidicola, A. gossypii, also harbors both Hamiltonella defensa and Arsenophonus sp. Quantitative PCR (qPCR) results revealed that the populations of the endosymbionts increased throughout nymphal development up to adult emergence; thereafter, populations of Buchnera and Arsenophonus were diminished while the density of H. defensa constantly increased. Buchnera reduction caused prolonged development and no progeny production. Furthermore, secondary symbiont reduction led to reduction of the total life span and intrinsic rate of natural increase as well as appearance of the deformed dead offspring in comparison with the control insects. Reduction of the secondary symbionts did not affect parasitism rate of the aphid by the parasitic wasp Aphidius matricariae. Together these findings showed that H. defensa and Arsenophonus contributed to the fitness of A. gossypii by enhancing its performance, but not through parasitoid resistance.

Differential accumulation of leucine and methionine in red and green pea aphids leads to different fecundity in response to nitrogen fertilization

BACKGROUND

Nitrogen fertilization affects plants directly and herbivorous insects indirectly. Although insect species and even genotypes are known to differ in their responses to nitrogen fertilization, the physiological and molecular mechanisms remain unclear. This study assessed the fecundity and related regulatory signaling pathways in the green and red morphs of pea aphid (Acyrthosiphon pisum) feeding on Medicago truncatula with and without nitrogen fertilization.

RESULTS

Nitrogen fertilization significantly increased foliar amino acid concentrations and consequently increased the concentrations of several individual essential amino acids in body tissue of the green morph. The increased concentration of Leu, Ile, Met and Val was consistent with enhanced biosynthesis of these amino acids in the endosymbiont Buchnera. Under nitrogen fertilization, Leu and Met accumulated in the green morph enhanced the target of rapamycin (TOR) signaling pathway, which consequently increased fecundity by promoting vitellogenin synthesis. In the red morph, however, nitrogen fertilization did not change the concentration of essential amino acids, TOR signaling or fecundity.

CONCLUSION

Specific amino acids accumulation and the nutrient transduction pathway in pea aphids are responsible for genotype-specific fecundity in response to nitrogen fertilization, which could be used as potential target for pest control. © 2018 Society of Chemical Industry.

Targeting symbiosis-related insect genes by RNAi in the pea aphid-Buchnera symbiosis

The growth and reproduction of phloem sap-feeding insects requires the sustained function of intracellular bacteria localized in specialized cells known as bacteriocytes, giving the potential to target the bacterial symbiosis as a novel strategy for controlling sap-feeding insect pests. We focused on two genes in the pea aphid Acyrthosiphon pisum, amiD and ldcA1, which were acquired horizontally from bacteria and have the annotated function to degrade immunogenic bacterial peptidoglycan. We hypothesized that AmiD and LdcA1 function to eliminate peptidoglycan fragments released by the bacterial symbiont Buchnera inhabiting the bacteriocytes, thereby protecting the Buchnera from host attack. Consistent with this hypothesis, expression of amiD and ldcA1 was enriched in bacteriocytes and varied significantly with aphid age, conforming to an inverse curvilinear relationship for amiD and negative linear relationship for ldcA1. RNAi against amiD and ldcA1 administered orally to larval pea aphids caused a significant reduction in Buchnera abundance and activity, accompanied by depressed aphid growth rates. For RNAi experiments, the aphids were co-administered with dsRNA against an aphid nuclease nuc1, protecting the dsRNA against non-specific degradation. These experiments demonstrate that selective suppression of insect symbiosis-related gene function can reduce the performance of an insect pest. Phylogenetic analysis identified amiD and ldcA1 in sequenced genomes of other aphid species, and amiD in related groups of phloem-feeding insects, offering the opportunity for specific controls against a range of insect pests.

Probing behavior of aposymbiotic green peach aphid (Myzus persicae) on susceptible Solanum tuberosum and resistant Solanum stoloniferum plants

The green peach aphid, Myzus persicae Sulzer (Hemiptera: Aphididae) is one of the potato important pests; it is the most efficient vector of potato viruses. Myzus persicae harbors the endosymbiotic bacteria Buchnera aphidicola which supplements their diet. There is increasing evidence that B. aphidicola is involved in plant-aphid interactions and we previously demonstrated that B. aphidicola disruption (aposymbiosis) affected the probing behavior of M. persicae on radish plants, delaying host plant acceptance. In this work, we evaluated the effect of aposymbiosis on the probing behavior of M. persicae on 2 Solanum species with different compatibility with M. persicae, Solanum tuberosum (susceptible) and Solanum stoloniferum (resistant) with the electrical penetration graph technique (EPG). To disrupt B. aphidicola, rifampicin was administered to aphids through artificial diets. Aposymbiotic aphids, on both plant species, showed increased pathway activities, mechanical problems with the stylets, and delayed salivation in the phloem. The extended time in derailed stylet mechanics affected the occurrence of most other probing activities; it delayed the time to the first phloem phase and prevented ingestion from the phloem. The effect of aposymbiosis was more evident in the compatible interaction of M. persicae-S. tuberosum, than in the incompatible interaction with S. stoloniferum, which generated the M. persicae-S. tuberosum interaction to become incompatible. These results confirm that B. aphidicola is involved in the plant-aphid interaction in relation to plant acceptance, presumably through a role in stylets penetration in the plant.

A metagenomic approach from aphid's hemolymph sheds light on the potential roles of co-existing endosymbionts

BACKGROUND

Aphids are known to live in symbiosis with specific bacteria, called endosymbionts which can be classified as obligate or accessory. Buchnera aphidicola is generally the only obligatory symbiont present in aphids, supplying essential nutrients that are missing in the plants phloem to its host. Pentalonia nigronervosa is the main vector of the banana bunchy top virus, one of the most damageable viruses in banana. This aphid is carrying two symbionts: B. aphidicola (BPn) and Wolbachia sp. (wPn). The high occurrence of Wolbachia in the banana aphid raises questions about the role it plays in this insect. The goal of this study was to go further in the understanding of the role played by the two symbionts in P. nigronervosa. To do so, microinjection tests were made to see the effect of wPn elimination on the host, and then, high-throughput sequencing of the haemolymph was used to analyze the gene content of the symbionts.

RESULTS

We observed that the elimination of wPn systematically led to the death of aphids, suggesting that the bacterium could play a mutualistic role. In addition, we identify and annotate 587 and 250 genes for wPn and BPn, respectively, through high-throughput sequencing. Analysis of these genes suggests that the two bacteria are working together for the production of several essential nutrients. The most striking cases are for lysin and riboflavin which are usually provided by B. aphidicola alone to the host. In the banana aphid, the genes involved in the production pathways of these metabolites are shared between the two bacteria making them both essential for the survival of the aphid host.

CONCLUSIONS

Our results suggest that a co-obligatory symbiosis between B. aphidicola and Wolbachia occurs in the banana aphid, the two bacteria acting together to supply essential nutrients to the host. This is, to our knowledge, the first time Wolbachia is reported to play an essential role in aphids.

Suppression of plant defenses by a Myzus persicae (green peach aphid) salivary effector protein

The complex interactions between aphids and their host plant are species-specific and involve multiple layers of recognition and defense. Aphid salivary proteins, which are released into the plant during phloem feeding, are a likely mediator of these interactions. In an approach to identify aphid effectors that facilitate feeding from host plants, eleven Myzus persicae (green peach aphid) salivary proteins and the GroEL protein of Buchnera aphidicola, a bacterial endosymbiont of this aphid species, were expressed transiently in Nicotiana tabacum (tobacco). Whereas two salivary proteins increased aphid reproduction, expression of three other aphid proteins and GroEL significantly decreased aphid reproduction on N. tabacum. These effects were recapitulated in stable transgenic Arabidopsis thaliana plants. Further experiments with A. thaliana expressing Mp55, a salivary protein that increased aphid reproduction, showed lower accumulation of 4-methoxyindol-3-ylmethylglucosinolate, callose and hydrogen peroxide in response to aphid feeding. Mp55-expressing plants also were more attractive for aphids in choice assays. Silencing Mp55 gene expression in M. persicae using RNA interference approaches reduced aphid reproduction on N. tabacum, A. thaliana, and N. benthamiana. Together, these results demonstrate a role for Mp55, a protein with as-yet-unknown molecular function, in the interaction of M. persicae with its host plants.

Aphis (Hemiptera: Aphididae) species groups found in the Midwestern United States and their contribution to the phylogenetic knowledge of the genus

A phylogeny of the genus Aphis Linnaeus, 1 758 was built primarily from specimens collected in the Midwest of the United States. A data matrix was constructed with 68 species and 41 morphological characters with respective character states of alate and apterous viviparous females. Dendrogram topologies of analyses performed using UPGMA (Unweighted Pair Group Method with Arithmetic Mean), Maximum Parsimony and Bayesian analysis of Cytochrome Oxidase I, Elongation Factor 1-α and primary endosymbiont Buchnera aphidicola 16S sequences were not congruent. Bayesian analysis strongly supported most terminal nodes of the phylogenetic trees. The phylogeny was strongly supported by EF1-α, and analysis of COI and EF1-α molecular data combined with morphological characters. It was not supported by single analysis of COI or Buchnera aphidicola 16S. Results from the Bayesian phylogeny show 4 main species groups: asclepiadis, fabae, gossypii, and middletonii. Results place Aphis and species of the genera Protaphis Börner, 1952, Toxoptera Koch, and Xerobion Nevsky, 1928 in a monophyletic clade. Morphological characters support this monophyly as well. The phylogeny shows that the monophyletic clade of the North American middletonii species group belong to the genus Protaphis: P. debilicornis (Gillette & Palmer, 1929), comb. nov., P. echinaceae (Lagos and Voegtlin, 2009), comb. nov., and P. middletonii (Thomas, 1879). The genus Toxoptera should be considered a subgenus of Aphis (stat. nov.). The analysis also indicates that the current genus Iowana Frison, 1954 should be considered a subgenus of Aphis (stat. nov.).

Exploring the bacterial microbiota associated with native South American species of Aphis (Hemiptera: Aphididae)

Aphids harbor a variety of bacterial endosymbionts, including the obligate symbiont Buchnera aphidicola and diverse facultative symbionts. The former supplies its host with essential amino acids. The latter are not indispensable for insect survival, but often improve their host's fitness. To date, the study of such associations was restricted to aphids of Holarctic origin. The bacterial microbiota of seven Aphis species from Argentina was investigated. The presence of B. aphidicola was assessed by specific PCR. Additional symbionts were identified through PCR with eubacterial universal primers, cloning, and sequencing of nearly complete 16S rRNA gene, intergenic spacer region, and partial 23S rRNA gene and subjected to phylogenetic analysis. Infection with B. aphidicola was confirmed in every species analyzed. The facultative symbiont Serratia symbiotica was detected in Aphis malalhuina Mier Durante, Nieto Nafría & Ortego, 2003, Aphis senecionicoides Blanchard, 1944, and Aphis schinifoliae Blanchard, 1939, while Hamiltonella defensa was identified in Aphis mendocina Mier Durante, Ortego & Nieto Nafría, 2006. Arsenophonus sp. was found infecting Aphis melosae Mier Durante & Ortego, 1999, and a new, undescribed Aphis sp. In Aphis danielae Remaudière, 1994, no facultative symbionts could be recorded. When analyzing the highly conserved 16S rRNA gene, the phylogenetic tree grouped the S. symbiotica, H. defensa, and Arsenophonus isolates into three well-defined clusters showing little variability among clones corresponding to the same aphid host species. This article reports for the first time the endosymbionts associated with aphids native to South America. Despite their geographic origin, the qualitative composition of their microbiota revealed no evident differences from that described for aphids in the Northern Hemisphere.

Evolutionary relationships of Pemphigus and allied genera (Hemiptera: Aphididae: Eriosomatinae) and their primary endosymbiont, Buchnera aphidicola

Aphids harbor primary endosymbionts, Buchnera aphidicola, in specialized cells within their body cavities. Aphids and Buchnera have strict mutualistic relationships in nutrition exchange. This ancient association has received much attention from researchers who are interested in endosymbiotic evolution. Previous studies have found parallel phylogenetic relationships between non-galling aphids and Buchnera at lower taxonomic levels (genus, species). To understand whether relatively isolated habitats such as galls have effect on the parallel relationships between aphids and Buchnera, the present paper investigated the phylogenetic relationships of gall aphids from Pemphigus and allied genera, which induce pseudo-galls or galls on Populus spp. (poplar) and Buchnera. The molecular phylogenies inferred from three aphid genes (COI, COII and EF-1α) and two Buchnera genes (gnd, 16S rRNA gene) indicated significant congruence between aphids and Buchnera at generic as well as interspecific levels. Interestingly, both aphid and Buchnera phylogenies supported three main clades corresponding to the galling locations of aphids, namely leaf, the joint of leaf blade and petiole, and branch of the host plant. The results suggest phylogenetic conservatism of gall characters, which indicates gall characters are more strongly affected by aphid phylogeny, rather than host plants.

The combined effects of bacterial symbionts and aging on life history traits in the pea aphid, Acyrthosiphon pisum

While many endosymbionts have beneficial effects on hosts under specific ecological conditions, there can also be associated costs. In order to maximize their own fitness, hosts must facilitate symbiont persistence while preventing symbiont exploitation of resources, which may require tight regulation of symbiont populations. As a host ages, the ability to invest in such mechanisms may lessen or be traded off with demands of other life history traits, such as survival and reproduction. Using the pea aphid, Acyrthosiphon pisum, we measured survival, lifetime fecundity, and immune cell counts (hemocytes, a measure of immune capacity) in the presence of facultative secondary symbionts. Additionally, we quantified the densities of the obligate primary bacterial symbiont, Buchnera aphidicola, and secondary symbionts across the host's lifetime. We found life history costs to harboring some secondary symbiont species. Secondary symbiont populations were found to increase with host age, while Buchnera populations exhibited a more complicated pattern. Immune cell counts peaked at the midreproductive stage before declining in the oldest aphids. The combined effects of immunosenescence and symbiont population growth may have important consequences for symbiont transmission and maintenance within a host population.

Pea aphid promotes amino acid metabolism both in Medicago truncatula and bacteriocytes to favor aphid population growth under elevated CO2

Rising atmospheric CO(2) levels can dilute the nitrogen (N) resource in plant tissue, which is disadvantageous to many herbivorous insects. Aphids appear to be an exception that warrants further study. The effects of elevated CO(2) (750 ppm vs. 390 ppm) were evaluated on N assimilation and transamination by two Medicago truncatula genotypes, a N-fixing-deficient mutant (dnf1) and its wild-type control (Jemalong), with and without pea aphid (Acyrthosiphon pisum) infestation. Elevated CO(2) increased population abundance and feeding efficiency of aphids fed on Jemalong, but reduced those on dnf1. Without aphid infestation, elevated CO(2) increased photosynthetic rate, chlorophyll content, nodule number, biomass, and pod number for Jemalong, but only increased pod number and chlorophyll content for dnf1. Furthermore, aphid infested Jemalong plants had enhanced activities of N assimilation-related enzymes (glutamine synthetase, Glutamate synthase) and transamination-related enzymes (glutamate oxalate transaminase, glutamine phenylpyruvate transaminase), which presumably increased amino acid concentration in leaves and phloem sap under elevated CO(2). In contrast, aphid infested dnf1 plants had decreased activities of N assimilation-related enzymes and transmination-related enzymes and amino acid concentrations under elevated CO(2). Furthermore, elevated CO(2) up-regulated expression of genes relevant to amino acid metabolism in bacteriocytes of aphids associated with Jemalong, but down-regulated those associated with dnf1. Our results suggest that pea aphids actively elicit host responses that promote amino acid metabolism in both the host plant and in its bacteriocytes to favor the population growth of the aphid under elevated CO(2).

Deep sequencing of the transcriptomes of soybean aphid and associated endosymbionts

BACKGROUND

The soybean aphid has significantly impacted soybean production in the U.S. Transcriptomic analyses were conducted for further insight into leads for potential novel management strategies.

METHODOLOGY/PRINCIPAL FINDINGS

Transcriptomic data were generated from whole aphids and from 2,000 aphid guts using an Illumina GAII sequencer. The sequence data were assembled de novo using the Velvet assembler. In addition to providing a general overview, we demonstrate (i) the use of the Multiple-k/Multiple-C method for de novo assembly of short read sequences, followed by BLAST annotation of contigs for increased transcript identification: From 400,000 contigs analyzed, 16,257 non-redundant BLAST hits were identified; (ii) analysis of species distributions of top non-redundant hits: 80% of BLAST hits (minimum e-value of 1.0-E3) were to the pea aphid or other aphid species, representing about half of the pea aphid genes; (iii) comparison of relative depth of sequence coverage to relative transcript abundance for genes with high (membrane alanyl aminopeptidase N) or low transcript abundance; (iv) analysis of the Buchnera transcriptome: Transcripts from 57.6% of the genes from Buchnera aphidicola were identified; (v) identification of Arsenophonus and Wolbachia as potential secondary endosymbionts; (vi) alignment of full length sequences from RNA-seq data for the putative salivary gland protein C002, the silencing of which has potential for aphid management, and the putative Bacillus thuringiensis Cry toxin receptors, aminopeptidase N and alkaline phosphatase.

CONCLUSIONS/SIGNIFICANCE

THIS STUDY PROVIDES THE MOST COMPREHENSIVE DATA SET TO DATE FOR SOYBEAN APHID GENE EXPRESSION: This work also illustrates the utility of short-read transcriptome sequencing and the Multiple-k/Multiple-C method followed by BLAST annotation for rapid identification of target genes for organisms for which reference genome sequences are not available, and extends the utility to include the transcriptomes of endosymbionts.

Multiple origins of endosymbiosis within the Enterobacteriaceae (γ-Proteobacteria): convergence of complex phylogenetic approaches

BACKGROUND

The bacterial family Enterobacteriaceae gave rise to a variety of symbiotic forms, from the loosely associated commensals, often designated as secondary (S) symbionts, to obligate mutualists, called primary (P) symbionts. Determination of the evolutionary processes behind this phenomenon has long been hampered by the unreliability of phylogenetic reconstructions within this group of bacteria. The main reasons have been the absence of sufficient data, the highly derived nature of the symbiont genomes and lack of appropriate phylogenetic methods. Due to the extremely aberrant nature of their DNA, the symbiotic lineages within Enterobacteriaceae form long branches and tend to cluster as a monophyletic group. This state of phylogenetic uncertainty is now improving with an increasing number of complete bacterial genomes and development of new methods. In this study, we address the monophyly versus polyphyly of enterobacterial symbionts by exploring a multigene matrix within a complex phylogenetic framework.

RESULTS

We assembled the richest taxon sampling of Enterobacteriaceae to date (50 taxa, 69 orthologous genes with no missing data) and analyzed both nucleic and amino acid data sets using several probabilistic methods. We particularly focused on the long-branch attraction-reducing methods, such as a nucleotide and amino acid data recoding and exclusion (including our new approach and slow-fast analysis), taxa exclusion and usage of complex evolutionary models, such as nonhomogeneous model and models accounting for site-specific features of protein evolution (CAT and CAT+GTR). Our data strongly suggest independent origins of four symbiotic clusters; the first is formed by Hamiltonella and Regiella (S-symbionts) placed as a sister clade to Yersinia, the second comprises Arsenophonus and Riesia (S- and P-symbionts) as a sister clade to Proteus, the third Sodalis, Baumannia, Blochmannia and Wigglesworthia (S- and P-symbionts) as a sister or paraphyletic clade to the Pectobacterium and Dickeya clade and, finally, Buchnera species and Ishikawaella (P-symbionts) clustering with the Erwinia and Pantoea clade.

CONCLUSIONS

The results of this study confirm the efficiency of several artifact-reducing methods and strongly point towards the polyphyly of P-symbionts within Enterobacteriaceae. Interestingly, the model species of symbiotic bacteria research, Buchnera and Wigglesworthia, originated from closely related, but different, ancestors. The possible origins of intracellular symbiotic bacteria from gut-associated or pathogenic bacteria are suggested, as well as the role of facultative secondary symbionts as a source of bacteria that can gradually become obligate maternally transferred symbionts.

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.

Systemic analysis of the symbiotic function of Buchnera aphidicola, the primary endosymbiont of the pea aphid Acyrthosiphon pisum

Buchnera aphidicola is the primary obligate intracellular symbiont of most aphid species. B. aphidicola and aphids have been evolving in parallel since their association started, about 150 Myr ago. Both partners have lost their autonomy, and aphid diversification has been confined to smaller ecological niches by this co-evolution. B. aphidicola has undergone major genomic and biochemical changes as a result of adapting to intracellular life. Several genomes of B. aphidicola from different aphid species have been sequenced in the last decade, making it possible to carry out analyses and comparative studies using system-level in silico methods. This review attempts to provide a systemic description of the symbiotic function of aphid endosymbionts, particularly of B. aphidicola from the pea aphid Acyrthosiphon pisum, by analyzing their structural genomic properties, as well as their genetic and metabolic networks.

Post-Pleistocene radiation of the pea aphid complex revealed by rapidly evolving endosymbionts

Adaptation to different resources has the potential to cause rapid species diversification, but few studies have been able to quantify the time scale of recent adaptive radiations. The pea aphid, Acyrthosiphon pisum, a model of speciation for host-specialized parasites, consists of several biotypes (races or species) living on distinct legume hosts. To document this radiation, we used rapidly evolving sequences from Buchnera, the maternally transmitted bacterial endosymbiont of aphids. Analyses of Buchnera pseudogene sequences revealed that 11 host-associated biotypes sort mostly into distinct matrilines despite low sequence divergence. A calibration based on divergence times of 7 sequenced genomes of Buchnera allowed us to date the last maternal ancestor of these biotypes between 8,000 and 16,000 years, with a burst of diversification at an estimated 3,600-9,500 years. The recency of this diversification, which is supported by microsatellite data, implies that the pea aphid complex ranks among the most rapid adaptive radiations yet documented. This diversification coincides with post-Pleistocene warming and with the domestication and anthropogenic range expansion of several of the legume hosts of pea aphids. Thus, we hypothesize that the new availability or abundance of resources triggered a cascade of divergence events in this newly formed complex.

Two species of symbiotic bacteria present in the soybean aphid (Hemiptera: Aphididae)

Aphids, which feed solely on plant phloem sap, have developed symbiotic associations with bacteria that provide them with the amino acids that are lacking in phloem. Three soybean aphid (Aphis glycines Mat samura) populations were screened for the presence of Buchnera aphidicola and three common species of secondary aphid symbionts (Serratia symbiotica, Hamiltonella defensa, and Regiella insecticola). Diagnostic polymerase chain reaction and subsequent DNA sequencing showed the presence of two species of symbiotic bacteria present in all three soybean aphid populations tested: B. aphidicola and Arsenophonus sp. Although Buchnera is commonly found in aphids, Arsenophonus is most commonly found in whiteflies (Hemiptera: Aleyrodidae), making the soybean aphid unique among aphids that have been tested for the presence of Arsenophonus.

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.

Evolution of the secondary symbiont "Candidatus serratia symbiotica" in aphid species of the subfamily lachninae

Buchnera aphidicola BCc, the primary endosymbiont of the aphid Cinara cedri (subfamily Lachninae), is losing its symbiotic capacity and might be replaced by the coresident "Candidatus Serratia symbiotica." Phylogenetic and morphological analyses within the subfamily Lachninae indicate two different "Ca. Serratia symbiotica" lineages and support the longtime coevolution of both symbionts in C. cedri.

Antibiotics, primary symbionts and wing polyphenism in three aphid species

The possible role of the primary Buchnera symbionts in wing polyphenism is examined in three aphid species. Presumptive winged aphids were fed on antibiotic-treated beans to destroy these symbionts. As previously reported, this leads to inhibited growth and low/zero fecundity. When such treatment is applied to the short-day-induced gynoparae (the winged autumn migrant) of the black bean aphid, Aphis fabae, it also causes many insects to develop as wingless or winged/wingless intermediate adult forms (apterisation). However, whilst antibiotic treatment of crowd-induced, long-day winged forms of the pea aphid, Acyrthosiphon pisum (a green and a pink clone) and the vetch aphid, Megoura viciae has similar effects on size and fecundity, it does not affect wing development. Food deprivation also promotes apterisation in A. fabae gynoparae but not in the crowd-induced winged morphs of the other two species. Thus, it appears that apterisation in A. fabae is not a direct effect of antibiotic treatment or a novel role for symbionts but is most likely related to impaired nutrition induced by the loss of the symbiont population.

Genetic evidence from mitochondrial, nuclear, and endosymbiont markers for the evolution of host plant associated species in the aphid genus Hyalopterus (Hemiptera: Aphididae)

Over the past several decades biologists' fascination with plant-herbivore interactions has generated intensive research into the implications of these interactions for insect diversification. The study of closely related phytophagous insect species or populations from an evolutionary perspective can help illuminate ecological and selective forces that drive these interactions. Here we present such an analysis for aphids in the genus Hyalopterus (Hemiptera: Aphididae), a cosmopolitan group that feeds on plants in the genus Prunus (Rosaceae). Hyalopterus currently contains two recognized species associated with different Prunus species, although the taxonomy and evolutionary history of the group is poorly understood. Using mitochondrial COI sequences, 16S rDNA sequences from the aphid endosymbiont Buchnera aphidicola, and nine microsatellite loci we investigated population structure in Hyalopterus from the most commonly used Prunus host species throughout the Mediterranean as well as in California, where the species H. pruni is an invasive pest. We found three deeply divergent lineages structured in large part by specific associations with plum, almond, and peach trees. There was no evidence that geographic or temporal barriers could explain the overall diversity in the genus. Levels of genetic differentiation are consistent with that typically attributed to aphid species and indicate divergence times older than the domestication of Prunus for agriculture. Interestingly, in addition to their typical hosts, aphids from each of the three lineages were frequently found on apricot trees. Apricot also appears to act as a resource mediated hybrid zone for plum and almond associated lineages. Together, results suggest that host plants have played a role in maintaining host-associated differentiation in Hyalopterus for as long as several million years, despite worldwide movement of host plants and the potential for ongoing hybridization.

Strict host-symbiont cospeciation and reductive genome evolution in insect gut bacteria

Host-symbiont cospeciation and reductive genome evolution have been identified in obligate endocellular insect symbionts, but no such example has been identified from extracellular ones. Here we first report such a case in stinkbugs of the family Plataspidae, wherein a specific gut bacterium is vertically transmitted via “symbiont capsule.” In all of the plataspid species, females produced symbiont capsules upon oviposition and their gut exhibited specialized traits for capsule production. Phylogenetic analysis showed that the plataspid symbionts constituted a distinct group in the γ-Proteobacteria, whose sister group was the aphid obligate endocellular symbionts Buchnera. Removal of the symbionts resulted in retarded growth, mortality, and sterility of the insects. The host phylogeny perfectly agreed with the symbiont phylogeny, indicating strict host-symbiont cospeciation despite the extracellular association. The symbionts exhibited AT-biased nucleotide composition, accelerated molecular evolution, and reduced genome size, as has been observed in obligate endocellular insect symbionts. These findings suggest that not the endocellular conditions themselves but the population genetic attributes of the vertically transmitted symbionts are probably responsible for the peculiar genetic traits of these insect symbionts. We proposed the designation “Candidatus Ishikawaella capsulata” for the plataspid symbionts. The plataspid stinkbugs, wherein the host-symbiont associations can be easily manipulated, provide a novel system that enables experimental approaches to previously untouched aspects of the insect-microbe mutualism. Furthermore, comparative analyses of the sister groups, the endocellular Buchnera and the extracellular Ishikawaella, would lead to insights into how the different symbiotic lifestyles have affected their genomic evolution.

Evidence of host-symbiont cospeciation in an insect gut symbiont suggests that long-term vertical transmission and population structure are central forces driving the genomic changes characteristic of insect nutritional symbionts.

A small microbial genome: the end of a long symbiotic relationship?

Intracellular bacteria are characterized by genome reduction. The 422,434-base pair genome of Buchnera aphidicola BCc, primary endosymbiont of the aphid Cinara cedri, is approximately 200 kilobases smaller than the previously sequenced B. aphidicola genomes. B. aphidicola BCc has lost most metabolic functions, including the ability to synthesize the essential amino acid tryptophan and riboflavin. In addition, most retained genes are evolving rapidly. Possibly, B. aphidicola BCc is losing its symbiotic capacity and is being complemented (and might be replaced) by the highly abundant coexisting secondary symbiont.

Hundreds of flagellar basal bodies cover the cell surface of the endosymbiotic bacterium Buchnera aphidicola sp. strain APS

Buchnera aphidicola is the endosymbiotic bacterium of the pea aphid. Due to its small genome size, Buchnera lacks many essential genes for autogenous life but obtains nutrients from the host. Although the Buchnera cell is nonmotile, it retains clusters of flagellar genes that lack the late genes necessary for motility, including the flagellin gene. In this study, we show that the flagellar genes are actually transcribed and translated and that the Buchnera cell surface is covered with hundreds of hook-basal-body (HBB) complexes. The abundance of HBB complexes suggests a role other than motility. We discuss the possibility that the HBB complex may serve as a protein transporter not only for the flagellar proteins but also for other proteins to maintain the symbiotic system.

The players in a mutualistic symbiosis: insects, bacteria, viruses, and virulence genes

Aphids maintain mutualistic symbioses involving consortia of coinherited organisms. All possess a primary endosymbiont, Buchnera, which compensates for dietary deficiencies; many also contain secondary symbionts, such as Hamiltonella defensa, which confers defense against natural enemies. Genome sequences of uncultivable secondary symbionts have been refractory to analysis due to the difficulties of isolating adequate DNA samples. By amplifying DNA from hemolymph of infected pea aphids, we obtained a set of genomic sequences of H. defensa and an associated bacteriophage. H. defensa harbors two type III secretion systems, related to those that mediate host cell entry by enteric pathogens. The phage, called APSE-2, is a close relative of the previously sequenced APSE-1 but contains intact homologs of the gene encoding cytolethal distending toxin (cdtB), which interrupts the eukaryotic cell cycle and which is known from a variety of mammalian pathogens. The cdtB homolog is highly expressed, and its genomic position corresponds to that of a homolog of stx (encoding Shiga-toxin) within APSE-1. APSE-2 genomes were consistently abundant in infected pea aphids, and related phages were found in all tested isolates of H. defensa, from numerous insect species. Based on their ubiquity and abundance, these phages appear to be an obligate component of the H. defensa life cycle. We propose that, in these mutualistic symbionts, phage-borne toxin genes provide defense to the aphid host and are a basis for the observed protection against eukaryotic parasites.

Functional genomics of Buchnera and the ecology of aphid hosts

In many animal groups, mutualistic bacterial symbionts play a central role in host ecology, by provisioning rare nutrients and thus enabling specialization on restricted diets. Among such symbionts, genomic studies are most advanced for Buchnera, the obligate symbiont of aphids, which feed on phloem sap. The contents of the highly reduced Buchnera genomes have verified its role in aphid nutrition. Comparisons of Buchnera gene sets indicate ongoing, irreversible gene losses that are expected to affect aphid nutritional needs. Furthermore, almost all regulatory genes have been eliminated, raising the question of whether and how gene expression responds to environmental change. Microarray studies on genome-wide expression indicate that Buchnera has evolved some constitutive changes in gene expression: homologues of heat stress genes have elevated transcript levels in Buchnera (relative to other bacteria) even in the absence of stress. Additionally, the microarray results indicate that responses to heat stress and to amino acid availability are both few and modest. Observed responses are consistent with control by the few ancestral regulators retained in the genome. Initial studies on the role of host genes in mediating the symbiosis reveal distinctive expression patterns in host cells harbouring Buchnera. In the near future, a complete genome of pea aphid will accelerate progress in understanding the functional integration of aphid and Buchnera genomes. Although information for other insect symbioses is relatively limited, studies on symbionts of carpenter ants and tsetse flies indicate many similarities to Buchnera.

Regulation of transcription in a reduced bacterial genome: nutrient-provisioning genes of the obligate symbiont Buchnera aphidicola

Buchnera aphidicola, the obligate symbiont of aphids, has an extremely reduced genome, of which about 10% is devoted to the biosynthesis of essential amino acids needed by its hosts. Most regulatory genes for these pathways are absent, raising the question of whether and how transcription of these genes responds to the major shifts in dietary amino acid content encountered by aphids. Using full-genome microarrays for B. aphidicola of the host Schizaphis graminum, we examined transcriptome responses to changes in dietary amino acid content and then verified behavior of individual transcripts using quantitative reverse transcriptase PCR. The only gene showing a consistent and substantial (>twofold) response was metE, which underlies methionine biosynthesis and which is the only amino acid biosynthetic gene retaining its ancestral regulator (metR). In another aphid host, Acyrthosiphon pisum, B. aphidicola has no functional metR and shows no response in metE transcript levels to changes in amino acid concentrations. Thus, the only substantial transcriptional response involves the one gene for which an ancestral regulator is retained. This result parallels that from a previous study on heat stress, in which only the few genes retaining the global heat shock promoter showed responses in transcript abundance. The irreversible losses of transcriptional regulators constrain ability to alter gene expression in the context of environmental fluctuations affecting the symbiotic partners.

Transcriptome analysis of the aphid bacteriocyte, the symbiotic host cell that harbors an endocellular mutualistic bacterium, Buchnera

Aphids possess bacteriocytes, cells specifically differentiated to harbor obligatory mutualistic bacteria of the genus Buchnera, which have lost many genes that are essential for common bacterial functions. To understand the host's role in maintaining the symbiotic relationship, bacteriocytes were isolated from the pea aphid, Acyrthosiphon pisum, and the host transcriptome was investigated by using EST analysis and real-time quantitative RT-PCR. A number of genes were highly expressed specifically in the bacteriocyte, including (i) genes for amino acid metabolism, including those for biosynthesis of amino acids that Buchnera cannot produce, and those for utilization of amino acids that Buchnera can synthesize; (ii) genes related to transport, including genes for mitochondrial transporters and a gene encoding Rab, a G protein that regulates vesicular transport; and (iii) genes for putative lysozymes that degrade bacterial cell walls. Significant up-regulation of i clearly indicated that the bacteriocyte is involved in the exchange of amino acids between the host aphid and Buchnera, the key metabolic process in the symbiotic system. Conspicuously high expression of ii and iii shed light on previously unknown aspects of the host-Buchnera interactions in the symbiotic system.

Changing partners in an obligate symbiosis: a facultative endosymbiont can compensate for loss of the essential endosymbiont Buchnera in an aphid

Almost all aphids harbour an endosymbiotic bacterium, Buchnera aphidicola, in bacteriocytes. Buchnera synthesizes essential nutrients and supports growth and reproduction of the host. Over the long history of endosymbiosis, many essential genes have been lost from the Buchnera genome, resulting in drastic genome reduction and the inability to live outside the host cells. In turn, when deprived of Buchnera, the host aphid suffers retarded growth and sterility. Buchnera and the host aphid are often referred to as highly integrated almost inseparable mutualistic partners. However, we discovered that, even after complete elimination of Buchnera, infection with a facultative endosymbiotic gamma-proteobacterium called pea aphid secondary symbiont (PASS) enabled survival and reproduction of the pea aphid. In the Buchnera-free aphid, PASS infected the cytoplasms of bacteriocytes that normally harbour Buchnera, establishing a novel endosymbiotic system. These results indicate that PASS can compensate for the essential role of Buchnera by physiologically and cytologically taking over the symbiotic niche. By contrast, PASS negatively affected the growth and reproduction of normal host aphids by suppressing the essential symbiont Buchnera. These findings illuminate complex symbiont-symbiont and host-symbiont interactions in an endosymbiotic system, and suggest a possible evolutionary route to novel obligate endosymbiosis by way of facultative endosymbiotic associations.

A genomic perspective on nutrient provisioning by bacterial symbionts of insects

Many animals show intimate interactions with bacterial symbionts that provision hosts with limiting nutrients. The best studied such association is that between aphids and Buchnera aphidicola, which produces essential amino acids that are rare in the phloem sap diet. Genomic studies of Buchnera have provided a new means for inferring metabolic capabilities of the symbionts and their likely contributions to hosts. Despite evolutionary reduction of genome size, involving loss of most ancestral genes, Buchnera retains capabilities for biosynthesis of all essential amino acids. In contrast, most genes duplicating amino acid biosynthetic capabilities of hosts have been eliminated. In Buchnera of many aphids, genes for biosynthesis of leucine and tryptophan have been transferred from the chromosome to distinctive plasmids, a feature interpreted as a mechanism for overproducing these amino acids through gene amplification. However, the extent of plasmid-associated amplification varies between and within species, and plasmid-borne genes are sometimes fewer in number than single copy genes on the (polyploid) main chromosome. This supports the broader interpretation of the plasmid location as a means of achieving regulatory control of gene copy number and/or transcription. Buchnera genomes have eliminated most regulatory sequences, raising the question of the extent to which gene expression is moderated in response to changing demands imposed by host nutrition or other factors. Microarray analyses of the Buchnera transcriptome reveal only slight changes in expression of nutrition-related genes in response to shifts in host diet, with responses less dramatic than those observed for the related nonsymbiotic species, Escherichia coli.

Consequences of reductive evolution for gene expression in an obligate endosymbiont

The smallest cellular genomes are found in obligate symbiotic and pathogenic bacteria living within eukaryotic hosts. In comparison with large genomes of free-living relatives, these reduced genomes are rearranged and have lost most regulatory elements. To test whether reduced bacterial genomes incur reduced regulatory capacities, we used full-genome microarrays to evaluate transcriptional response to environmental stress in Buchnera aphidicola, the obligate endosymbiont of aphids. The 580 genes of the B. aphidicola genome represent a subset of the 4500 genes known from the related organism, Escherichia coli. Although over 20 orthologues of E. coli heat stress (HS) genes are retained by B. aphidicola, only five were differentially expressed after near-lethal heat stress treatments, and only modest shifts were observed. Analyses of upstream regulatory regions revealed loss or degradation of most HS (sigma32) promoters. Genomic rearrangements downstream of an intact HS promoter yielded upregulation of a functionally unrelated and an inactivated gene. Reanalyses of comparable experimental array data for E. coli and Bacillus subtilis revealed that genome-wide differential expression was significantly lower in B. aphidicola. Our demonstration of a diminished stress response validates reports of temperature sensitivity in B. aphidicola and suggests that this reduced bacterial genome exhibits transcriptional inflexibility.

Facultative bacterial symbionts in aphids confer resistance to parasitic wasps

Symbiotic relationships between animals and microorganisms are common in nature, yet the factors controlling the abundance and distributions of symbionts are mostly unknown. Aphids have an obligate association with the bacterium Buchnera aphidicola (the primary symbiont) that has been shown to contribute directly to aphid fitness. In addition, aphids sometimes harbor other vertically transmitted bacteria (secondary symbionts), for which few benefits of infection have been previously documented. We carried out experiments to determine the consequences of these facultative symbioses in Acyrthosiphon pisum (the pea aphid) for vulnerability of the aphid host to a hymenopteran parasitoid, Aphidius ervi, a major natural enemy in field populations. Our results show that, in a controlled genetic background, infection confers resistance to parasitoid attack by causing high mortality of developing parasitoid larvae. Compared with uninfected controls, experimentally infected aphids were as likely to be attacked by ovipositing parasitoids but less likely to support parasitoid development. This strong interaction between a symbiotic bacterium and a host natural enemy provides a mechanism for the persistence and spread of symbiotic bacteria.

Comparative molecular evolution of primary (Buchnera) and secondary symbionts of aphids based on two protein-coding genes

A+T content, phylogenetic relationships, codon usage, evolutionary rates, and ratio of synonymous versus non-synonymous substitutions have been studied in partial sequences of the atpD and aroQ/pheA genes of primary ( Buchnera) and secondary symbionts of aphids and a set of selected non-symbiotic bacteria, belonging to the five subdivisions of the Proteobacteria. Compared to the homologous genes of the last group, both genes belonging to Buchnera behave in a similar way, showing a higher A+T content, forming a monophyletic group, a loss in codon bias, especially in third base position, an evolutionary acceleration and an increase in the number of non-synonymous substitutions, confirming previous results reported elsewhere for other genes. When available, these properties have been partly observed with the secondary symbionts, but with values that are intermediate between Buchnera and free living Proteobacteria. They show high A+T content, but not as high as Buchnera, a non-solved phylogenetic position between Buchnera, and the other gamma-Proteobacteria, a loss in codon bias, again not as high as in Buchnera and a significant evolutionary acceleration in the case of the three atpD genes, but not when considering aroQ/pheA genes. These results give support to the hypothesis that they are symbionts at different stages of the symbiotic accommodation to the host.

50 million years of genomic stasis in endosymbiotic bacteria

Comparison of two fully sequenced genomes of Buchnera aphidicola, the obligate endosymbionts of aphids, reveals the most extreme genome stability to date: no chromosome rearrangements or gene acquisitions have occurred in the past 50 to 70 million years, despite substantial sequence evolution and the inactivation and loss of individual genes. In contrast, the genomes of their closest free-living relatives, Escherichia coli and Salmonella spp., are more than 2000-fold more labile in content and gene order. The genomic stasis of B. aphidicola, likely attributable to the loss of phages, repeated sequences, and recA, indicates that B. aphidicola is no longer a source of ecological innovation for its hosts.

An experimental validation of orphan genes of Buchnera, a symbiont of aphids

Although Buchnera sp. APS, an intracellular symbiont of pea aphids, is a close relative of Escherichia coli, its genome has been extensively modified because of its prolonged intracellular life. In our previous studies on the Buchnera genome, computer analysis predicted three "orphan" genes, yba2, yba3, and yba4, which are open reading frames (ORFs) with no homologs in the database. In this paper, we successfully validated all these orphan genes by RT-PCR and Northern hybridization. The present study also revealed that yba3 and yba4 formed an operon, suggesting that they function in concert. Sequences around transcriptional start sites suggests that these genes are under the control of sigma 70. In view of codon usage and AT bias observed in these genes, it is likely that Buchnera have maintained them for an evolutionarily long time.

Accumulation of species-specific amino acid replacements that cause loss of particular protein functions in Buchnera, an endocellular bacterial symbiont

Endosymbiotic bacteria live in animal cells and are transmitted vertically at the time of the host's reproduction. In view of their small and asexual populations with infrequent chances of recombination, these endocellular bacteria are expected to accumulate mildly deleterious mutations. Previous studies showed that the DNA sequences of these bacteria evolved faster than those of free-living bacteria. In this study, we compared all the ORFs of Buchnera, an endocellular bacterial symbiont of aphids, with those of 34 other prokaryotic organisms and estimated the effect of the accelerated evolution of Buchnera on the functions of its proteins. It was revealed that Buchnera proteins contain many mutations at the sites where sequences are conserved in their orthologues in many other organisms. In addition, amino acid replacements at the conserved sites are mostly changes to physicochemically different amino acids. These results suggest that functions and conformations of Buchnera proteins have been seriously impaired or strongly modified. Indeed, extensive loss of functional motifs was observed in some Buchnera proteins. In many Buchnera proteins mutations were not detected evenly throughout each molecule but tended to accumulate in some functional units, possibly leading to loss of specific functions. As Buchnera has an unusual and limited gene repertory, it is conceivable that the manner of interactions among its proteins has been changed, and thus, functional constraints over their amino acid residues have also been changed during evolution. This may account for the loss of some functional units only in the Buchnera proteins. We obtained evidence that amino acid replacements in Buchnera were not always deleterious, but neutral or, in some cases, even positively selected.

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.

Mutualists and parasites: how to paint yourself into a (metabolic) corner

Eukaryotes have developed an elaborate series of interactions with bacteria that enter their bodies and/or cells. Genome evolution of symbiotic and parasitic bacteria multiplying inside eukaryotic cells results in both convergent and divergent changes. The genome sequences of the symbiotic bacteria of aphids, Buchnera aphidicola, and the parasitic bacteria of body louse and humans, Rickettsia prowazekii, provide insights into these processes. Convergent genome characteristics include reduction in genome sizes and lowered G+C content values. Divergent evolution was recorded for amino acid and cell wall biosynthetic genes. The presence of pseudogenes in both genomes provides examples of recent gene inactivation events and offers clues to the process of genome deterioration and host-cell adaptation.

Genome interdependence in insect-bacterium symbioses

Symbioses between unicellular and multicellular organisms have contributed significantly to the evolution of life on Earth. As exemplified by several studies of bacterium-insect symbioses, modern genomic techniques are providing exciting new information about the molecular basis and the biological roles of these complex relationships, revealing for instance that symbionts have lost many genes for functions that are provided by the host, but that they can provide amino acids that the host cannot synthesize.

Cospeciation between bacterial endosymbionts (Buchnera) and a recent radiation of aphids (Uroleucon) and pitfalls of testing for phylogenetic congruence

Previous studies of phylogenetic congruence between aphids and their symbiotic bacteria (Buchnera) supported long-term vertical transmission of symbionts. However, those studies were based on distantly related aphids and would not have revealed horizontal transfer of symbionts among closely related hosts. Aphid species of the genus Uroleucon are closely related phylogenetically and overlap in geographic ranges, habitats, and parasitoids. To examine support for congruence of phylogenies of Buchnera and Uroleucon, sequences from four mitochondrial, one nuclear, and one endosymbiont gene (trpB) were obtained. Congruence of phylogenies based on pooled aphid genes with phylogenies based on trpB was highly significant: Most nodes resolved by trpB corresponded to nodes resolved by the pooled aphid genes. Furthermore, no nodes were both inconsistent between the trees and strongly supported in both trees. Two kinds of analyses testing the null hypothesis of perfect congruence between pairwise combinations of datasets and tree topologies were performed: the Kishino-Hasegawa test and the likelihood-ratio test. Both tests indicated significant disagreement among most pairwise combinations of mitochondrial, nuclear, and symbiont datasets. Because rampant recombination among mitochondrial genomes of different aphid species is unlikely, inaccurate assumptions in the evolutionary models underlying these tests appear to be causing the hypothesis of a shared history to be incorrectly rejected. Moreover, trpB was more consistent with the aphid genes as a set than any single aphid gene was with the others, suggesting that the symbionts show the same phylogeny as the aphids. Overall, analyses support the interpretation that symbionts and aphids have undergone strict cospeciation, with no horizontal transmission of symbionts even among closely related, ecologically similar aphid hosts.

Acetone preservation: a practical technique for molecular analysis

In attempts to establish a convenient and reliable method for field collection and archival preservation of insects and their endosymbiotic microorganisms for molecular analysis, acetone, ethanol, and other organic solvents were tested for DNA preservability of the pea aphid Acyrthosiphon pisum and its intracellular symbiotic bacterium Buchnera sp. After 6 months' storage, not only the band of high-molecular-size DNA but also the bands of rRNA were well preserved in acetone, ethanol, 2-propanol, diethyl ether and ethyl acetate. Polymerase chain reaction (PCR) assays confirmed that the DNA of both the insects and their symbionts was well preserved in these solvents. In contrast, methanol and chloroform showed poor DNA preservability. When water-containing series of acetone and ethanol were examined for DNA preservability, acetone was apparently more robust against water contamination than ethanol. Considering that most biological materials contain high amounts of water, acetone may be a more recommendable preservative for DNA analysis than ethanol which has been widely used for this purpose. The DNA of various insects could be preserved in acetone at room temperature in good condition for several years. In addition to the DNA of the host insects, the DNA of their endosymbionts, including Buchnera and other mycetocyte symbionts, Wolbachia, and gut bacteria, was amplified by PCR after several years of acetone storage. The RNA and protein of the pea aphid and its endosymbiont were also preserved for several years in acetone. After 2 years' storage in acetone, proteins of A. pisum could be analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting, and the endosymbiotic bacteria were successfully detected by immunohistochemistry and in situ hybridization on the tissue sections.