Fitness effects of two facultative endosymbiotic bacteria on the pea aphid,Acyrthosiphon pisum,and the blue alfalfa aphid,A. kondoi

The effects of two bacterial endosymbionts, designated PASS and PAR, were evaluated on the pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera:Aphididae), in which they occur facultatively, and on the blue alfalfa aphid, A. kondoi Shinji, in which these bacteria have not been found in natural populations. Subclones of pea aphids and blue alfalfa aphids, derived from parent aphid clones that did not contain PASS or PAR, were infected with one or both bacteria, generating PASS- and/or PAR-positive subclones with minimal genetic differences from the parent clones. Under laboratory conditions at 20 °C, PAR consistently reduced the fecundity (by between 19 and 60%) of subclones derived from three different parent pea aphid clones on bur clover, Medicago hispida Gaertn. PAR had intermediate effects on pea aphids reared on sweet pea, Lathyrus odoratus L., and had no significant effect on pea aphids on alfalfa, Medicago sativa L. The effect of PASS was either neutral or negative, depending on parent clone as well as host plant. Also at 20 °C, PASS reduced fecundity (70–77%) and longevity (40–48%), and increased the age of first reproduction (by up to 1.5 days) of blue alfalfa aphid reared on alfalfa and clover. PAR had a less dramatic effect (e.g., 30–39% reduction in fecundity) on these traits of blue alfalfa aphid. In contrast, PAR and PASS increased the fitness of pea aphid subclones of one parent clone reared for three generations at 25 °C on each of the three test plants. Without facultative bacteria, fecundity of the parent clone was reduced to a mean total of < 6 offspring per adult at this elevated temperature, but with PASS or PAR, mean total fecundity of its subclones was > 35. However, this ameliorative effect of facultative bacteria at 25 °C was not found for two other sets of parent clones and their derived subclones. Alate production in pea aphids was significantly increased in large populations of two PASS- and PAR-positive subclones relative to their parent clones. Attempts to transmit PASS or PAR horizontally, i.e., from aphid to aphid via feeding on host plants (bur clover), were unsuccessful.     

Does variation in host plant association and symbiont infection of pea aphid populations induce genetic and behaviour differentiation of its main parasitoid, Aphidius ervi?

The host-associated differentiation (HAD) hypothesis states that higher trophic levels in parasitic associations should exhibit similar divergence in case of host sympatric speciation. We tested HAD on populations of Aphidius ervi the main parasitoid of the pea aphid Acyrthosiphon pisum, emerging from host populations specialized on either alfalfa or red clover. Host and parasitoid populations were assessed for genetic variation and structure, while considering geography, host plant and host aphid protective symbionts Regiella insecticola and Hamiltonella defensa as potential covariables. Cluster and hierarchical analyses were used to assess the contribution of these variables to population structure, based on genotyping pea aphids and associated A. ervi with microsatellites, and host aphid facultative symbionts with 16S rDNA markers. Pea aphid genotypes were clearly distributed in two groups closely corresponding with their plant origins, confirming strong plant associated differentiation of this aphid in North America. Overall parasitism by A. ervi averaged 21.5 % across samples, and many parasitized aphids producing a wasp hosted defensive bacteria, indicating partial or ineffective protective efficacy of these symbionts in the field. The A. ervi population genetic data failed to support differentiation according to the host plant association of their pea aphid host. Potential for parasitoid specialization was also explored in experiments where wasps from alfalfa and clover aphids were reciprocally transplanted on alternate hosts, the hypothesis being that wasp behaviour and parasitic stages should be most adapted to their host of origin. Results revealed higher probability of oviposition on the alfalfa aphids, but higher adult emergence success on red clover aphids, with no interaction as expected under HAD. We conclude that our study provides no support for the HAD in this system. We discuss factors that might impair A. ervi specialization on its divergent aphid hosts on alfalfa and clover.     

EVOLUTION OF AN APHID-PARASITOID INTERACTION: VARIATION IN RESISTANCE TO PARASITISM AMONG APHID POPULATIONS SPECIALIZED ON DIFFERENT PLANTS

The evolution of associations between herbivorous insects and their parasitoids is likely to be influenced by the relationship between the herbivore and its host plants. If populations of specialized herbivorous insects are structured by their host plants such that populations on different hosts are genetically differentiated, then the traits affecting insect-parasitoid interactions may exhibit an associated structure. The pea aphid (Acyrthosiphon pisum) is a herbivorous insect species comprised of genetically distinct groups that are specialized on different host plants (Via 1991a, 1994). Here, we examine how the genetic differentiation of pea aphid populations on different host plants affects their interaction with a parasitoid wasp, Aphidius ervi. We performed four experiments. (1) By exposing pea aphids from both alfalfa and clover to parasitoids from both crops, we demonstrate that pea aphid populations that are specialized on alfalfa are successfully parasitized less often than are populations specialized on clover. This difference in parasitism rate does not depend upon whether the wasps were collected from alfalfa or clover fields. (2) When we controlled for potential differences in aphid and parasitoid behavior between the two host plants and ensured that aphids were attacked, we found that pea aphids from alfalfa were still parasitized less often than pea aphids from clover. Thus, the difference in parasitism rates is not due to behavior of either aphids or wasps, but appears to be a physiologically based difference in resistance to parasitism. (3) Replicates of pea aphid clones reared on their own host plant and on a common host plant, fava bean, exhibited the same pattern of resistance as above. Thus, there do not appear to be nutritional or secondary chemical effects on the level of physiological resistance in the aphids due to feeding on clover or alfalfa, and therefore the difference in resistance on the two crops appears to be genetically based. (4) We assayed for genetic variation in resistance among individual pea aphid clones collected from clover fields and found no detectable genetic variation for resistance to parasitism within two populations sampled from clover. This is in contrast to Henter and Via's (1995) report of abundant genetic variation in resistance to this parasitoid within a pea aphid population on alfalfa. Low levels of genetic variation may be one factor that constrains the evolution of resistance to parasitism in the populations of pea aphids from clover, leading them to remain more susceptible than populations of the same species from alfalfa.     

Predation Determines Different Selective Pressure on Pea Aphid Host Races in a Complex Agricultural Mosaic

Field assessments were conducted to examine the interplay between host plant and predation in complex agricultural mosaic on pea aphid clover and alfalfa races. In one experiment, we examined the relative fitness on clover race (CR) and alfalfa race (AR) pea aphids on broad bean, red clover and alfalfa alone. But because clover is typically grown in a more complex agricultural mosaic with alfalfa and broad bean, a second experiment was conducted to assess the fitness consequences under predation in a more complex agricultural field setting that also included potential apparent competition with AR pea aphids. In a third experiment we tested for the effect of differential host race density on the fitness of the other host race mediated by a predator effect. CR pea aphids always had fitness losses when on broad bean (had lower fitness on broad bean relative to red clover) and fitness benefits when on red clover (higher fitness on red clover relative to broad bean), whether or not in apparent competition with alfalfa race aphids on bean and alfalfa. AR suffered fitness loss on both alfalfa and bean in apparent competition with CR on clover. Therefore we can conclude that the predation rate between host races was highly asymmetrical. The complexity of the agricultural mosaic thus can influence prey selection by predators on different host plants. These may have evolutionary consequences through context dependent fitness benefits on particular host plants.     

Host-based divergence in populations of the pea aphid: insights from nuclear markers and the prevalence of facultative symbionts

In North America, the pea aphid Acyrthosiphon pisum encompasses ecologically and genetically distinct host races that offer an ideal biological system for studies on sympatric speciation. In addition to its obligate symbiont Buchnera, pea aphids harbour several facultative and phylogenetically distant symbionts. We explored the relationships between host races of A. pisum and their symbiotic microbiota to gain insights into the historical process of ecological specialization and symbiotic acquisition in this aphid. We used allozyme and microsatellite markers to analyse the extent of genetic differentiation between populations of A. pisum on pea, alfalfa and clover in France. In parallel, we examined: (i) the distribution of four facultative symbionts; and (ii) the genetic variation in the Buchnera genome across host-associated populations of A. pisum. Our study clearly demonstrates that populations of A. pisum on pea, clover and alfalfa in France are genetically divergent, which indicates that they constitute distinct host races. We also found a very strong association between host races of A. pisum and their symbiotic microbiota. We stress the need for phylogeographic studies to shed light on the process of host-race formation and acquisition of facultative symbionts in A. pisum. We also question the effects of these symbionts on aphid host fitness, including their role in adaptation to a host plant.     

Removal of a specialization-associated symbiont does not affect aphid fitness

Symbioses are ubiquitous in nature. However, these interactions, in particular those which are facultative from the perspective of the host, are still poorly understood. One major question is whether facultative symbionts affect host fitness, and in particular whether they can benefit the host. In pea aphid, host plant specialization was found to be associated with the presence of Pea Aphid U‐type Symbiont [Proc. R. Soc. Lond. B 270 (2003) S209]. Host and symbiont genomes are normally co‐transmitted, making it difficult to determine which is responsible for causing this variation in fitness. Here antibiotic treatment is used to selectively remove the symbiont, thus partitioning symbiont and host genomes. I find that PAUS is not responsible for causing host plant specialization. Examination of aphids in which PAUS presence has been naturally manipulated also supports this conclusion. Additional work will be necessary to determine the forces maintaining this symbiont in host populations, and whether it benefits the host insect under different conditions.     

Diversity of bacteria associated with natural aphid populations

The bacterial communities of aphids were investigated by terminal restriction fragment length polymorphism and denaturing gradient gel electrophoresis analysis of 16S rRNA gene fragments generated by PCR with general eubacterial primers. By both methods, the gamma-proteobacterium Buchnera was detected in laboratory cultures of six parthenogenetic lines of the pea aphid Acyrthosiphon pisum and one line of the black bean aphid Aphis fabae, and one or more of four previously described bacterial taxa were also detected in all aphid lines except one of A. pisum. These latter bacteria, collectively known as secondary symbionts or accessory bacteria, comprised three taxa of gamma-proteobacteria (R-type [PASS], T-type [PABS], and U-type [PAUS]) and a rickettsia (S-type [PAR]). Complementary analysis of aphids from natural populations of four aphid species (A. pisum [n = 74], Amphorophora rubi [n = 109], Aphis sarothamni [n = 42], and Microlophium carnosum [n = 101]) from a single geographical location revealed Buchnera and up to three taxa of accessory bacteria, but no other bacterial taxa, in each aphid. The prevalence of accessory bacterial taxa varied significantly among aphid species but not with the sampling month (between June and August 2000). These results indicate that the accessory bacterial taxa are distributed across multiple aphid species, although with variable prevalence, and that laboratory culture does not generally result in a shift in the bacterial community in aphids. Both the transmission patterns of the accessory bacteria between individual aphids and their impact on aphid fitness are suggested to influence the prevalence of accessory bacterial taxa in natural aphid populations.     

Symbiont modifies host life-history traits that affect gene flow

The evolution of herbivore-host plant specialization requires low levels of gene flow between populations on alternate plant species. Accordingly, selection for host plant specialization is most effective when genotypes have minimal exposure to, and few mating opportunities with individuals from, alternate habitats. Maternally transmitted bacterial symbionts are common in insect herbivores and can influence host fecundity under a variety of conditions. Symbiont-mediated effects on host life-history strategies, however, are largely unknown. Here, we show that the facultative bacterial symbiont Candidatus Regiella insecticola strikingly alters both dispersal and mating in the pea aphid, Acyrthosiphon pisum. Pea aphids containing Regiella produced only half the number of winged offspring in response to crowding and, for two out of three aphid lineages, altered the timing of sexual reproduction in response to conditions mimicking seasonal changes, than did aphids lacking Regiella. These symbiont-associated changes in dispersal and mating are likely to have played a key role in the initiation of genetic differentiation and in the evolution of pea aphid-host plant specialization. As symbionts are widespread in insects, symbiont-induced life history changes may have promoted specialization, and potentially speciation, in many organisms.     

Occurrence and Transmission of Facultative Endosymbionts in Aphids

The occurrence of a secondary bacterial symbiont (PASS) of pea aphid, Acyrthosiphon pisum (Harris), was detected by polymerase chain reaction (PCR) with specific nucleotide primers based on PASS 16S rDNA nucleotide sequences from over 80% (50/57) of clones of pea aphid collected from widely separated locations in California. PASS was also detected by PCR in both red and green phenotypes of rose aphid, Macrosiphum rosae (L.), but not in six other species of aphids examined, including blue alfalfa aphid (A. kondoi Shinji). The nucleotide sequences of the PCR-amplified, partial 16S rDNAs (1060 bp) from pea aphid and rose aphid were identical and 99.9% similar to the published 16S rDNA of PASS. PASS and a recently described new rickettsia of pea aphid (PAR) were transmitted by needle injection of hemolymph from positive pea aphid clones into negative clones and into blue alfalfa aphids. Both PASS and PAR were maintained in the offspring of some of the injected mother aphids via high rate of maternal transmission. Received: 18 September 1996 / Accepted: 30 September 1996     

An aphid-borne bacterium allied to the secondary symbionts of whitefly

Bacterial 16S rDNA amplified by PCR from the pea aphid Acyrthosiphon pisum included a sequence with >98% similarity to secondary symbionts in the whitefly Bemisia tabaci. The 'pea aphid Bemisia-like bacterium' (PABS) and B. tabaci secondary symbionts are estimated to have diverged 17-34 million years ago, a time considerably more recent than the common ancestor of aphids and whitefly and suggestive of horizontal transmission of this bacterial lineage. PABS was scored in both the gut and ovaries of aphids by PCR and identified as a small rod by in situ hybridisation. PABS was not universal in pea aphids: 2/3 laboratory strains and 13/35 of field aphids were PABS-positive. It is suggested that the incidence of PABS in pea aphids is determined by the balance between loss (processes may include occasional failure of vertical transmission and selection against PABS-positive aphids) and horizontal transfer between insects.     

Facultative bacterial endosymbionts benefit pea aphids Acyrthosiphon pisum under heat stress

Abstract 1. Natural populations of pea aphids in California contain at least two facultative bacterial secondary symbionts (pea aphid secondary symbiont, PASS, or pea aphid rickettsia, PAR) in a range of frequencies throughout the state.
2. Two pea aphid clones without either of these facultative associates failed to reproduce in the first 8 days after the final moult if they had been heat-stressed for a period of about 4 h at 39 °C as 1-day-old larvae in the laboratory.
3. Aphids infected artificially with PASS, however, were able to produce up to 48% of the normal complement of offspring produced by PASS-positive aphids that had not been heat-stressed. Clones infected artificially with PAR did not have the same advantage as those with PASS after heat stress.
4. In aphids without PASS or PAR, heat stress reduced the number of bacteriocytes (in which the obligate primary symbiont, Buchnera , resides) to 7% of non-heat-stressed aphids, while aphids with only PASS retained 70% of their bacteriocytes. Bacteriocytes in aphids with PAR but not PASS were reduced to 42% of controls.
5. When larvae were heat-stressed as older instars (5 days old), a similar pattern emerged, though the effect of heat stress was less extreme. Clones containing PASS produced the most offspring, three to 14 times as many as aphids without PASS or PAR. Aphids with PAR only, or PASS and PAR together, had reduced or no advantage over aphids without facultative symbionts.
6. Aphids of all clones that had been heat-stressed as later instars gave birth to a variable number of stillborn offspring. Aphids without facultative symbionts produced the most stillborn larvae.
7. Field studies showed a higher incidence of PASS in aphids collected in California in summer compared with aphids from the same sites collected 2–4 months earlier. The difference was significant in two of three widely dispersed locations.     

Diversity and geographic distribution of secondary endosymbiotic bacteria in natural populations of the pea aphid,Acyrthosiphon pisum

In addition to the essential intracellular symbiotic bacterium Buchnera, several facultative endosymbiotic bacteria called collectively secondary symbionts (S-symbionts) have been identified from the pea aphid Acyrthosiphon pisum. We conducted an extensive and systematic survey of S-symbionts in Japanese local populations of A. pisum using a specific PCR detection technique. Five S-symbionts of A. pisum, PASS, PAUS, PABS, Rickettsia and Spiroplasma, and two facultative endosymbionts universally found in various insects, Wolbachia and Arsenophonus, were targeted. Of 119 isofemale strains originating from 81 localities, 66.4% of the strains possessed either of four S-symbionts: PASS (38.7%); PAUS (16.0%); Rickettsia (8.4%); and Spiroplasma (3.4%), while 33.6% of the strains contained only Buchnera. PABS, Wolbachia and Arsenophonus were not detected from the Japanese strains of A. pisum. In order to understand intra- and interpopulational diversity of S-symbiont microbiota in detail, 858 insects collected from 43 localities were examined for infection with the four S-symbionts. It was demonstrated that different S-symbionts coexist commonly in the same local populations, but double infections with two S-symbionts were rarely detected. Notably, the S-symbionts exhibited characteristic geographical distribution patterns: PASS at high frequencies all over Japan; PAUS at high frequencies mainly in the northeastern part of Japan; and Rickettsia and Spiroplasma at low frequencies sporadically in the southwestern part of Japan. These results indicate that the geographical distribution and infection frequency of the S-symbionts, in particular PAUS, might be affected by environmental and/or historical factors. Statistical analyses suggested that the distribution of PAUS infection might be related to host plant species, temperature and precipitation.     

Survival to Parasitoids in an Insect Hosting Defensive Symbionts: A Multivariate Approach to Polymorphic Traits Affecting Host Use by Its Natural Enemy

Insect parasitoids and their insect hosts represent a wide range of parasitic trophic relations that can be used to understand the evolution of biotic diversity on earth. Testing theories of coevolution between hosts and parasites is based on factors directly involved in host susceptibility and parasitoid virulence. We used controlled encounters with potential hosts of the Aphidius ervi wasp to elucidate behavioral and other phenotypic traits of host Acyrthosiphon pisum that most contribute to success or failure of parasitism. The host aphid is at an advanced stage of specialization on different crop plants, and exhibits intra-population polymorphism for traits of parasitoid avoidance and resistance based on clonal variation of color morph and anti-parasitoid bacterial symbionts. Randomly selected aphid clones from alfalfa and clover were matched in 5 minute encounters with wasps of two parasitoid lineages deriving from hosts of each plant biotype in a replicated transplant experimental design. In addition to crop plant affiliation (alfalfa, clover), aphid clones were characterized for color morph (green, pink), Hamiltonella defensa and Regiella insecticola symbionts, and frequently used behaviors in encounters with A. ervi wasps. A total of 12 explanatory variables were examined using redundancy analysis (RDA) to predict host survival or failure to A. ervi parasitism. Aphid color was the best univariate predictor, but was poorly predictive in the RDA model. In contrast, aphid host plant and symbionts were not significant univariate predictors, but significant predictors in the multivariate model. Aphid susceptibility to wasp acceptance as reflected in host attacks and oviposition clearly differed from its suitability to parasitism and progeny development. Parasitoid progeny were three times more likely to survive on clover than alfalfa host aphids, which was compensated by behaviorally adjusting eggs invested per host. Strong variation of the predictive power of intrinsic (body color) and extrinsic traits (symbionts, host plant), indicate that host variables considered as key predictors of outcomes strongly interact and cannot be considered in isolation.     

Linking the bacterial community in pea aphids with host-plant use and natural enemy resistance

Abstract.  1. Pea aphids, Acyrthosiphon pisum , harbour a range of facultative accessory bacteria (secondary symbionts), including those informally known as PASS (R-type), PAR, PABS (T-type), and PAUS (U-type).
2. To explore the relationship between possession of these bacteria and ecologically important traits of A. pisum , correlations between the accessory bacteria found in 47 parthenogenetic clones of A. pisum and the host plant on which each clone was collected and its susceptibility to natural enemies were surveyed.
3. The bacterial complement varied with plant of collection. PAUS (U) was present in all of 12 clones affiliated to Trifolium but was otherwise rare, while PABS (T) and PASS (R) occurred at significantly higher frequency in clones from Lotus and Vicia , respectively, than clones from other plants.
4. Possession of PABS (T) was associated strongly with resistance to the parasitoid Aphidius eadyi and weakly with resistance to Aphidius ervi . Aphids carrying PAUS (U) were more resistant to the fungal pathogen Pandora ( Erynia ) neoaphidis , although this correlation was complicated by a strong association with host-plant use.     

Behavioural correlates of genetic divergence due to host specialization in the pea aphid, Acyrthosiphon pisum

Host specialization plays a central role in the diversification of herbivorous insects and yet we know very little about the evolution of this trait. Populations of the pea aphid, Acyrthosiphon pisum (Harris), are specialized and locally adapted to either alfalfa or clover. Preference for either plant produces assortative mating, and gene flow between alfalfa and clover populations is consistently restricted (Via, 1991a,b, 1994). Behavioural studies of freely-moving aphids on the plant surface for 30 min suggest that host preference is chemically mediated, involves chemoreception, and is most likely due to secondary compounds located in the epidermal or mesophyll cells. Pea aphids do not distinguish between hosts and non-hosts at a distance but determine whether the plant is suitable or not after only a short probe. Thirty-minute recordings of the activities of aphid stylets using an electrical monitoring system (EPG), where aphids are attached to a gold wire with silver paint, provide a different picture, suggesting that EPG experiments do not accurately reflect natural behaviour during the first 30 min of the aphid-plant interaction.     

Effects of bacterial secondary symbionts on host plant use in pea aphids

Aphids possess several facultative bacterial symbionts that have important effects on their hosts'' biology. These have been most closely studied in the pea aphid (Acyrthosiphon pisum), a species that feeds on multiple host plants. Whether secondary symbionts influence host plant utilization is unclear. We report the fitness consequences of introducing different strains of the symbiont Hamiltonella defensa into three aphid clones collected on Lathyrus pratensis that naturally lack symbionts, and of removing symbionts from 20 natural aphid–bacterial associations. Infection decreased fitness on Lathyrus but not on Vicia faba, a plant on which most pea aphids readily feed. This may explain the unusually low prevalence of symbionts in aphids collected on Lathyrus. There was no effect of presence of symbiont on performance of the aphids on the host plants of the clones from which the H. defensa strains were isolated. Removing the symbiont from natural aphid–bacterial associations led to an average approximate 20 per cent reduction in fecundity, both on the natural host plant and on V. faba, suggesting general rather than plant-species-specific effects of the symbiont. Throughout, we find significant genetic variation among aphid clones. The results provide no evidence that secondary symbionts have a major direct role in facilitating aphid utilization of particular host plant species.     

Facultative symbiont infections affect aphid reproduction

Some bacterial symbionts alter their hosts reproduction through various mechanisms that enhance their transmission in the host population. In addition to its obligatory symbiont Buchnera aphidicola, the pea aphid Acyrthosiphon pisum harbors several facultative symbionts influencing several aspects of host ecology. Aphids reproduce by cyclical parthenogenesis whereby clonal and sexual reproduction alternate within the annual life cycle. Many species, including the pea aphid, also show variation in their reproductive mode at the population level, with some lineages reproducing by cyclical parthenogenesis and others by permanent parthenogenesis. While the role of facultative symbionts has been well studied during the parthenogenetic phase of their aphid hosts, very little is known on their possible influence during the sexual phase. Here we investigated whether facultative symbionts modulate the capacity to produce sexual forms in various genetic backgrounds of the pea aphid with controlled symbiont composition and also in different aphid genotypes from natural populations with previously characterized infection status and reproductive mode. We found that most facultative symbionts exhibited detrimental effects on their hosts fitness under sex-inducing conditions in comparison with the reference lines. We also showed that the loss of sexual phase in permanently parthenogenetic lineages of A. pisum was not explained by facultative symbionts. Finally, we demonstrated that Spiroplasma infection annihilated the production of males in the host progeny by inducing a male-killing phenotype, an unexpected result for organisms such as aphids that reproduce primarily through clonal reproduction.     

Population differentiation and genetic variation in performance on eight hosts in the pea aphid complex

Phytophagous insects frequently use multiple host-plant species leading to the evolution of specialized host-adapted populations and sometimes eventually to speciation. Some insects are confronted with a large number of host-plant species, which may provide complex routes of gene flow between host-adapted populations. The pea aphid (Acyrthosiphon pisum) attacks a broad range of plants in the Fabaceae and it is known that populations on Trifolium pratense and Medicago sativa can be highly specialized at exploiting these species. To find out whether adaptation to a broad range of co-occurring hosts has occurred, we tested the performance of pea aphid clones collected from eight host-plant genera on all of these plants in a reciprocal transfer experiment. We provide evidence for pervasive host-plant specialization. The high performance of all aphid clones on Vicia faba suggests that this host plant could be a site of gene flow between different populations that could limit further host-associated divergence. The genetic variance in host-plant usage was partitioned into within- and among-population components, which represent different levels of host adaptation. Little evidence of within-population trade-offs in performance on different plant species was found.     

Ecological specialization correlates with genotypic differentiation in sympatric host-populations of the pea aphid

The pea aphid, Acyrthosiphon pisum, encompasses distinct host races specialized on various Fabaceae species, but the extent of genetic divergence associated with ecological specialization varies greatly depending on plant and geographic origins of aphid populations. Here, we studied the genetic structure of French sympatric pea aphid populations collected on perennial (pea and faba bean) and annual (alfalfa and red clover) hosts using 14 microsatellite loci. Classical and Bayesian population genetics analyses consistently identified genetic clusters mostly related to plant origin: the pea/faba bean cluster was highly divergent from the red clover and the alfalfa ones, indicating they represent different stages along the continuum of genetic differentiation. Some genotypes were assigned to a cluster differing from the one expected from their plant origin while others exhibited intermediate genetic characteristics. These results suggest incomplete barriers to gene flow. However, this limited gene flow seems insufficient to prevent ecological specialization and genetic differentiation in sympatry.     

Diversity of Bacteria Associated with Natural Aphid Populations

The bacterial communities of aphids were investigated by terminal restriction fragment length polymorphism and denaturing gradient gel electrophoresis analysis of 16S rRNA gene fragments generated by PCR with general eubacterial primers. By both methods, the γ-proteobacterium Buchnera was detected in laboratory cultures of six parthenogenetic lines of the pea aphid Acyrthosiphon pisum and one line of the black bean aphid Aphis fabae, and one or more of four previously described bacterial taxa were also detected in all aphid lines except one of A. pisum. These latter bacteria, collectively known as secondary symbionts or accessory bacteria, comprised three taxa of γ-proteobacteria (R-type [PASS], T-type [PABS], and U-type [PAUS]) and a rickettsia (S-type [PAR]). Complementary analysis of aphids from natural populations of four aphid species (A. pisum [n = 74], Amphorophora rubi [n = 109], Aphis sarothamni [n = 42], and Microlophium carnosum [n = 101]) from a single geographical location revealed Buchnera and up to three taxa of accessory bacteria, but no other bacterial taxa, in each aphid. The prevalence of accessory bacterial taxa varied significantly among aphid species but not with the sampling month (between June and August 2000). These results indicate that the accessory bacterial taxa are distributed across multiple aphid species, although with variable prevalence, and that laboratory culture does not generally result in a shift in the bacterial community in aphids. Both the transmission patterns of the accessory bacteria between individual aphids and their impact on aphid fitness are suggested to influence the prevalence of accessory bacterial taxa in natural aphid populations.