The ecological significance of arthropod vectors of plant, animal, and human pathogens.

Vector management is a cornerstone in the fight against vector-borne pathogens. However, the impact on ecosystem functioning of reducing or eliminating arthropod vector populations remains poorly understood. Vectors are members of complex ecological communities, and recent studies suggest that their population suppression alters food web dynamics (bottom-up and top-down trophic cascades), inter- and intraspecific competition, and plant pollination. Other possible overlooked roles are also proposed. In this review, with examples from vectors of plant, animal, and human pathogens, we highlight that, although the ecological roles of most vector species might be redundant with other non-vector species, changes in vector abundance alter biotic interactions and, thus, are unlikely to be neutral in terms of ecosystem functioning.

Occurrence data for the two cryptic species of Cacopsylla pruni (Hemiptera: Psylloidea).

BACKGROUND: Cacopsylla pruni is a psyllid that has been known since 1998 as the vector of the bacterium 'Candidatus Phytoplasma prunorum', responsible for the European stone fruit yellows (ESFY), a disease that affects species of Prunus. This disease is one of the major limiting factors for the production of stone fruits, most notably apricot (Prunus armeniaca) and Japanese plum (P. salicina), in all EU stone fruit-growing areas. The psyllid vector is widespread in the Western Palearctic and evidence for the presence of the phytoplasma that it transmits to species of Prunus has been found in 15 of the 27 EU countries.Recent studies showed that C. pruni is actually composed of two cryptic species that can be differentiated by molecular markers. A literature review on the distribution of C. pruni was published in 2012, but it only provided presence or absence information at the country level and without distinction between the two cryptic species.Since 2012, numerous new records of the vector in several European countries have been published. We ourselves have acquired a large amount of data from sampling in France and other European countries. We have also carried out a thorough systematic literature review to find additional records, including all the original sources mentioning C. pruni (or its synonyms) since the first description by Scopoli in 1763. Our aim was to create an exhaustive georeferenced occurrence catalogue, in particular in countries that are occasionally mentioned in literature with little detail. Finally, for countries that seem suitable for the proliferation of C. pruni (USA, Canada, Japan, China etc.), we dug deeper into literature and reliable sources (e.g. published checklists) to better substantiate its current absence from those regions.Information on the distribution ranges of these vector psyllids is of crucial interest in order to best predict the vulnerability of stone fruit producing countries to the ESFY threat in the foreseeable future. NEW INFORMATION: We give free access to a unique file of 1975 records of all occurrence data in our possession concerning C. pruni, that we have gathered through more than twenty years of sampling efforts in Europe or through intensive text mining.We have made every effort to retrieve the source information for the records extracted from literature (1201 records). Thus, we always give the title of the original reference, together with the page(s) citing C. pruni and, if possible, the year of sampling. To make the results of this survey publicly available, we give a URL to access the literature sources. In most cases, this link allows free downloads of a PDF file.We also give access to information extracted from GBIF (162 exploitable data points on 245 occurrences found in the database), which we thoroughly checked and often supplemented to make the information more easily exploitable.We give access to our own unpublished georeferenced and genotyped records from 612 samples taken over the last 20 years in several European countries (Switzerland, Belgium, Netherlands, Spain etc.). These include two countries (Portugal and North Macedonia), for which the presence of C. pruni had not been reported before. As our specimens have been genotyped (74 sites with species A solely, 202 with species B solely and 310 with species A+B), our new data enable a better overview of the geographical distribution of the two cryptic species at the Palaearctic scale.

Alfalfa leaf curl virus is efficiently acquired by its aphid vector Aphis craccivora but inefficiently transmitted.

Alfalfa leaf curl virus (ALCV) is the first geminivirus for which aphid transmission was reported. Transmission by Aphis craccivora was determined previously to be highly specific and circulative. Using various complementary techniques, the transmission journey of ALCV was monitored from its uptake from infected plant tissues up to the head of its vector. ALCV was shown to be restricted to phloem tissues using fluorescence in situ hybridization (FISH) and electropenetrography (EPG) monitoring of virus acquisition. Furthermore, the virus is heterogeneously distributed in phloem tissues, as revealed by FISH and quantitative PCR of viral DNA acquired by EPG-monitored aphids. Despite the efficient ingestion of viral DNA, about 106 viral DNA copies per insect in a 15 h feeding period on ALCV-infected plants, the individual maximum transmission rate was 12 %. Transmission success was related to a critical viral accumulation, around 1.6×107 viral DNA copies per insect, a threshold that generally needed more than 48 h to be reached. Moreover, whereas the amount of acquired virus did not decrease over time in the whole aphid body, it declined in the haemolymph and heads. ALCV was not detected in progenies of viruliferous aphids and did not affect aphid fitness. Compared to geminiviruses transmitted by whiteflies or leafhoppers, or to luteoviruses transmitted by aphids, the transmission efficiency of ALCV by A. craccivora is low. This result is discussed in relation to the aphid vector of this geminivirus and the agroecological features of alfalfa, a hardy perennial host plant.

Co-Acquired Nanovirus and Geminivirus Exhibit a Contrasted Localization within Their Common Aphid Vector.

Single-stranded DNA (ssDNA) plant viruses belong to the families Geminiviridae and Nanoviridae. They are transmitted by Hemipteran insects in a circulative, mostly non-propagative, manner. While geminiviruses are transmitted by leafhoppers, treehoppers, whiteflies and aphids, nanoviruses are transmitted exclusively by aphids. Circulative transmission involves complex virus-vector interactions in which epithelial cells have to be crossed and defense mechanisms counteracted. Vector taxa are considered a relevant taxonomic criterion for virus classification, indicating that viruses can evolve specific interactions with their vectors. Thus, we predicted that, although nanoviruses and geminiviruses represent related viral families, they have evolved distinct interactions with their vector. This prediction is also supported by the non-structural Nuclear Shuttle Protein (NSP) that is involved in vector transmission in nanoviruses but has no similar function in geminiviruses. Thanks to the recent discovery of aphid-transmitted geminiviruses, this prediction could be tested for the geminivirus alfalfa leaf curl virus (ALCV) and the nanovirus faba bean necrotic stunt virus (FBNSV) in their common vector, Aphis craccivora. Estimations of viral load in midgut and head of aphids, precise localization of viral DNA in cells of insect vectors and host plants, and virus transmission tests revealed that the pathway of the two viruses across the body of their common vector differs both quantitatively and qualitatively.

Multi-scale spatial genetic structure of the vector-borne pathogen 'Candidatus Phytoplasma prunorum' in orchards and in wild habitats.

Inferring the dispersal processes of vector-borne plant pathogens is a great challenge because the plausible epidemiological scenarios often involve complex spread patterns at multiple scales. The spatial genetic structure of 'Candidatus Phytoplasma prunorum', responsible for European stone fruit yellows disease, was investigated by the application of a combination of statistical approaches to genotype data of the pathogen sampled from cultivated and wild compartments in three French Prunus-growing regions. This work revealed that the prevalence of the different genotypes is highly uneven both between regions and compartments. In addition, we identified a significant clustering of similar genotypes within a radius of 50 km or less, but not between nearby wild and cultivated Prunus. We also provide evidence that infected plants are transferred between production areas, and that both species of the Cacopsylla pruni complex can spread the pathogen. Altogether, this work supports a main epidemiological scenario where 'Ca. P. prunorum' is endemic in - and generally acquired from - wild Prunus by its immature psyllid vectors. The latter then migrate to shelter plants that epidemiologically connect sites less than 50 km apart by later providing infectious mature psyllids to their "migration basins". Such multi-scale studies could be useful for other pathosystems.

A framework for estimating the effects of sequential reproductive barriers: Implementation using Bayesian models with field data from cryptic species.

Determining how reproductive barriers modulate gene flow between populations represents a major step toward understanding the factors shaping the course of speciation. Although many indices quantifying reproductive isolation (RI) have been proposed, they do not permit the quantification of cross-direction-specific RI under varying species frequencies and over arbitrary sequences of barriers. Furthermore, techniques quantifying associated uncertainties are lacking, and statistical methods unrelated to biological process are still preferred for obtaining confidence intervals and P-values. To address these shortcomings, we provide new RI indices that model changes in gene flow for both directions of hybridization, and we implement them in a Bayesian model. We use this model to quantify RI between two species of the psyllid Cacopsylla pruni based on field genotypic data for mating individuals, inseminated spermatophores and progeny. The results showed that preinsemination isolation was strong, mildly asymmetric, and indistinguishably different between study sites despite large differences in species frequencies; that postinsemination isolation strongly affected the more common hybrid type; and that cumulative isolation was close to complete. In the light of these results, we discuss how these developments can strengthen comparative RI studies.

Mixed xylem and phloem sap ingestion in sheath-feeders as normal dietary behavior: Evidence from the leafhopper Scaphoideus titanus.

In phytophagous piercing-sucking insects, salivary sheath-feeding species are often described as xylem- or phloem-sap feeding specialists. Because these two food sources have very different characteristics, two feeding tactics are often associated with this supposed specialization. Studying the feeding behavior of insects provides substantial information on their biology, ecology, and evolution. Furthermore, study of feeding behavior is of primary importance to elucidate the transmission ability of insects that act as vectors of plant pathogens. In this study, we compared the durations of ingestion performed in xylem versus phloem by a leafhopper species, Scaphoideus titanus Ball, 1932. This was done by characterizing and statistically analyzing electrical signals recorded using the electropenetrography technique, derived from the feeding behaviors of males and females. We identified three groups of S. titanus based on their feeding behavior: 1) a group that reached the phloem quickly and probed for a longer time in phloem tissue than the other groups, 2) a group that reached the xylem quickly and probed for a longer time in xylem tissue than the other groups, and 3) a group where individuals did not ingest much sap. In addition, the numbers and durations of waveforms representing ingestion of xylem and phloem saps differed significantly depending on the sex of the leafhopper, indicating that the two sexes exhibit different feeding behaviors. Males had longer phloem ingestion events than did females, which indicates that males are greater phloem feeders than females. These differences are discussed, specifically in relation to hypotheses about evolution of sap feeding and phytoplasma transmission from plant to plant.

First finding of a dual-meaning X wave for phloem and xylem fluid ingestion: Characterization of Scaphoideus titanus (Hemiptera: Cicadellidae) EPG waveforms.

The leafhopper Scaphoideus titanus (Hemiptera: Cicadomorpha: Cicadellidae), an invasive deltocephaline species introduced into Europe from North America, is the vector of the most important phytoplasma disease in European viticulture, flavescence dorée. In this first electropenetrography (EPG) study of S. titanus, we characterized its feeding waveforms and defined their biological meanings. Four typical waveform phases (pathway, X wave, sustained ingestion, and interruption) and four families within those phases (A, B, C, and N) were characterized using DC EPG technology. We proposed biological meanings for these waveforms based on excreta pH-ingestion correlations, presence of X waves, and comparison with previous AC, DC, and AC-DC EPG waveforms conducted on Cicadomorpha. We observed that sustained (i.e., >10min) ingestion by a deltocephaline leafhopper can occur from both xylem and phloem vascular cells. Waveform C2x represented ingestion of xylem fluid, and two waveforms represented behaviors when stylets were inserted into phloem sieve elements: C2p variant 1 (C2p-1), which may represent salivation (perhaps simultaneous with ingestion), and C2p variant 2 (C2p-2), which represented active ingestion. Furthermore, we found that the EPG-recorded X wave has a dual meaning by occurring prior to sustained ingestion from either phloem or xylem. This X wave was very similar in appearance to the model X wave of sharpshooters, an entirely different leafhopper subfamily, Cicadellinae. All cicadellines are obligate xylem-ingesters. Such a "dual-meaning X wave" will provide insights into how the feeding tactics of S. titanus relate to other sheath-feeding hemipterans, and will provide support for future research to clarify the role of this leafhopper as a vector of plant pathogens.

Molecular test to assign individuals within the Cacopsylla pruni complex.

Crop protection requires the accurate identification of disease vectors, a task that can be made difficult when these vectors encompass cryptic species. Here we developed a rapid molecular diagnostic test to identify individuals of Cacopsyllapruni (Scopoli, 1763) (Hemiptera: Psyllidae), the main vector of the European stone fruit yellows phytoplasma. This psyllid encompasses two highly divergent genetic groups that are morphologically similar and that are characterized by genotyping several microsatellite markers, a costly and time-consuming protocol. With the aim of developing species-specific PCR primers, we sequenced the Internal Transcribed Spacer 2 (ITS2) on a collection of C. pruni samples from France and other European countries. ITS2 sequences showed that the two genetic groups represent two highly divergent clades. This enabled us to develop specific primers for the assignment of individuals to either genetic group in a single PCR, based on ITS2 amplicon size. All previously assigned individuals yielded bands of expected sizes, and the PCR proved efficient on a larger sample of 799 individuals. Because none appeared heterozygous at the ITS2 locus (i.e., none produced two bands), we inferred that the genetic groups of C. pruni, whose distribution is partly sympatric, constitute biological species that have not exchanged genes for an extended period of time. Other psyllid species (Cacopsylla, Psylla, Triozidae and Aphalaridae) failed to yield any amplicon. These primers are therefore unlikely to produce false positives and allow rapid assignment of C. pruni individuals to either cryptic species.

Multilocus sequence analysis reveals the genetic diversity of European fruit tree phytoplasmas and supports the existence of inter-species recombination.

The genetic diversity of three temperate fruit tree phytoplasmas 'Candidatus Phytoplasma prunorum', 'Ca. P. mali' and 'Ca. P. pyri' has been established by multilocus sequence analysis. Among the four genetic loci used, the genes imp and aceF distinguished 30 and 24 genotypes, respectively, and showed the highest variability. Percentage of substitution for imp ranged from 50 to 68 % according to species. Percentage of substitution varied between 9 and 12 % for aceF, whereas it was between 5 and 6 % for pnp and secY. In the case of 'Ca P. prunorum' the three most prevalent aceF genotypes were detected in both plants and insect vectors, confirming that the prevalent isolates are propagated by insects. The four isolates known to be hypo-virulent had the same aceF sequence, indicating a possible monophyletic origin. Haplotype network reconstructed by eBURST revealed that among the 34 haplotypes of 'Ca. P. prunorum', the four hypo-virulent isolates also grouped together in the same clade. Genotyping of some Spanish and Azerbaijanese 'Ca. P. pyri' isolates showed that they shared some alleles with 'Ca. P. prunorum', supporting for the first time to our knowledge, the existence of inter-species recombination between these two species.

Mapping and validation of QTLs for resistance to aphids and whiteflies in melon.

Aphis gossypii and Bemisia tabaci are severe hemipteran pests of melon crops and breeding for resistance to both insects is required to reduce pesticide use. Resistance was evaluated for its effect on behaviour and biotic potential of both hemipterans in a population of recombinant inbred lines (RILs) derived from the cross Védrantais x PI 161375. Insect variability was considered using two A. gossypii clones and two B. tabaci populations. Two additive QTLs affected the whiteflies. Four additive QTLs and two couples of epistatic QTLs affected the aphids. Amongst them, a major QTL affects both behaviour and biotic potential of A. gossypii and therefore a same R gene induces both antixenosis and antibiosis. This major QTL colocalizes with the Vat gene belonging to the NBS-LRR gene family. No loci affected both aphids and whiteflies contrary to what was observed for the Mi1.2 gene, a NBS-LRR gene in tomato. Original populations with different allelic compositions at QTLs affecting A. gossypii were built by one inter-crossing of RILs used for the mapping process. The genetic background was shown homogeneous between these populations what allowed validating QTLs and investigating the effect of allelic combinations at QTLs. Effects of QTLs were stronger than expected and some QTLs had a wider spectrum than expected. This strategy of validation appeared rapid and low cost.

Efficient transmission of 'Candidatus phytoplasma prunorum' Is delayed by eight months due to a long latency in its host-alternating vector.

Understanding at which spatiotemporal scale a disease causes significant secondary spread has both theoretical and practical implications. We investigated this issue in the case of European stone fruit yellows (ESFY), a quarantine vector-borne phytoplasma disease of Prunus trees. Our work was focused on the processes underlying disease spread: the interplay between the life cycles of the pathogen ('Candidatus Phytoplasma prunorum') and of the vector (Cacopsylla pruni). We demonstrated experimentally that C. pruni has only one generation per year and we showed that, at least in southeastern France, C. pruni migrates between conifers in mountainous regions (where it overwinters) and Prunus spp. at lower altitude (where it breeds). In acquisition-inoculation experiments performed with C. pruni over its period of presence on Prunus spp., both immature and mature C. pruni were hardly infectious (0.6%) despite effective phytoplasma acquisition and multiplication. We demonstrated that most immature vectors born on infected plants reach their maximum phytoplasma load (10(7) genomes per insect) only after migrating to conifers and that, after a life-long retention of the phytoplasma, their transmission efficiency was very high (60%) at the end of winter (when they migrate back to their Prunus host). Thus, most transmissions occur only after an effective latency of 8 months, following vector migrations and overwintering on conifers in mountainous regions. From this transmission cycle, we can infer that local secondary spread of ESFY in apricot orchards is marginal, and recommend that disease management strategies take more into account the processes occurring at a regional scale, including the role of wild Prunus spp. in ESFY epidemics.

Nine polymorphic microsatellite loci from the psyllid Cacopsylla pruni (Scopoli), the vector of European stone fruit yellows.

Cacopsylla pruni is the vector of European stone fruit yellows, a quarantine disease of Prunus trees. Nine polymorphic microsatellite markers were developed from enriched DNA libraries. Allelic variability was assessed in a collection of 149 females obtained from five localities covering a large geographical area in France. The number of detected alleles ranged from 8 to 37. Within the localities, observed and expected heterozygosities averaged across loci ranged from 0.39 to 0.55, and from 0.68 to 0.81, respectively. A heterozygote deficiency was detected for almost all loci, possibly due to a high null allele frequency. Other possible causes of the homozygote excess (mode of reproduction, inbreeding, assortative mating or Wahlund effect) are discussed. These variable microsatellite loci can provide tools to assess overall genetic variation in this important vector species. They will be used to search for population structure and migration patterns of C. pruni.

Identifying risk factors for European stone fruit yellows from a survey.

ABSTRACT European stone fruit yellows (ESFY) is becoming a major economic problem for Prunus growers in Europe. The causal agent ("Candidatus Phytoplasma prunorum") and its vector (Cacopsylla pruni) have been identified, but the present knowledge of the risk factors for this disease relies, at best, on specific experiments. To assess the relative significance of several factors correlated with ESFY incidence in the field, an exhaustive survey was performed on apricot and Japanese plum orchards in the Crau plain (France). After a preliminary multivariate exploration of the data, we used a logistic regression model to analyze and predict the cumulative number of diseased trees on the basis of a set of quantitative (age, planting density, and area of the orchard) and categorical variables (species, cultivar, and rootstock). Because of the nature of the data, we used an overdispersed binomial model and we developed a parametric bootstrap procedure based on the beta-binomial distribution to obtain confidence intervals. Our results indicated that the age, species, and cultivar of the scion were the major factors explaining the observed number of diseased trees. The planting density and the rootstocks used in the zone under study were less significant, and the area of the orchard had no effect. The residuals of the model showed that some explanatory variables had not been taken into account, because part of the remaining variability could be explained by a grower effect. The spatial distribution of the residuals suggested that one of the reasons for this grower effect was the correlation between orchards closer than 100 m, possibly caused by the flight behavior of infectious vectors.

Impact of melon accessions resistant to aphids on the demographic potential of silverleaf whitefly.

Bemisia tabaci (Gennadius) and Aphis gossypii Glover are devastating melon, Cucumis melo L., pests. The geographic areas where they occur overlap, and the same chemicals are used to control both of them. Therefore, to reduce pesticide use, it would be necessary to breed melon lines that simultaneously express a resistance to both insects. Female survival; the time when reproduction starts, peaks, and ends; the number of female offspring at the reproductive peak; and total reproduction (S) were determined under semicontrolled conditions for B. tabaci kept in clip-cages on a susceptible melon genotype Vedrantais, and 12 potential resistant accessions, particularly genotypes expressing the Vat gene controlling resistance to A. gossypii. By using the Lewontin triangular reproductive function and Bootstrapping, the intrinsic rate of increase (r) and its variance were calculated. Statistical analysis showed that the parameter S was as relevant as r for discriminating between the melon accessions. Three genotypes were potential genitors of resistance to the whitefly: PI 161375, PI 414723, and PI 532841. Those possessing the Vat gene were either resistant (PI 161375 and PI 414723) or susceptible (Margot, IsoVat R, and AR 5). This demonstrated the ineffectiveness of Vat against B. tabaci. In this article, we propose a strategy to breed lines that express resistance to aphids and whiteflies on the short-term.

Binding of the insecticidal lectin Concanavalin A in pea aphid, Acyrthosiphon pisum (Harris) and induced effects on the structure of midgut epithelial cells.

Concanavalin A (lectin from Canavalia ensiformis L., ConA) has previously been shown to act as a feeding inhibitor for Acyrthosiphon pisum, the pea aphid. In the present study a range of histochemical and biochemical techniques were used to elucidate the target tissues and binding sites of the lectin in the aphid. Diet uptake was evaluated using a radioactive tracer (14C-methylated inulin) and demonstrated that adults were capable of ingesting high quantities of the toxin (approx. 1 microg over a 48 h period). Electophoretic analysis and enzyme-linked immuno-sorbent assay of honeydew samples confirmed these results and further demonstrated that only small levels of ConA were excreted. Histofluorescence and immunolocalisation studies on nymphs revealed that the stomach was the primary target for ConA. At concentrations up to 400 microg ml(-1), lectin binding only occurred in the stomach region, however, at high concentrations (800 microg ml(-1)) the whole digestive tract was stained, although there was no evidence of binding in either the oesophagus or rectum. In addition to binding, there was evidence to suggest that ConA was also causing systemic effects in that the lectin appeared to cross the intestinal epithelial barrier. Immunohistochemical and electron microscopy studies revealed that ConA induced severe cellular swelling of the epithelial cells, accompanied by hypersecretion and a progressive detachment of the apical membrane; however, the striated border itself did not appear to be directly affected. Furthermore, there was no lysis of the epithelium, nor loss of integrity of the epithelial cells themselves. Our results suggest that ConA interacts with glycosylated receptors at the surface of the stomach epithelial cells, interfering with normal metabolism and cell function, resulting in a rapid feedback response on feeding behaviour. Whilst our results provide a much greater understanding regarding the modes of action of ConA in insects, they suggest that different lectins, including other mannose binding lectins, have different modes of action at the cellular levels, and thus generalizations should be treated with caution.