Spatial and temporal genetic variability in French populations of the peach-potato aphid, Myzus persicae

Predicting the seasonal flight activity of Myzus persicae, the main aphid vector of Virus Yellows in sugar beet

BACKGROUND

Virus Yellows (VY), a disease caused by several aphid-borne viruses, is a major threat to the global sugar beet production. Following the ban of neonicotinoid-based seed treatments against aphids in Europe, increased efforts are needed to monitor and forecast aphid population spread during the sugar beet growing season. In particular, predicting aphid flight seasonal activity could allow anticipation of the timing and intensity of crop colonisation and contribute to the proper implementation of management methods. Forecasts should be made early enough to assess risk, but can be updated as the season progresses to refine management. Based on a long-term suction-trap dataset gathered between 1978 and 2014, we built and evaluated a set of models to predict the flight activity features of the main VY vector, Myzus persicae, at any location in the French sugar beet production area (c. 4 × 105  ha). Flight onset dates, length of flight period and cumulative abundance of flying aphids were predicted using climatic and land-use predictors as well as geographical position.

RESULTS

Our predictions outperformed current models published in the literature. The importance of the predictor variables varied according to the predicted flight feature but winter and early spring temperature always played a major role. Forecasts based on temperature were made more accurate by adding predictors related to aphid winter reservoirs. In addition, updating the model parameters to take advantage of new weather data acquired during the season improved the flight forecast.

CONCLUSION

Our models can be used as a tool for the mitigation in sugar beet crops. © 2023 Society of Chemical Industry.

Mitochondrial genetic diversity of Thrips tabaci (Thysanoptera: Thripidae) in onion growing regions of the United States

Onion thrips (Thrips tabaci Lindeman, Thysanoptera: Thripidae) causes severe damage to many horticultural and agronomic crops worldwide. It also acts as a vector of several plant viruses. T. tabaci is a key pest of Allium cepa in the United States. However, there is limited information available on the genetic variation within and between T. tabaci populations in the United States and its key evolutionary parameters. In the current study, 83 T. tabaci specimens were collected from A. cepa from 15 different locations comprising four states of the United States. A total of 92 mtCOI gene sequences of T. tabaci from A. cepa were analyzed to understand the genetic diversity and structure of T. tabaci collected from onion host. Seven distinct haplotypes of T. tabaci infesting A. cepa were identified from the current collection, while nine T. tabaci sequences retrieved from GenBank comprised 5 haplotypes. Overall, 15 haplotypes of T. tabaci infesting A. cepa were identified in the world that includes the ten haplotypes in the United States. In the phylogenetic analysis, all the populations collected during the study clustered with thelytokous lineage, while T. tabaci sequences retrieved from GenBank corresponded to leek-associated arrhenotokous lineage. The highest genetic variation was found in Elba and Malheur populations with 3 haplotypes identified in each. The results suggest that haplotypes 1 and 7 are more frequently prevailing haplotypes in the north-western United States, with haplotype 1 being the predominant all over the country. The eastern United States appears to have a more diverse group of haplotypes. The populations from Hungary constituted distinct haplotypes and a haplotype from Kingston linked it with the predominant haplotype.

Likelihood of Extreme Early Flight of Myzus persicae (Hemiptera: Aphididae) Across the UK

Myzus persicae (Sulzer, Hemiptera: Aphididae) is a major global crop pest; it is the primary aphid vector for many damaging viruses and has developed resistance to most insecticides. In temperate regions, the risk of widespread crop infection and yield loss is heightened following warm winters, which encourage rapid population growth and early flight. Estimates of the frequency and magnitude of warm winters are, therefore, helpful for understanding and managing this risk. However, it is difficult to quantify the statistical distribution of climate events, particularly extremes, because climate observations represent just a small sample of the possible climate variations in a region. The purpose of this study was to establish a large-scale relationship between temperature and M. persicae observations across the UK and apply this to a very large ensemble of climate model simulations, which better sample the variability in climate, to quantify the current likelihood of extreme early M. persicae flight across the UK. The timing of M. persicae flight was shown to be significantly related to January-February mean temperature, where a 1°C warmer/cooler temperature relates to about 12 d earlier/later flight. Climate model simulations predict 40% likelihood of experiencing a year with unprecedented early M. persicae flight during the next decade in the UK. Results from this method can help crop managers assess the long-term viability of crops and management practices across the UK and provide early warning information for targeting pest surveillance activities on the locations and timings at highest risk of early M. persicae flight.

Spatial Variation in Australian Neonicotinoid Usage and Priorities for Resistance Monitoring

Australia is the third largest exporting country of cereals and a leader in other major commodity crops, yet little data exist on pesticide usage patterns in agriculture. This knowledge gap limits the management of off-target chemical impacts, such as the evolution of pesticide resistance. Here, for the first time, we quantify spatial patterns in neonicotinoid applications in Australia by coalescing land use data with sales and market research data contributed by agrichemical and agribusiness companies. An example application to resistance management is explored through the development of recommendations for the cosmopolitan crop pest, Myzus persicae (Sulzer) (Hemiptera: Aphididae), utilizing spatial statistical models. This novel dataset identified Australian neonicotinoid usage patterns, with most neonicotinoid products in Australia applied as cereal, canola, cotton and legume seed treatments and soil applications in sugarcane. Importantly, there were strong regional differences in pesticide applications, which will require regionally specific strategies to manage off-target impacts. Indeed, the estimated spatial grid of neonicotinoid usage demonstrated a statistically significant influence on the distribution of M. persicae neonicotinoid resistance, indicating off-target impacts are unevenly distributed in space. Future research on neonicotinoid usage will be supported by the spatial grids generated and made available through this study. Overall, neonicotinoid pesticides are widely relied upon throughout Australia's plant production systems but will face increasing pressure from resistance evolution, emerging research on off-target impacts, and stricter regulatory pressures.

Structuration of multilocus genotypes associated with insecticide resistance of the peach potato aphid, Myzus persicae (Sulzer), in potato fields in southern Belgium

BACKGROUND

The peach potato aphid, Myzus persicae, has developed resistance to many insecticides. In Belgium, M. persicae is one of the most common aphids in potato fields and one of the most effective virus vectors. We monitored resistance mutations to pyrethroids, carbamates and neonicotinoids and related these results to microsatellite genotyping to provide information to support the choice of management tactics.

RESULTS

Most of the 254 aphids tested (97.6%) displayed at least one mutation conferring resistance to pyrethroids (L1014F, M918L and M918T) and 36.2% additionally carried the modified acetylcholinesterase (MACE) carbamates resistance making them resistant to two insecticide action modes. Ten mutation combinations were detected, two of which were frequent and a strong linkage was found between MACE and M918L mutations. The R81T mutation conferring resistance to neonicotinoids was not detected. Microsatellites highlighted a moderate genetic diversity [69 multilocus genotypes (MLG) detected], severe deviations from Hardy-Weinberg expectations, a highly significant excess of heterozygotes and linkage disequilibrium between all pairs of loci. A structuration of MLGs in association with the mutation combinations was observed. Genetic differentiation was mainly not significant between sampling locations and most MLGs were geographically widespread. These results suggest the likely coexistence of parthenogenesis (obligatory or facultative) and sexual reproduction, and the existence of 'old' parthenogenetic overwintering asexual lineages.

CONCLUSION

The results of this monitoring at a regional scale provide useful information on insecticide resistance, genetic diversity and reproductive modes, and highlight the need to reduce the insecticide selection pressure and to implement mitigating techniques.

Population Genetic Diversity and Structure of Thrips tabaci (Thysanoptera: Thripidae) on Allium Hosts in China, Inferred From Mitochondrial COI Gene Sequences

Thrips tabaci Lindeman is a widely distributed agricultural pest China, which causes damage to many vegetables and cash crops. However, the population genetic variation of this pest in China remains unknown. In this study, the genetic diversity and structure of T. tabaci on Allium hosts collected from 12 geographic locations were evaluated based on mitochondrial cytochrome oxidase subunit I (COI) sequences. Six haplotypes were identified in 247 T. tabaci individuals from 12 geographic locations. All the identified T. tabaci haplotypes were thelytokous populations. The strongest genetic differentiation and relatively low gene flow were found between QHXN and other locations, which might be due to geographic barriers, such as high altitude Qinghai-Tibet Plateau. The lowest genetic variation was found in eastern and southern regions, with only one haplotype identified. The Mantel test showed no correlation between genetic distance and geographical distances. High gene flow between locations with substantial geographical distances suggested that migration of T. tabaci across China might be facilitated through human activities. The results of demographic analysis suggested that T. tabaci in China have undergone a recent demographic expansion. The possible influences of T. tabaci invasion history and human activities on the current haplotype geographical distribution were interpreted and the implications of these findings for T. tabaci management were discussed.

Spatial and Temporal Genetic Diversity of the Peach Potato Aphid Myzus persicae (Sulzer) in Tunisia

The peach potato aphid, Myzus persicae (Sulzer), is a worldwide pest of many crops, and the most important aphid pest of peach and potato crops in Tunisia, mainly due to virus transmission, for which insecticides are frequently applied. We studied the genetic structure of M. persicae populations in Tunisia, in order to further our understanding of the biotic and abiotic factors shaping populations and to predict their evolutionary responses to the present management practices. We monitored peach orchards and seed potato crops in different seasons and regions from 2011–2013 and in 2016 (19 populations), assessing the genetic diversity of M. persicae at six microsatellite loci. Temporal and spatial changes in the frequency and distribution of 397 genotypes in 548 sampled aphids were studied. Only 37 genotypes were found more than once (clonal amplification), as most genotypes were found only once (91.60% in peach; 88.73% in potato crops). A similarly high genetic diversity was observed in aphids sampled from peach (G/N = 0.76; Ho = 0.617) and potato (G/N = 0.70; Ho = 0.641). Only a weak genetic differentiation among populations was found, mainly between geographic locations. Clustering analysis revealed genotypes to be grouped mainly according to host plant. The availability of the primary host, high proportion of unique genotypes, high genetic diversity and lack of structuring suggest that the aphid reproduces mainly through cyclical parthenogenesis in Tunisia. On the other hand, we provide a farm-scale study that shows how easily M. persicae can colonize different areas and hosts, which may have important implications in relation to plant virus vectoring.

Using host-associated differentiation to track source population and dispersal distance among insect vectors of plant pathogens

Small, mobile insects are notoriously challenging to track across landscapes and manage in agricultural fields. However, genetic differentiation among insect populations and host plants acquired through host-associated differentiation could be exploited to infer movement within crop systems and damage potential. Although many insects exhibit host-associated differentiation, management strategies for insect vectors of plant pathogens assume a homogenous population. Nevertheless, phenotypic changes derived from host-associated differentiation could manifest in altered behavior or physiology affecting the likelihood of vector-pathogen-plant interactions, or the subsequent efficiency of pathogen transmission. We used SNPs to assess genotypic structure and host-associated differentiation in the cowpea aphid, Aphis craccivora Koch (Hemiptera: Aphididae). To do so, we sampled A. craccivora across the Midwestern United States. from two host plants, alfalfa (Medicago sativa) and black locust (Robinia pseudoacacia)-putative source populations for winged migrants. Simultaneously, we sampled winged A. craccivora landing in pumpkin fields where they transmit viruses. Structure analyses supported host-associated differentiation by identifying two major genotypic groups: an alfalfa group containing a single multilocus genotype and a locust group containing all others. Winged locust-group aphids landed at a much greater magnitude within focal fields during year 2 than year 1, while those in the alfalfa group remained fairly consistent. Spatial autocorrelation analyses indicated locust-group aphid movement was characterized by small-scale dispersal during year 2, likely originating from populations within 10 km. We also detected strong temporal differences in colonization from the two host plants. Early in the summer, most winged aphids (79.4%) derived from the locust group, whereas late in the summer more (58.3%) were from the alfalfa group. Because early crop growth stages are more susceptible to damage from aphid-vectored viruses, these data implicate locust as the more important source and illustrate how host-associated differentiation can be used to track dispersal and inform management of heterogeneous pest populations.

Chemical Ecology and Sociality in Aphids: Opportunities and Directions

Aphids have long been recognized as good phytochemists. They are small sap-feeding plant herbivores with complex life cycles that can involve cyclical parthenogenesis and seasonal host plant alternation, and most are plant specialists. Aphids have distinctive traits for identifying and exploiting their host plants, including the expression of polyphenisms, a form of discrete phenotypic plasticity characteristic of insects, but taken to extreme in aphids. In a relatively small number of species, a social polyphenism occurs, involving sub-adult "soldiers" that are behaviorally or morphologically specialized to defend their nestmates from predators. Soldiers are sterile in many species, constituting a form of eusociality and reproductive division of labor that bears striking resemblances with other social insects. Despite a wealth of knowledge about the chemical ecology of non-social aphids and their phytophagous lifestyles, the molecular and chemoecological mechanisms involved in social polyphenisms in aphids are poorly understood. We provide a brief primer on aspects of aphid life cycles and chemical ecology for the non-specialists, and an overview of the social biology of aphids, with special attention to chemoecological perspectives. We discuss some of our own efforts to characterize how host plant chemistry may shape social traits in aphids. As good phytochemists, social aphids provide a bridge between the study of insect social evolution sociality, and the chemical ecology of plant-insect interactions. Aphids provide many promising opportunities for the study of sociality in insects, and to understand both the convergent and novel traits that characterize complex sociality on plants.

Responses of holocyclic and anholocyclic Rhopalosiphum padi populations to low-temperature and short-photoperiod induction

The different life cycles of aphid species make these organisms good models for studying the short‐term consequences of sex. The bird cherry‐oat aphid Rhopalosiphum padi has a wide geographic distribution and correspondingly different life cycles. In this study, the life cycles of R. padi collected from six different regions in China were characterized experimentally by comparing the responses of holocyclic and anholocyclic populations to low‐temperature and short‐photoperiod induction. Clones collected from Chuzhou, Taian, and Taigu consistently reproduced via obligate parthenogenesis, whereas clones from Hami and Baicheng were holocyclic in their response, and those from Lanzhou were both holocyclic and anholocyclic. Prolonged exposure to low temperature and a short photoperiod (LS) had negative effects on the offspring of anholocyclic aphids with regard to adult lifespan, total longevity, and fecundity compared with aphids maintained at a normal temperature and a long photoperiod (NL). Holocyclic LS R. padi had longer developmental times at all nymph stages, a shorter adult lifespan, shorter total longevity, and a lower fecundity than NL counterparts. The adult prereproduction period of gynoparae was significantly longer than that of virginoparae, and the total longevity of gynoparae was significantly shorter than that of virginoparae. Moreover, the net reproductive and gross reproduction rates, as well as the total fecundity, were roughly fivefold higher in virginoparae than in gynoparae, indicating that there is the short‐term cost of sex. When maintained on their secondary host (Triticum aestivum), gynoparae, males, and oviparae produced by holocyclic populations could survive, and gynoparae produced oviparae. However, under NL conditions, oviparae could not produce overwintering eggs on the secondary host, whereas a few overwintering eggs were generated by oviparae under LS conditions. Taken together, these results illuminate the complexity of insect responses and contribute to a complete understanding of the aphid life cycle and its evolution.

The Distribution and Host Shifts of Cotton-Melon Aphids in Northern China

Aphis gossypii Glover (Hemiptera: Aphididae) is a serious pest of cotton in northern China. A microsatellite analysis was used to characterize the genetic structure of A. gossypii populations from different geographic, host plant, and seasonal populations in 2014. Among 906 individuals, 507 multilocus genotypes were identified, with genotypic richness values of 0.07–1.00 for the populations. We observed moderate levels of genetic differentiation among geographic populations (F_ST = 0.103; 95% confidence interval: 0.065–0.145) and host plant populations (F_ST = 0.237; 95% confidence interval: 0.187–0.296). A Mantel test of isolation by distance revealed no significant correlations between Slatkin’s linearized F_ST and the natural logarithm of geographic distance. A Bayesian analysis of population genetic structures identified three clusters. An analysis of molecular variance revealed significant differences among the three clusters (F = 0.26596, P < 0.0001), among seasons (F = 0.04244, P = 0.00381), and among host populations (F = 0.12975, P = 0.0029). Thus, the A. gossypii populations in northern China exhibit considerable genotypic diversity. Additionally, our findings indicated that the 31 analyzed populations could be classified as one of three host biotypes (i.e., cotton, cucumber, and pomegranate biotypes). There were also clear seasonal effects on population genetic structure diversity among aphids collected from Anyang.

High Spatial Genetic Structure and Genetic Diversity in Chinese Populations of Sitobion miscanthi (Hemiptera: Aphididae)

The wheat aphid, Sitobion miscanthi Takahashi, a serious wheat pest, was previously considered to be highly migratory and anholocyclic in China. We recorded 69 alleles and 346 multilocus genotypes among 708 aphid individuals from 12 populations in China using 5 microsatellite loci. This genotypic diversity indicates that at least some holocyclic lineages exist. Bayesian clustering analysis revealed that there are two differentiated genetic groups of S. misanthi, one northern and one southern, in China. Principal coordinates analysis of population genetic distance, pairwise F(ST)'s, and network analysis of individual minimum spanning distance also supported the division. Low levels of migration were detected between the northern and southern sampling sites, but the high genetic differentiation does not support the hypothesis S. miscanthi overwinters in the south and migrates to the north in the spring annually.

Identification of the Population Structure of Myzus persicae (Hemiptera: Aphididae) on Peach Trees in China Using Microsatellites

In this study, we characterized the genetic structure of Myzus persicae (Sulzer) (Hemiptera: Aphididae) populations in China using microsatellites. We expected that these data will reveal the genetic relationships among various populations of M. persicae and will be of value in the development of better methods for pest control. Four hundred sixty individuals from 23 areas over 13 provinces were collected in the early spring of 2010, all from their primary host, Prunus persicae. The markers analyzed were highly polymorphic, as demonstrated by the expected heterozygosity value (He = 0.861) and the Polymorphism Information Content (PIC = 0.847), which indicated that M. persicae maintains a high level of genetic diversity. Analysis of molecular variance revealed an intermediate level of population differentiation among M. persicae populations (F(ST) = 0.1215). Geographic isolation existed among these populations, and, consequently, the genetic structure of the populations was split into a southern group and a northern group divided by the Yangtse River.

Genetic diversity of Sitobion avenae (Homoptera: Aphididae) populations from different geographic regions in China

Sitobion avenae is a major agricultural pest of wheat in China. Using microsatellite markers, we studied the potential gene flow, genetic diversity, genetic differentiation, and genetic structure of seven S. avenae populations from different regions of China (Beijing, Hebei, Henan, Hubei, Jiangsu, Shandong, and Shanxi provinces). The populations from Henan, Shandong, and Jiangsu showed high levels of genic and genotypic diversity. By contrast, the genic diversity in the Beijing and Hebei populations was much lower. Despite this low genic diversity, the genotypic diversity of the Beijing population was higher than that of all of the other populations, except those from Jiangsu and Shandong. Overall, the genetic divergence among the seven S. avenae populations tested was high, though there was almost no differentiation between the Shandong and Henan populations. We observed significant negative correlation between the strength of gene flow and the geographic distances among populations. Based on genetic analysis, the seven S. avenae populations studied can be divided into four distinct clusters; (i) Hubei, (ii) Shanxi, (iii) Beijing and Hebei, and (iv) Shandong, Henan, and Jiangsu. The present results provide a basis for potentially optimizing integrated pest management (IPM) programs in China, through adapting control methods that target biological traits shared by various populations of the same genotype.

Genetic diversity and insecticide resistance during the growing season in the green peach aphid (Hemiptera: Aphididae) on primary and secondary hosts: a farm-scale study in Central Chile

The seasonal dynamics of neutral genetic diversity and the insecticide resistance mechanisms of insect pests at the farm scale are still poorly documented. Here this was addressed in the green peach aphid Myzus persicae (Sulzer) (Hemiptera: Aphididae) in Central Chile. Samples were collected from an insecticide sprayed peach (Prunus persica L.) orchard (primary host), and a sweet-pepper (Capsicum annum var. grossum L.) field (secondary host). In addition, aphids from weeds (secondary hosts) growing among these crops were also sampled. Many unique multilocus genotypes were found on peach trees, while secondary hosts were colonized mostly by the six most common genotypes, which were predominantly sensitive to insecticides. In both fields, a small but significant genetic differentiation was found between aphids on the crops vs. their weeds. Within-season comparisons showed genetic differentiation between early and late season samples from peach, as well as for weeds in the peach orchard. The knock-down resistance (kdr) mutation was detected mostly in the heterozygote state, often associated with modified acetylcholinesterase throughout the season for both crops. This mutation was found in high frequency, mainly in the peach orchard. The super-kdr mutation was found in very low frequencies in both crops. This study provides farm-scale evidence that the aphid M. persicae can be composed of slightly different genetic groups between contiguous populations of primary and secondary hosts exhibiting different dynamics of insecticide resistance through the growing season.

Molecular analysis reveals high compartmentalization in aphid-primary parasitoid networks and low parasitoid sharing between crop and noncrop habitats

The ecosystem service of insect pest regulation by natural enemies, such as primary parasitoids, may be enhanced by the presence of uncultivated, semi-natural habitats within agro-ecosystems, although quantifying such host-parasitoid interactions is difficult. Here, we use rRNA 16S gene sequencing to assess both the level of parasitism by Aphidiinae primary parasitoids and parasitoid identity on a large sample of aphids collected in cultivated and uncultivated agricultural habitats in Western France. We used these data to construct ecological networks to assess the level of compartmentalization between aphid and parasitoid food webs of cultivated and uncultivated habitats. We evaluated the extent to which uncultivated margins provided a resource for parasitoids shared between pest and nonpest aphids. We compared the observed quantitative ecological network described by our molecular approach to an empirical qualitative network based on aphid-parasitoid interactions from traditional rearing data found in the literature. We found that the molecular network was highly compartmentalized and that parasitoid sharing is relatively rare between aphids, especially between crop and noncrop compartments. Moreover, the few cases of putative shared generalist parasitoids were questionable and could be due to the lack of discrimination of cryptic species or from intraspecific host specialization. Our results suggest that apparent competition mediated by Aphidiinae parasitoids is probably rare in agricultural areas and that the contribution of field margins as a source of these biocontrol agents is much more limited than expected. Further large-scale (spatial and temporal) studies on other crops and noncrop habitats are needed to confirm this.

Inferring the population structure of Myzus persicae in diverse agroecosystems using microsatellite markers

Diverse agroecosystems offer phytophagous insects a wide choice of host plants. Myzus persicae is a polyphagous aphid common in moderate climates. During its life cycle it alternates between primary and secondary hosts. A spatial genetic population structure may arise due to environmental factors and reproduction modes. The aim of this work was to determine the spatial and temporal genetic population structure of M. persicae in relation to host plants and climatic conditions. For this, 923 individuals of M. persicae collected from six plant families between 2005 and 2008 in south-eastern Spain were genotyped for eight microsatellite loci. The population structure was inferred by neighbour-joining, analysis of molecular variance (AMOVA) and Bayesian analyses. Moderate polymorphism was observed for the eight loci in almost all the samples. No differences in the number of alleles were observed between primary and secondary hosts or between geographical areas. The proportion of unique genotypes found in the primary host was similar in the north (0.961 ± 0.036) and the south (0.987 ± 0.013), while in the secondary host it was higher in the north (0.801 ± 0.159) than in the south (0.318 ± 0.063). Heterozygosity excess and linkage disequilibrium suggest a high representation of obligate parthenogens in areas with warmer climate and in the secondary hosts. The F ST-values pointed to no genetic differentiation of M. persicae on the different plant families. F ST-values, AMOVA and Bayesian model-based cluster analyses pointed to a significant population structure that was related to primary and secondary hosts. Differences between primary and secondary hosts could be due to the overrepresentation of parthenogens on herbaceous plants.

Spatial and temporal genetic variation in Tunisian field populations of Macrosiphum euphorbiae (Thomas)

The potato aphid Macrosiphum euphorbiae (Thomas) is a major pest of several economic crops in Tunisia. Using 10 microsatellites, we analyzed five populations of M. euphorbiae sampled during the 2004-2005 solanaceous and cucurbit season (April through September) from five geographic origins. From 235 aphids, 61 different multilocus genotypes were identified of which three genotypes MLG1, MLG2, and MLG42 were predominant on all host plants and regions. MLG1 and MLG2 genotypes were detected in 2004 and did not reappear in spring 2005, while the genotype MLG42 was detected only in 2005. All populations showed significant deviation from Hardy-Weinberg equilibrium even in data sets including one individual per genotype. χ(2) independence tests and analysis of molecular variance showed no significant differentiation among populations collected on different host plant and from different geographic origin, but differences between populations from two successive years were significant. Factorial component analysis corroborates these results. The probable causes of this seasonal variation were discussed.

Climate effects on life cycle variation and population genetic architecture of the black bean aphid, Aphis fabae

Aphid species may exhibit different reproductive modes ranging from cyclical to obligate parthenogenesis. The distribution of life cycle variation in aphids is generally determined by ecological forces, mainly climate, because only sexually produced diapausing eggs can survive harsh winters or periods of absence of suitable host plants. Aphids are thus interesting models to investigate intrinsic and environmental factors shaping the competition among sexual and asexual lineages. We conducted a Europe-wide sampling of black bean aphids, Aphis fabae, and combined population genetic analyses based on microsatellite data with an experimental determination of life cycle strategies. Aphids were collected from broad beans (Vicia faba) as well as some Chenopodiaceae, but we detected no genetic differentiation between aphids from different host plants. Consistent with model predictions, life cycle variation was related to climate, with aphids from areas with cold winters investing more in sexual reproduction than aphids from areas with mild winters. Accordingly, only populations from mild areas exhibited a clear genetic signature of clonal reproduction. These differences arise despite substantial gene flow over large distances, which was evident from a very low geographic population structure and a lack of isolation-by-distance among 18 sites across distances of more than 1000 km. There was virtually no genetic differentiation between aphids with different reproductive modes, suggesting that new asexual lineages are formed continuously. Indeed, a surprising number of A. fabae genotypes even from colder climates produced some parthenogenetic offspring under simulated winter conditions. From this we predict that a shift to predominantly asexual reproduction could take place rapidly under climate warming.

Levels of clonal mixing in the black bean aphid Aphis fabae, a facultative ant mutualist

Aphids are a worldwide pest and an important model in ecology and evolution. Little is known, however, about the genetic structure of their colonies at a microgeographic level. For example, it remains largely unknown whether most species form monoclonal or polyclonal colonies. Here, we present the first detailed study on levels of clonal mixing in a nonsocial facultative ant mutualist, the black bean aphid Aphis fabae. In contrast to the earlier suggestion that colonies of this species are generally monoclonal, we found that across two subspecies of the black bean aphid, A. fabae cirsiiacanthoidis and A. fabae fabae, 32% and 67% of the aphid colonies were in fact polyclonal, consisting of a mix of up to four different clones, which resulted in an overall average relatedness within colonies of 0.90 and 0.79 in the two subspecies. Data further show that the average relatedness in A. f. cirsiiacanthoidis remained relatively constant throughout the season, which means that clonal erosion due to clonal selection more or less balanced with the influx of new clones from elsewhere. Nevertheless, relatedness tended to decrease over the lifetime of a given colony, implying that clonal mixing primarily resulted from the joining of pre-existing colonies as opposed to via simultaneous host colonisation by several foundresses. Widespread clonal mixing is argued to affect the ecology and evolution of the aphids in various important ways, for example with respect to the costs and benefits of group living, the evolution of dispersal and the interaction with predators as well as with the ant mutualists.

Weak spatial and temporal population genetic structure in the rosy apple aphid, Dysaphis plantaginea, in French apple orchards

We used eight microsatellite loci and a set of 20 aphid samples to investigate the spatial and temporal genetic structure of rosy apple aphid populations from 13 apple orchards situated in four different regions in France. Genetic variability was very similar between orchard populations and between winged populations collected before sexual reproduction in the fall and populations collected from colonies in the spring. A very small proportion of individuals (∼2%) had identical multilocus genotypes. Genetic differentiation between orchards was low (F(ST)<0.026), with significant differentiation observed only between orchards from different regions, but no isolation by distance was detected. These results are consistent with high levels of genetic mixing in holocyclic Dysaphis plantaginae populations (host alternation through migration and sexual reproduction). These findings concerning the adaptation of the rosy apple aphid have potential consequences for pest management.

Why are there so few aphid clones?

In Europe, aphids contribute significantly to the so-called 'aerial plankton' during the spring to autumn months (growing season), although individual flight behaviour has been found, especially from molecular ecological studies, to be species-specific in terms of migratory range (ambit). Many of these species individuals may be assumed to be clonal in origin, that is, derived from a single asexual foundress. We are presently studying two specialist aphid species on Tansy, Tanacetum vulgare L. from samples collected in Jena, Germany - Macrosiphoniella tanacetaria (Kaltenbach) and Metopeurum fuscoviride Stroyan, using microsatellite markers. On plotting the number of sets of different multilocus genotypes or MLGs (i.e. multiple clonal repeats: 1, 2, 3 copies, etc.), against the frequency of their occurrence, a negative exponential relationship was found, with populations of both species consisting mostly of single (i.e. unique) or low number repeats rather than larger multiple copy (clonal) MLG repeats. To test this further, microsatellite data collected from a previous study on M. tanacetaria in Jena in the year 2000 and on samples of the Grain aphid, Sitobion avenae (F.), collected in the UK in 1997/8, the latter both in the field and from 12.2 m high suction traps, were examined in the same way. Again, similar relations were found, with most MLGs occurring as unique or low copy number repeats. The data are briefly discussed in the light of our evidence, as well as that of other similar studies on other aphid species, relating aphid molecular genetic data to aphid life cycle, behaviour and ecology.

Reproductive performance of asexual clones of the peach-potato aphid, ( Myzus persicae, Homoptera: Aphididae), colonising Scotland in relation to host plant and field ecology

The population of peach-potato aphid, Myzus persicae (Sulzer), in Scotland comprises large numbers of a few superclones with much smaller numbers of other clones, and the reason for their differential success has yet to be elucidated. In the current study, the reproduction of lineages derived from these clones was measured by counting the numbers of offspring produced by a one-day-old nymph after 15 days. This was measured on four plant species, including local agricultural hosts and at two different temperatures (14 and 18 degrees C). There were significant differences in clonal lineage reproduction on different hosts and at different temperatures and amongst clonal lineages on the same hosts at the same temperature. Lineages of local insecticide sensitive clones did not have the best reproductive potential; instead, a recently introduced clonal lineage carrying MACE insecticide resistance was the best reproducer. The clonal lineage with the lowest reproductive potential also carried insecticide resistance, but this was kdr. A lineage from a local insecticide-sensitive clone was the least affected by reduced temperature. There was evidence of host plant specialisation in some of the clonal lineages.

Climate and agricultural context shape reproductive mode variation in an aphid crop pest

In aphids, reproductive mode is generally assumed to be selected for by winter climate. Sexual lineages produce frost-resistant eggs, conferring an advantage in regions with cold winters, while asexual lineages predominate in regions with mild winters. However, habitat and resource heterogeneities are known to exert a strong influence on sex maintenance and might modulate the effect of climate on aphid reproductive strategies. We carried out a hierarchical sampling in northern France to investigate whether reproductive mode variation of the aphid Rhopalosiphum padi is driven by winter climate conditions, by habitat and resource heterogeneities represented by a range of host plants or by both factors. We confirmed the coexistence in R. padi populations of two genetic clusters associated with distinct reproductive strategies. Asexual lineages predominated, whatever the surveyed year and location. However, we detected a between-year variation in the local contribution of both clusters, presumably associated with preceding winter severity. No evidence for host-driven niche differentiation was found in the field on six Poaceae among sexual and asexual lineages. Two dominant multilocus genotypes ( approximately 70% of the sample), having persisted over a 10-year period, were equally abundant on different plant species and locations, indicating their large ecological tolerance. Our results fit theoretical predictions of the influence of winter climate on the balance between sexual and asexual lineages. They also highlight the importance of current agricultural practices which seem to favour a small number of asexual generalist genotypes and their migration across large areas of monotonous environments.

Tracking the global dispersal of a cosmopolitan insect pest, the peach potato aphid

Background

Global commerce and human transportation are responsible for the range expansion of various insect pests such as the plant sucking aphids. High resolution DNA markers provide the opportunity to examine the genetic structure of aphid populations, identify aphid genotypes and infer their evolutionary history and routes of expansion which is of value in developing management strategies. One of the most widespread aphid species is the peach-potato aphid Myzus persicae, which is considered as a serious pest on various crops in many parts of the world. The present study examined the genetic variation of this aphid at a world scale and then related this to distribution patterns. In particular, 197 aphid parthenogenetic lineages from around the world were analysed with six microsatellite loci.

Results

Bayesian clustering and admixture analysis split the aphid genotypes into three genetic clusters: European M. persicae persicae, New Zealand M. persicae persicae and Global M. persicae nicotianae. This partition was supported by F_ST and genetic distance analyses. The results showed two further points, a possible connection between genotypes found in the UK and New Zealand and globalization of nicotianae associated with colonisation of regions where tobacco is not cultivated. In addition, we report the presence of geographically widespread clones and for the first time the presence of a nicotianae genotype in the Old and New World. Lastly, heterozygote deficiency was detected in some sexual and asexual populations.

Conclusion

The study revealed important genetic variation among the aphid populations we examined and this was partitioned according to region and host-plant. Clonal selection and gene flow between sexual and asexual lineages are important factors shaping the genetic structure of the aphid populations. In addition, the results reflected the globalization of two subspecies of M. persicae with successful clones being spread at various scales throughout the world. A subspecies appears to result from direct selection on tobacco plants. This information highlights the ultimate ability of a polyphagous aphid species to generate and maintain ecologically successful gene combinations through clonal propagation and the role of human transportation and global commerce for expanding their range.

Allelic and genotypic diversity in long-term asexual populations of the pea aphid, Acyrthosiphon pisum in comparison with sexual populations

Many aphid species exhibit geographical variation in the mode of reproduction that ranges from cyclical parthenogenesis with a sexual phase to obligate parthenogenesis (asexual reproduction). Theoretical studies predict that organisms reproducing asexually should maintain higher allelic diversity per locus but lower genotypic diversity than organisms reproducing sexually. To corroborate this hypothesis, we evaluated genotypic and allelic diversities in the sexual and asexual populations of the pea aphid, Acyrthosiphon pisum (Harris). Microsatellite analysis revealed that populations in central Japan are asexual, whereas populations in northern Japan are obligatorily sexual. No mixed populations were detected in our study sites. Phylogenetic analysis using microsatellite data and mitochondrial cytochrome oxidase subunit I (COI) gene sequences revealed a long history of asexuality in central Japan and negated the possibility of the recent origin of the asexual populations from the sexual populations. Asexual populations exhibited much lower genotypic diversity but higher allelic richness per locus than did sexual populations. Asexual populations consisted of a few predominant clones that were considerably differentiated from one another. Sexual populations on alfalfa, an exotic plant in Japan, were most closely related to asexual populations associated with Vicia sativa L. The alfalfa-associated sexual populations harboured one COI haplotype that was included in the haplotype clade of the asexual populations. Available evidence suggests that the sexuality of the alfalfa-associated populations has recently been restored through the northward migration and colonization of alfalfa by V. sativa-associated lineages. Therefore, our results support the theoretical predictions and provide a new perspective on the origin of sexual populations.

Bad housekeeping: why do aphids leave their exuviae inside the colony?

Background

Animals can gain protection against predators and parasites by living in groups. The encounter-dilution effect provides protection when the probability of detection of a group does not increase in proportion to group size (i.e. encounter effect), so that predators do not offset the encounter effect by attacking more members of the group (i.e. dilution effect). In this paper, we propose a novel mechanism by which prey insects could gain by producing decoys that act as multiple targets for predators or parasitoids if these decoys are recognised as preys or hosts and negatively affect the patch foraging strategy of these predators and parasitoids. Such a decoy mechanism could be present in aphid colonies in which aphid exuviae are recognised and attacked by Aphidiine wasps.

Results

We conducted a behavioural study to evaluate the effect of exuviae on parasitoid patch residence time and egg allocation in experimental aphid patches with or without exuviae. We showed that exuviae are recognised and attacked at the same level as aphids when both are present in the patch. While parasitism rate was not significantly lower in patches with exuviae when the parasitoid left the patch, the time wasted by parasitoids to handle exuviae did influence the patch residence time. As a consequence, the attack rate on the live aphids was lower in patches that contain exuviae.

Conclusion

Aphids had more time available to flee and thus each individual might gain protection against parasitoids by leaving their exuviae near and within the colony. These results demonstrate that the encounter-dilution effect provided by living in a group can be enhanced by extra-materials that act as decoy for natural enemies.

Seasonal and annual genotypic variation and the effect of climate on population genetic structure of the cereal aphid Sitobion avenae in northern France

Changes in the genetic structure and genotypic variation of the aphid Sitobion avenae collected from cereal crops in northern France were examined by analysing variation at five microsatellite loci across several years and seasons. Little regional and temporal differentiation was detected, as shown by very low FST among populations. Repeated genotypes, significant heterozygote deficits, positive FIS values and frequent linkage disequilibria were found in nearly all samples, suggesting an overall pattern of reproductive mode variation in S. avenae populations. In addition, samples from Brittany (Bretagne) showed greater signs of asexual reproduction than those from the north of France, indicating a trend toward increasing sexuality northward. These patterns of reproductive variation in S. avenae are consistent with theoretical models of selection of aphid reproductive modes by climate. Contrasting with little changes in allelic frequencies, genotypic composition varied substantially in time and, to a lesser extent, in space. An important part of changes in genotypic arrays was due to the variation in frequency distribution of common genotypes, i.e. those that were found at several instances in the samples. Genotypic composition was also shown to vary according to climate, as genotypic diversity in spring was significantly correlated with the severity of the previous winter and autumn. We propose that the genetic homogeneity among S. avenae populations shown here across large temporal and spatial scales is the result of two forces: (i) migration conferred by high dispersal capabilities, and (ii) selection over millions of hectares of cereals (mostly wheat) bred from a narrow genetic base.

Distribution of common genotypes of Myzus persicae (Hemiptera: Aphididae) in Greece, in relation to life cycle and host plant

Microsatellite genotyping was used to identify common clones in populations of the Myzus persicae group from various hosts and regions in mainland Greece and southern Italy and to compare their distribution and occurrence on tobacco and other crops. Common clones were defined as genotypes collected at more than one time or in more than one population; and, therefore, unlikely to be participating in the annual sexual phase on peach. Sixteen common genotypes were found, accounting for 49.0% of the 482 clonal lineages examined. Eight of these genotypes were subjected, in the laboratory, to short days and found to continue parthenogenetic reproduction, i.e. they were anholocyclic. Four of the six commonest genotypes were red, and one of these accounted for 29.6% of the samples from tobacco and 29.4% of those from overwintering populations on weeds. All six commonest genotypes were found on weeds and five of them both on tobacco and on other field crops. In mainland Greece, the distribution of common clones corresponded closely with that of anholocyclic lineages reported in a previous study of life cycle variation. Common genotypes were in the minority in the commercial peach-growing areas in the north, except on weeds in winter and in tobacco seedbeds in early spring, but predominated further south, away from peach trees. This contrasts with the situation in southern Italy, reported in a previous paper, where peaches were available for the sexual phase, yet all samples from tobacco were of common genotypes.

Temporal dynamics and linkage disequilibrium in natural Caenorhabditis elegans populations

Caenorhabditis elegans is a major laboratory model system yet a newcomer to the field of population genetics, and relatively little is known of its biology in the wild. Recent studies of natural populations at a single time point revealed strong spatial population structure and suggested that these populations may be very dynamic. We have therefore studied several natural C. elegans populations over time and genotyped them at polymorphic microsatellite loci. While some populations appear to be genetically stable over the course of observation, others seem to go extinct, with full replacement of multilocus genotypes upon regrowth. The frequency of heterozygotes indicates that outcrossing occurs at a mean frequency of 1.7% and is variable between populations. However, in genetically stable populations, linkage disequilibrium between different chromosomes can be maintained over several years at a level much higher than expected from the heterozygote frequency. C. elegans seems to follow metapopulation dynamics, and the maintenance of linkage disequilibrium despite a low yet significant level of outcrossing suggests that selection may act against the progeny of outcrossings.

Temporal and spatial incidence of alleles conferring knockdown resistance to pyrethroids in the peach-potato aphid, Myzus persicae (Hemiptera: Aphididae), and their association with other insecticide resistance mechanisms

The peach-potato aphid, Myzus persicae (sulzer), is an important arable pest species throughout the world. Extensive use of insecticides has led to the selection of resistance to most chemical classes including organochlorines, organophosphates, carbamates and pyrethroids. Resistance to pyrethroids is often the result of mutations in the para-type sodium channel protein (knockdown resistance or kdr). In M. persicae, knockdown resistance is associated with two amino-acid substitutions, L1014F (kdr) and M918T (super-kdr). In this study, the temporal and spatial distributions of these mutations, diagnosed using an allelic discriminating polymerase chain reaction assay, were investigated alongside other resistance mechanisms (modified acetylcholinesterase (MACE) and elevated carboxylesterases). Samples were collected from the UK, mainland Europe, Zimbabwe and south-eastern Australia. The kdr mutation and elevated carboxylesterases were widely distributed and recorded from nearly every country. MACE and super-kdr were widespread in Europe but absent from Australian samples. The detection of a strongly significant heterozygote excess for both kdr and super-kdr throughout implies strong selection against individuals homozygous for these resistance mutations. The pattern of distribution found in the UK seemed to indicate strong selection against the super-kdr (but not the kdr) mutation in any genotype, in the absence of insecticide pressure. There was a significant association (linkage disequilibrium) between different resistance mechanisms, which was probably promoted by a lack of recombination due to parthenogenesis.

Temporal dynamics of genotypic diversity reveal strong clonal selection in the aphid Myzus persicae

Parthenogenetic organisms often harbour substantial genotypic diversity. This diversity may be the result of recurrent formations of new clones, or it may be maintained by environmental heterogeneity acting on ecological differences among clones. In aphids, both processes may be important because obligate and cyclical parthenogens can form mixed populations. Using microsatellites, I analysed the temporal dynamics of clonal diversity in such a population of the aphid Myzus persicae over a 1-year period. The frequency distribution of clonal genotypes was very skewed, with many rare and few common clones. The relative frequencies of common clones underwent strong and rapid changes indicative of intense clonal selection. Differences in their host associations suggest that these shifts may partly be caused by changes in the abundance of annual host plants. Other selective factors of potential importance are also discussed. New, sexually produced genotypes made a minor contribution to clonal diversity, consistent with the observed heterozygote excess characteristic of predominantly asexual populations in M. persicae.

The attack of the clones: tracking the movement of insecticide-resistant peach-potato aphids Myzus persicae (Hemiptera: Aphididae)

Myzus persicae (Sulzer) collected in Scotland were characterized for four microsatellite loci, intergenic spacer fingerprints and the resistance mechanisms modified acetylcholinesterase (MACE), overproduced carboxylesterase and knockdown resistance (kdr). Microsatellite polymorphisms were used to define a limited number of clones that were either fully susceptible to insecticides or possessed characteristic combinations of resistance mechanisms. Within these clones, intergenic spacer fingerprints could either be very consistent or variable, with the latter indicating ongoing evolution within lineages, most likely derived from the same zygote. Two clones (termed A and B) possessed all three resistance mechanisms and predominated at sites treated with insecticides. Their appearance on seed potatoes and oilseed rape in Scotland in 2001 coincided with extensive insecticide use and severe control failures. Clones C, I and J, with no or fewer resistance mechanisms, were found in samples from 1995 and were dominant at untreated sites in 2001. A comparison of Scottish collections with those from other UK and non-UK sites provides insight into the likely origins, distribution and dynamics of M. persicae clones in a region where asexual (anholocyclic) reproduction predominates, but is vulnerable to migration by novel genotypes from areas of Europe where sexual (holocyclic) reproduction occurs.

Does insecticide resistance alone account for the low genetic variability of asexually reproducing populations of the peach-potato aphid Myzus persicae?

The typical life cycle of aphids includes several parthenogenetic generations and a single sexual generation (cyclical parthenogenesis), but some species or populations are totally asexual (obligate parthenogenesis). Genetic variability is generally low in these asexually reproducing populations, that is, few genotypes are spread over large geographic areas. Both genetic drift and natural selection are often invoked to account for this low genetic variability. The peach-potato aphid, Myzus persicae, which encompasses both cyclical and obligate parthenogens, has developed several insecticide resistance mechanisms as a consequence of intense insecticide use since the 1950s. We collected asexually reproducing M. persicae from oilseed rape and examined genetic variability at eight microsatellite loci and three insecticide resistance genes to determine whether their genetic structure was driven by drift and/or selection. We identified only 16 multilocus microsatellite genotypes among 255 individuals. One clone, which combined two insecticide resistance mechanisms, was frequently detected in all populations whatever their location over a large geographical area (the northern half of France). These unexpected findings suggest that drift is not the unique cause of this low variability. Instead, the intensification of both insecticide treatments and oilseed rape cultivation may have favored a few genotypes. Thus, we propose that selective pressures resulting from human activities have considerably modified the genetic structure of M. persicae populations in northern France in a relatively short period of time.

Admixed sexual and facultatively asexual aphid lineages at mating sites

Cyclically parthenogenetic organisms may have facultative asexual counterparts. Such organisms, including aphids, are therefore interesting models for the study of ecological and genetic interactions between lineages differing in reproductive mode. Earlier studies on aphids have revealed major differences in the genetic outcomes of populations that are possibly resulting mostly either from sexual or from asexual reproduction. Besides, notable gene flow between sexual and asexual derivatives has been suspected, which could lead to the emergence of new asexual lineages. The present study examines the interplay between these lineages and is based on analyses of population structure of individuals that may contribute to the pool of sexual reproductive forms in the host alternating aphid Rhopalosiphum padi. Using a Bayesian assignment method, we first show that the sexual forms of R. padi on mating sites encompass two genetically distinct clusters of individuals in the western part of France. The first cluster included unique genotypes of sexual lineages, while the second cluster included facultatively asexual lineages in numerous copies, the reproductive mode of the two clusters being confirmed by reference clones. Sexual reproductive forms produced by sexual and facultatively asexual lineages are thus admixed at mating sites which gives a large opportunity for the two clusters to mate with each other. Nevertheless, this study also highlights, as previously demonstrated, that the two clusters retained high genetic differentiation. Possible explanations for the inferred limited genetic exchanges are advanced in the discussion, but further dedicated investigations are required to solve this paradox.

Evidence for gene flow and local clonal selection in field populations of the grain aphid (Sitobion avenae) in Britain revealed using microsatellites

Samples of the grain aphid, Sitobion avenae (F.), a major European pest of cereals, were collected in June and July 1997 from fields sown with winter wheat in a rough transect south-west of Rothamsted, UK. These aphids were genotyped at four microsatellite loci known from previous studies to be highly polymorphic. Allelic frequencies were similar between samples collected in the fields and in the 12.2 m high suction trap at Rothamsted, and there were many widespread genotypes (clones), providing evidence that the species is highly migratory. However, field samples were found to display a high level of genotypic heterogeneity (= variable clonal composition), most probably the result of clonal selection. The suction trap genotypes sample were slightly different from the field samples, indicative of the inclusion of genotypes from plant hosts (cereals and grasses, Poaceae) other than winter wheat and/or genotype-biased emigration from the field. The relevance of these data to modelling of aphid outbreaks is briefly discussed.

Genetic diversity and insecticide resistance of Myzus persicae (Hemiptera: Aphididae) populations from tobacco in Chile: evidence for the existence of a single predominant clone

The tobacco-feeding race of Myzus persicae (Sulzer), formerly known as M. nicotianae Blackman, was introduced into Chile during the last decade. In order to evaluate the genetic diversity and insecticide resistance status of Chilean tobacco aphid populations, a field survey was conducted in 35 tobacco fields covering a 300 km latitudinal survey. The populations sampled were characterized using microsatellite markers and morphometric multivariate analysis. Insecticide resistance levels were assessed through a microplate esterase assay and the mutation status of the kdr gene. All samples collected corresponded to the same anholocyclic aphid genotype, and showed morphological variation within the range expected for the tobacco-feeding race of M. persicae. Esterase activity showed the level and variability expected for an R1 clone lacking mutations in the sodium channels (susceptible kdr), thus corresponding to a type slightly resistant to organophosphate and carbamate, and susceptible to pyrethroid insecticides.