Analysis of clonal diversity of the peach-potato aphid, Myzus persicae (Sulzer), in Scotland, UK and evidence for the existence of a predominant clone

Rapid specialization of counter defenses enables two-spotted spider mite to adapt to novel plant hosts

Genetic adaptation, occurring over a long evolutionary time, enables host-specialized herbivores to develop novel resistance traits and to efficiently counteract the defenses of a narrow range of host plants. In contrast, physiological acclimation, leading to the suppression and/or detoxification of host defenses, is hypothesized to enable broad generalists to shift between plant hosts. However, the host adaptation mechanisms used by generalists composed of host-adapted populations are not known. Two-spotted spider mite (TSSM; Tetranychus urticae) is an extreme generalist herbivore whose individual populations perform well only on a subset of potential hosts. We combined experimental evolution, Arabidopsis thaliana genetics, mite reverse genetics, and pharmacological approaches to examine mite host adaptation upon the shift of a bean (Phaseolus vulgaris)-adapted population to Arabidopsis. We showed that cytochrome P450 monooxygenases are required for mite adaptation to Arabidopsis. We identified activities of two tiers of P450s: general xenobiotic-responsive P450s that have a limited contribution to mite adaptation to Arabidopsis and adaptation-associated P450s that efficiently counteract Arabidopsis defenses. In approximately 25 generations of mite selection on Arabidopsis plants, mites evolved highly efficient detoxification-based adaptation, characteristic of specialist herbivores. This demonstrates that specialization to plant resistance traits can occur within the ecological timescale, enabling the TSSM to shift to novel plant hosts.

Aphids in focus: unravelling their complex ecology and evolution using genetic and molecular approaches

Aphids are renowned plant parasites of agriculture, horticulture and forestry, causing direct physical damage by sucking phloem and especially by transmission of plant pathogenic viruses. The huge yield loss they cause amounts to hundreds of millions of dollars globally, and because of this damage and the intense efforts expended on control, some 20 species are now resistant to pesticides worldwide. Aphids represent an ancient, mainly northern temperate group, although some species occur in the tropics, often as obligate asexual lineages or even asexual ‘species’. However, besides their notoriety as enemies of plant growers, aphids are also extremely interesting scientifically, especially at the molecular and genetic levels. They reproduce mainly asexually, one female producing 10–90 offspring in 7–10 days and therefore, theoretically, could produce billions of offspring in one growing season in the absence of mortality factors (i.e. climate/weather and antagonists). In this overview, we provide examples of what molecular and genetic studies of aphids have revealed concerning a range of topics, especially fine-grained ecological processes. Aphids, despite their apparently limited behavioural repertoire, are in fact masters (or, perhaps more accurately, mistresses) of adaptation and evolutionary flexibility and continue to flourish in a variety of ecosystems, including the agro-ecosystem, regardless of our best efforts to combat them.

Asexual parasites and their extraordinary host ranges

In diverse parasite taxa, from scale insects to root-knot nematodes, asexual lineages have exceptionally large host ranges, larger than those of their sexual relatives. Phylogenetic comparative studies of parasite taxa indicate that increases in host range and geographic range increase the probability of establishment of asexual lineages. At first pass, this convergence of traits appears counter-intuitive: intimate, antagonistic association with an enormous range of host taxa correlates with asexual reproduction, which should limit genetic variation within populations. Why would narrow host ranges favor sexual parasites and large host ranges favor asexual parasites? To take on this problem I link theory on ecological specialization to the two predominant hypotheses for the evolution of sex. I argue that both hypotheses predict a positive association between host range and the probability of invasion of asexual parasites, mediated either by variation in population size or in the strength of antagonistic coevolution. I also review hypotheses on colonization and the evolution of niche breadth in asexual lineages. I emphasize parasite taxa, with their diversity of reproductive modes and ecological strategies, as valuable assets in the hunt for solutions to the classic problems of the evolution of sex and geographic parthenogenesis.

Generalism in Nature…The Great Misnomer: Aphids and Wasp Parasitoids as Examples

In the present article we discuss why, in our view, the term ‘generalism’ to define the dietary breadth of a species is a misnomer and should be revised by entomologists/ecologists with the more exact title relating to the animal in question’s level of phagy—mono-, oligo, or polyphagy. We discard generalism as a concept because of the indisputable fact that all living organisms fill a unique ecological niche, and that entry and exit from such niches are the acknowledged routes and mechanisms driving ecological divergence and ultimately speciation. The term specialist is probably still useful and we support its continuing usage simply because all species and lower levels of evolutionary diverge are indeed specialists to a large degree. Using aphids and parasitoid wasps as examples, we provide evidence from the literature that even some apparently highly polyphagous agricultural aphid pest species and their wasp parasitoids are probably not as polyphagous as formerly assumed. We suggest that the shifting of plant hosts by herbivorous insects like aphids, whilst having positive benefits in reducing competition, and reducing antagonists by moving the target organism into ‘enemy free space’, produces trade-offs in survival, involving relaxed selection in the case of the manicured agro-ecosystem.

Aspects, Including Pitfalls, of Temporal Sampling of Flying Insects, with Special Reference to Aphids

Since the advent and widespread use of high-resolution molecular markers in the late 1970s, it is now well established that natural populations of insects are not necessarily homogeneous genetically and show variations at different spatial scales due to a variety of reasons, including hybridization/introgression events. In a similar vein, populations of insects are not necessarily homogenous in time, either over the course of seasons or even within a single season. This of course has profound consequences for surveys examining, for whatever reason/s, the temporal population patterns of insects, especially flying insects as mostly discussed here. In the present article, the topics covered include climate and climate change; changes in ecological niches due to changes in available hosts, i.e., essentially, adaptation events; hybridization influencing behaviour⁻host shifts; infection by pathogens and parasites/parasitoids; habituation to light, sound and pheromone lures; chromosomal/genetic changes affecting physiology and behaviour; and insecticide resistance. If such phenomena-i.e., aspects and pitfalls-are not considered during spatio-temporal study programmes, which is even more true in the light of the recent discovery of morphologically similar/identical cryptic species, then the conclusions drawn in terms of the efforts to combat pest insects or conserve rare and endangered species may be in error and hence end in failure.

Rapid transcriptional plasticity of duplicated gene clusters enables a clonally reproducing aphid to colonise diverse plant species

Background

The prevailing paradigm of host-parasite evolution is that arms races lead to increasing specialisation via genetic adaptation. Insect herbivores are no exception and the majority have evolved to colonise a small number of closely related host species. Remarkably, the green peach aphid, Myzus persicae, colonises plant species across 40 families and single M. persicae clonal lineages can colonise distantly related plants. This remarkable ability makes M. persicae a highly destructive pest of many important crop species.

Results

To investigate the exceptional phenotypic plasticity of M. persicae, we sequenced the M. persicae genome and assessed how one clonal lineage responds to host plant species of different families. We show that genetically identical individuals are able to colonise distantly related host species through the differential regulation of genes belonging to aphid-expanded gene families. Multigene clusters collectively upregulate in single aphids within two days upon host switch. Furthermore, we demonstrate the functional significance of this rapid transcriptional change using RNA interference (RNAi)-mediated knock-down of genes belonging to the cathepsin B gene family. Knock-down of cathepsin B genes reduced aphid fitness, but only on the host that induced upregulation of these genes.

Conclusions

Previous research has focused on the role of genetic adaptation of parasites to their hosts. Here we show that the generalist aphid pest M. persicae is able to colonise diverse host plant species in the absence of genetic specialisation. This is achieved through rapid transcriptional plasticity of genes that have duplicated during aphid evolution.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-016-1145-3) contains supplementary material, which is available to authorized users.

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.

Karyotype rearrangements and telomere analysis in Myzuspersicae (Hemiptera, Aphididae) strains collected on Lavandula sp. plants

Karyotype analysis of nine strains of the peach-potato aphid Myzuspersicae (Sulzer, 1776), collected on Lavandula sp. plants, evidenced showed that five of them had a standard 2n = 12 karyotype, one possessed a fragmentation of the X chromosome occurring at the telomere opposite to the NOR-bearing one and three strains had a chromosome number 2n = 11 due to a non-reciprocal translocation of an autosome A3 onto an A1 chromosome. Interestingly, the terminal portion of the autosome A1 involved in the translocation was the same in all the three strains, as evidenced by FISH with the histone cluster as a probe. The study of telomeres in the Myzuspersicae strain with the X fission evidenced that telomerase synthesised de novo telomeres at the breakpoints resulting in the stabilization of the chromosomal fragments. Lastly, despite the presence of a conserved telomerase, aphid genome is devoid of genes coding for shelterin, a complex of proteins involved in telomere functioning frequently reported as conserved in eukaryotes. The absence of this complex, also confirmed in the genome of other arthropods, suggests that the shift in the sequence of the telomeric repeats has been accompanied by other changes in the telomere components in arthropods in respect to other metazoans.

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.

Walking speed adaptation ability of Myzus persicae to different temperature conditions

Walking speeds were calculated for nine clones of the peach potato aphid Myzus persicae collected from three countries along a latitudinal cline of its European distribution from Sweden to Spain (Sweden, UK and Spain), and the effects of collection origin and intra and intergenerational acclimation were investigated. Walking speeds declined with decreasing temperature, with maximum performance at temperatures closest to acclimation temperature (fastest median walking speed of 5.8 cm min(-1) was recorded for clone UK 3, collected from the UK, at 25°C after acclimating to 25°C for one generation). Following acclimation at both 20°C and 25°C, walking ceased (as indicated by median walking speeds of 0.0 cm min(-1)) at temperatures as high as 7.5°C and 12.5°C. However, acclimation at 10°C enabled mobility to occur to temperatures as low as 0°C. There was no relationship between mobility and latitude of collection, suggesting that large scale mixing of aphids may occur across Europe. However, clonal variation was suggested, with clone UK 3 outperforming the majority of other clones across all temperatures at which mobility was maintained following acclimation at 10°C for one and three generations and at 25°C for one generation. The Scandinavian clones consistently outperformed their temperate and Mediterranean counterparts at the majority of temperatures following acclimation for three generations at 25°C.

Continuous occurrence of intra-individual chromosome rearrangements in the peach potato aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae)

Analysis of the holocentric mitotic chromosomes of the peach-potato aphid, Myzus persicae (Sulzer), from clones labelled 50, 51 and 70 revealed different chromosome numbers, ranging from 12 to 14, even within each embryo, in contrast to the standard karyotype of this species (2n = 12). Chromosome length measurements, combined with fluorescent in situ hybridization experiments, showed that the observed chromosomal mosaicisms are due to recurrent fragmentations of chromosomes X, 1 and 3. Contrary to what has generally been reported in the literature, X chromosomes were frequently involved in recurrent fragmentations, in particular at their telomeric ends opposite to the nucleolar organizer region. Supernumerary B chromosomes have been also observed in clones 50 and 51. The three aphid clones showed recurrent fissions of the same chromosomes in the same regions, thereby suggesting that the M. persicae genome has fragile sites that are at the basis of the observed changes in chromosome number. Experiments to induce males also revealed that M. persicae clones 50, 51 and 70 are obligately parthenogenetic, arguing that the reproduction by apomictic parthenogenesis favoured the stabilization and inheritance of the observed chromosomal fragments.

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.

The invasion route for an insect pest species: the tobacco aphid in the New World

Biological invasions are rapid evolutionary events in which populations are usually subject to a founder event during introduction followed by rapid adaptation to the new environment. Molecular tools and Bayesian approaches have shown their utility in exploring different evolutionary scenarios regarding the invasion routes of introduced species. We examined the situation for the tobacco aphid, Myzus persicae nicotianae, a recently introduced aphid species in Chile. Using seven microsatellite loci and approximate Bayesian computation, we studied populations of the tobacco aphid sampled from several American and European countries, identifying the most likely source populations and tracking the route of introduction to Chile. Our population genetic data are consistent with available historical information, pointing to an introduction route of the tobacco aphid from Europe and/or from other putative populations (e.g. Asia) with subsequent introduction through North America to South America. Evidence of multiple introductions to North America from different genetic pools, with successive loss of genetic diversity from Europe towards North America and a strong bottleneck during the southward introduction to South America, was also found. Additionally, we examined the special case of a widespread multilocus genotype that was found in all American countries examined. This case provides further evidence for the existence of highly successful genotypes or 'superclones' in asexually reproducing organisms.

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.

Physiological approach to explain the ecological success of 'superclones' in aphids: interplay between detoxification enzymes, metabolism and fitness

'Superclones' are predominant and time-persistent genotypes, exhibiting constant fitness across different environments. However, causes of this ecological success are still unknown. Therefore, we studied the physiological mechanisms that could explain this success, evaluating the effects of wheat chemical defences on detoxification enzymes [cytochrome P450 monooxygenases (P450), glutathione S-transferases (GST), esterases (EST)], standard metabolic rate (SMR), and fitness-related traits [adult body mass and intrinsic rate of increase (r(m))] of two 'superclones' (Sa1 and Sa2) of the grain aphid, Sitobion avenae. Additionally, we compared 'superclones' with a less-frequent genotype (Sa46). Genotypes were reared on three wheat cultivars with different levels of hydroxamic acids (Hx; wheat chemical defences). Detoxification enzymes and SMR did not differ between wheat hosts. However, GST and EST were different between 'superclones' and Sa46, while Sa1 showed a higher SMR than Sa2 or Sa46 (p=0.03). Differences between genotypes were found for r(m), which was higher for Sa1 than for Sa2 or Sa46. For all cases, genotype-host interactions were non-significant, except for aphid body mass. In conclusion, 'superclones' exhibit a broad host range, flat energetic costs for non-induced detoxification enzymes, and low variation in their reproductive performance on different defended hosts. However, physiological specialization of 'superclones' that could explain their ecological success was not evident in this study.

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.

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.

Clonal turnover of MACE-carrying peach-potato aphids (Myzus persicae (Sulzer), Homoptera: Aphididae) colonizing Scotland

Peach-potato aphids, Myzus persicae (Sulzer), collected in Scotland in the years 1995 and 2002-2004 were characterized using four microsatellite loci and three insecticide resistance mechanisms. From 868 samples, 14 multilocus genotypes were defined (designated clones A-N). Five of these (denoted A, B, H, M and N) carried modified acetylcholinesterase (MACE) resistance, the most recent resistance mechanism to have evolved in M. persicae. The current paper shows that the continued presence of MACE aphids is due to turnover, as clones A and B were replaced in field samples by clones H, M and N in later seasons. Thus, insecticide-resistant populations in Scotland can be attributed to multiple waves of rapid clone colonisations and not to the continued presence of stable resistant clones or mutation or sexual recombination in local populations. The MACE clones carried varying levels of the other insecticide resistance mechanisms, kdr and esterase. The presence of these mechanisms could alter the clones success in the field depending on insecticide spraying (positive selection) and resistance fitness costs (negative selection).

Evidence for widespread cryptic sexual generations in apparently purely asexual Andricus gallwasps

Oak gallwasps (Hymenoptera, Cynipidae, Cynipini) are one of seven major animal taxa that commonly reproduce by cyclical parthenogenesis (CP). A major question in research on CP taxa is the frequency with which lineages lose their sexual generations, and diversify as purely asexual radiations. Most oak gallwasp species are only known from an asexual generation, and secondary loss of sex has been conclusively demonstrated in several species, particularly members of the holarctic genus Andricus. This raises the possibility of widespread secondary loss of sex in the Cynipini, and of diversification within purely parthenogenetic lineages. We use two approaches based on analyses of allele frequency data to test for cryptic sexual generations in eight apparently asexual European species distributed through a major western palaearctic lineage of the gallwasp genus Andricus. All species showing adequate levels of polymorphism (7/8) showed signatures of sex compatible with cyclical parthenogenesis. We also use DNA sequence data to test the hypothesis that ignorance of these sexual generations (despite extensive study on this group) results from failure to discriminate among known but morphologically indistinguishable sexual generations. This hypothesis is supported: 35 sequences attributed by leading cynipid taxonomists to a single sexual adult morphospecies, Andricus burgundus, were found to represent the sexual generations of at least six Andricus species. We confirm cryptic sexual generations in a total of 11 Andricus species, suggesting that secondary loss of sex is rare in Andricus.

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 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.

Reproductive mode of grape phylloxera (Daktulosphaira vitifoliae, Homoptera: Phylloxeridae) in Europe: molecular evidence for predominantly asexual populations and a lack of gene flow between them

The genetic structure of European grape phylloxera populations, Daktulosphaira vitifoliae (Homoptera: Phylloxeridae), was analyzed using 6 polymorphic microsatellite markers. Genetic diversity data of 6 populations originating from northern and southern European viticultural regions was assessed for geographic differences, and the structure of 2 additional populations was examined in more detail, focusing on specific host plant and habitat characteristics. To test for "signatures" of clonal reproduction, different population genetic measures were applied to the data obtained from these populations. A total of 195 multilocus genotypes were detected in 360 individuals tested. Significant deviations from Hardy-Weinberg equilibrium, negative FIS values (from -0.148 to -0.658 per population), and the presence of multicopy genotypes revealed that the current major reproductive mode at each of the locations tested was asexual. The high genotypic diversity detected within and among populations, however, together with the occurrence of unique D. vitifoliae genotypes, indicates sexual recombination events took place, probably prior to the multiple introductions into Europe. The absence of overlapping genotypes between the sampling sites suggests low migration rates among the populations studied and implies that the main mode of insect dispersal is through infested plant material carried by human agency. The specific features of European D. vitifoliae habitats are illustrated to discuss the role of habitat and life cycle in the genetic structure of this globally important pest aphid species.

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.

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.

Mode of inheritance of the polymorphic esterases in Myzus persicae (Hemiptera: Aphididae) in Japan

Crossing experiments were conducted with 43 different parental clones of Myzus persicae (Sulzer) to clarify the genetic bases of esterase variations detected by electrophoresis. A total of 468 F1 clones obtained were analysed for variants at six polymorphic esterase loci, including one associated with resistance to organophosphorus insecticides (RAE). In addition, the effect of six different host plants on the activity of esterases was examined. The inheritance of variants at two loci could be explained by two-allele models, and that at three other loci by three-allele models, while segregation ratios at the RAE locus suggested a cumulative effect of the multiple genes. An interdependent assortment was found between two loci. The activity of the esterases detected in the alimentary canal was affected by the host plant. By combining the variants at the six polymorphic esterase loci with those in body colour, a total of 2592 colour-esterase forms were distinguishable. Thus esterases could be useful as genetic markers to study the genotypic composition and its dynamics of M. persicae populations.

Environmentally related patterns of reproductive modes in the aphidMyzus persicaeand the predominance of two ‘superclones’ in Victoria, Australia

Asexual organisms that naturally coexist with sexual relatives may hold the key to understanding the maintenance of sex and recombination, a long-standing problem in evolutionary biology. This situation applies to the peach-potato aphid, Myzus persicae, in southeastern Australia where cyclical parthenogens form mixed populations with obligate parthenogens. We collected M. persicae from several areas across Victoria, genotyped them at seven microsatellite loci and experimentally determined their reproductive mode. The geographic distribution of reproductive modes was correlated with two environmental variables that differentially affect obligate and cyclical parthenogens; obligate parthenogens were less frequent in areas with cold winters because they cannot produce frost-resistant eggs while cyclical parthenogens were limited by the availability of their primary host, peach, on which sexual reproduction takes place. Clonal diversity increased with the proportion of cyclical parthenogens in a sample because they tended to have unique microsatellite genotypes, whereas many obligate parthenogens were copies of the same genotype. Two obligately asexual genotypes stood out as being very abundant and widespread, one constituting 24% and the other 17.4% of the entire collection. Both of these highly successful genotypes were present in the majority of all collection sites. Genetic population structure was weak, albeit significant, with a multilocus FST of only 0.021 when samples were reduced to only one representative of each genotype. Interestingly, obligate parthenogens were, on average, more heterozygous and exhibited larger allele size differences between the two alleles at individual loci than cyclical parthenogens. This striking pattern could result from hybridization, for which we have no evidence, or may reflect the previously proposed model of biased mutational divergence of microsatellite alleles within asexual aphid lineages.

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

The peach-potato aphid, Myzus persicae (Sulzer), has a complex reproductive mode in which some lineages reproduce by continuous parthenogenesis, whereas others reproduce sexually once a year. The climate is thought to act directly on the reproductive mode, because sexual eggs are the only form that can resist frost in cold regions. Sexual reproduction necessitates an obligatory host alternation that may result in long-distance dispersal. Here, we examined the genetic variability at seven microsatellite loci of populations of M. persicae in France, where both reproductive modes occur. We provide clear genetic evidence that the breeding system affects genotypic variability, as cyclically parthenogenetic aphids are far more variable than their obligately parthenogenetic counterparts. A temporal decrease in genetic variability and a temporal genetic differentiation effect suggest the existence of selective factors that play an important role in shaping the genetic structure of M. persicae populations. Lastly, differences in the population structure between reproductive modes suggest that the migration associated with the change of host during sexual reproduction lowers the level of population differentiation.

Incidence of insecticide resistance alleles in sexually-reproducing populations of the peach-potato aphid Myzus persicae (Hemiptera: Aphididae) from southern France

Intensive chemical treatments have led to the development of a number of insecticide resistance mechanisms in the peach-potato aphid Myzus persicae (Sulzer). Some of these mechanisms are known to be associated with negative pleiotropic effects (resistance costs). Molecular and biochemical methods were used to determine the genotypes or phenotypes associated with four insecticide resistance mechanisms in single aphids from sexually-reproducing populations in southern France. The mechanisms considered were E4 and FE4 carboxylesterase overproduction, modified acetycholinesterase, and kdr and rdl resistance-associated mutations. A new method for determining individual kdr genotypes is presented. Almost all resistant individuals overproduced FE4 carboxylesterase, whereas modified acetylcholinesterase was rare. Both the kdr and rdl resistance mutations were present at high frequencies in French sexually-reproducing populations. The frequencies of insecticide resistance genes were compared before and after sexual reproduction in one peach orchard at Avignon to evaluate the potential impact of selection on the persistence of resistance alleles in the over-wintering phase. The frequencies of the kdr and rdl mutations varied significantly between autumn and spring sampling periods. The frequency of the kdr mutation increased, probably due to pyrethroid treatments at the end of the winter. Conversely, the frequency of the rdl mutation decreased significantly during winter, probably because of a fitness cost associated with this mutation.

Genetic architecture of sexual and asexual populations of the aphid Rhopalosiphum padi based on allozyme and microsatellite markers

Cyclical parthenogens, including aphids, are attractive models for comparing the genetic outcomes of sexual and asexual reproduction, which determine their respective evolutionary advantages. In this study, we examined how reproductive mode shapes genetic structure of sexual (cyclically parthenogenetic) and asexual (obligately parthenogenetic) populations of the aphid Rhopalosiphum padi by comparing microsatellite and allozyme data sets. Allozymes showed little polymorphism, confirming earlier studies with these markers. In contrast, microsatellite loci were highly polymorphic and showed patterns very discordant from allozyme loci. In particular, microsatellites revealed strong heterozygote excess in asexual populations, whereas allozymes showed heterozygote deficits. Various hypotheses are explored that could account for the conflicting results of these two types of genetic markers. A strong differentiation between reproductive modes was found with both types of markers. Microsatellites indicated that sexual populations have high allelic polymorphism and heterozygote deficits (possibly because of population subdivision, inbreeding or selection). Little geographical differentiation was found among sexual populations confirming the large dispersal ability of this aphid. In contrast, asexual populations showed less allelic polymorphism but high heterozygosity at most loci. Two alternative hypotheses are proposed to explain this heterozygosity excess: allele sequence divergence during long-term asexuality or hybrid origin of asexual lineages. Clonal diversity of asexual lineages of R. padi was substantial suggesting that they could have frozen genetic diversity from the pool of sexual lineages. Several widespread asexual genotypes were found to persist through time, as already seen in other aphid species, a feature seemingly consistent with the general-purpose genotype hypothesis.

Host-based genotype variation in insects revisited

Elucidation of the genetic variability of a model insect species, the grain aphid, Sitobion avenae (Fabricius), a predominantly asexual herbivore within the temperate agro-ecosystem tested, was initiated using molecular DNA markers (RAPDs). This revealed genetic profiles that appeared related to host adaptation at the specific level amongst the natural populations colonizing different grasses and cereals (Poaceae) within the same geographic location. These profiles were recorded either as 'specialist' genotypes found on specific grasses, or as 'generalist' genotypes colonizing several host types including cultivated cereals or native grasses. These findings are compared with analogous systems found amongst insect species, including at a higher trophic level, i.e. interactions between hymenopterous aphid parasitoids. As the aphids and their respective plant hosts occur in the same geographical region at the same time, this appears to be a rare example of the evolutionary transition leading to sympatric speciation in insects. Hence, this study highlights the importance of understanding not only the demographic parameters to genetic diversity, but also the more intricate correlation of genetic diversity to host types in agricultural environments.

Microsatellite variation in cyclically parthenogenetic populations of Myzus persicae in south-eastern Australia

We examined the population structure of the introduced aphid, Myzus persicae collected mainly from its primary host, Prunus persica, in south-east Australia. Myzus persicae has been present in Australia since at least 1893. Samples were collected in the spring of 1998 from two mainland and three Tasmanian localities and isofemale lines were established in the laboratory. The reproductive mode (life cycle), karyotype and 17-locus microsatellite genotype of each clone were determined. All populations showed significant population differentiation (F(ST) 0.058-0.202) even over small geographic distances (<50 km). All clones were karyotypically normal except for a subset of clones from one site that was exposed to the carbamate insecticide, Pirimor, the week prior to sampling. Those clones were heterozygous for an autosomal 1,3 translocation frequently associated in M. persicae with insecticide resistance. In contrast to other loci and despite being on different chromosomes, loci myz2(A) and M55(A) showed general and significant linkage disequilibrium. These loci may be affected by epistatic selection. We discuss the observed high clonal diversity, moderate but significant population differentiation, general conformance to Hardy-Weinberg equilibria and low linkage disequilibria with particular focus on the global population biology of M. persicae.

Evidence for predominant clones in a cyclically parthenogenetic organism provided by combined demographic and genetic analyses

Aphids are particularly interesting models in the study of genetic and demographic components of plant adaptation because of their breeding system which combines parthenogenesis and sexual reproduction (i.e. cyclical parthenogenesis), and the frequent emergence of host-adapted races reported in this group. In this paper, patterns of host adaptation were assessed on local populations of the aphid Sitobion avenae by following their demographic and genetic structure in a maize field for two consecutive years. The existence of putative generalist (polyphagous) or specialized (host-adapted) genotypes was also investigated by comparing the genotypic distribution of this aphid on maize and other cultivated host plants, using five microsatellite loci. Although population dynamics revealed strong variation in aphid abundance during the colonization period on maize, two genotypes identified at seven additional microsatellite loci were predominant and exhibited stable frequencies over cropping season and between years. Based on present and earlier studies, these two prevalent genotypes were shown to survive on different host plants other than maize, to colonize large geographical zones and to persist parthenogenetically for several years. All these data strongly suggest that these two genotypes are asexual generalist clones that could have been favoured by agricultural practices encountered in western Europe. Besides these two clones, a continual replacement of rare genotypes was observed on maize in both years. Hypotheses involving selection via aphid-plant interactions and natural enemies were proposed for explaining the disappearance of these genotypes on maize.

Some epidemiological approaches to the control of aphid-borne virus diseases in seed potato crops in northern Europe

An account is given of progress during the last 30 years in knowledge of the epidemiology of diseases caused by aphid-borne viruses in seed potato production areas of northern Europe. During this period, Potato leafroll virus (PLRV) and strains of Potato virus Y (PVY(O), PVY(N)) were consistently the most prevalent and harmful. The main factors influencing spread involve the amount of initial virus inoculum in seed crops, agricultural practices in relation to seed potato production, the status of aphids as colonising or transient vectors in the crop, variation in their life cycles and behaviour, effects of environmental factors on aphid population dynamics, and the interaction between aphid phenology and mature plant resistance. Lessons have been drawn from comparisons of the causes of outbreaks of PLRV and PVY, and various comprehensive models have been designed to predict virus spread and optimise control. The accuracy and reliability of virus detection, and chemical control of aphids have been considerably improved, but use of the latter has been severely affected by the development in Myzus persicae of resistance to most types of insecticide. It is suggested that more attention should be given to breeding virus-resistant cultivars that would lead to decreased pesticide use and thus promote more integrated environment-friendly strategies for control.