Reproductive mode and population genetic structure of the cereal aphid Sitobion avenae studied using phenotypic and microsatellite markers

As French populations of the aphid Sitobion avenae exhibit a range of reproductive modes, this species provides a good opportunity for studying the evolution of breeding system variation. The present analysis combined ecological and genetic investigations into the spatial distribution of variation in reproductive mode. Reproductive mode was characterized in 277 lineages of S. avenae from France, and these aphids were scored for five microsatellite loci. The analyses revealed strong geographical partitioning of breeding systems, with obligate asexuals mostly restricted to the south of France, while lineages producing sexual forms were more common in the north. Contrary to what might be anticipated for organisms with frequent parthenogenesis, there was substantial genic and genotypic diversity, even in the obligately asexual lineages. More than 120 different genotypes were detected among the 277 aphid lineages, with an average of 5.9 alleles per locus (range four to 16) and heterozygosity of 56.7%. As with previous studies of allozyme variation in aphids, most loci showed heterozygote deficits, and disequilibrium was common among allelic variants at different loci, even after removal of replicate copies of genotypes that might have been derived through clonal reproduction. Our results suggest that selection is important in structuring reproductive systems and genetic variation in French S. avenae. Canonical correspondence analysis was employed to examine the associations between genotypic and phenotypic variables, enabling the identification of alleles correlated with life-history traits.     

Predominance of sexual reproduction in Romanian populations of the aphid Sitobion avenae inferred from phenotypic and genetic structure

Models of coexistence of sexual and asexual lineages in aphids assume that obligate parthenogenetic lineages predominate in areas with mild winter climate because of their high reproductive output, while sexual lineages predominate in areas with severe winter because they produce eggs resistant to frost. To validate this hypothesis in natural conditions, the reproductive mode of populations of the aphid Sitobion avenae was assessed in two very contrasting climatic situations, Romania (severe winter) and Western France (mild winter). To achieve this, reproductive modes were inferred from both (1) the population composition in sexual and asexual forms in autumn, and (2) the genetic structure of Romanian and French populations of S. avenae using microsatellite markers. Romanian populations encompassed a high proportion of sexual forms and were characterised by a very high genotypic diversity and low linkage disequilibrium. In constrast, the French population showed frequent linkage disequilibria, low genetic diversity, and high level of clonal amplification with two asexual genotypes representing over 60% of the sample. In agreement with the model's predictions, these results clearly indicate that sexual reproduction in S. avenae is predominant under the continental climate of Romania, while asexual lineages prevail under the oceanic climate of Western France.     

Genetic structure of an aphid studied using microsatellites: cyclic parthenogenesis, differentiated lineages and host specialization

In a previous study, samples of the grain aphid Sitobion avenae (F.) were collected from wheat and adjacent cocksfoot hosts in a population thought to be primarily parthenogenetic, and DNA from individual aphids was analysed with a multilocus technique. Here we have applied single-locus microsatellites and a mitochondrial DNA marker to a subset of the same DNA extracts, and have made several additional inferences about important genetic and population processes in S. avenae . Microsatellite analysis indicated very high levels of genic and genotypic variation. S. avenae fell into three genotypic groups inferred to be almost noninterbreeding, while analysis of linkage and Hardy-Weinberg equilibria suggested high levels of sexual recombination within each genotypic group. Host specialization was evident: one lineage was found only on wheat, and one (bearing many alleles inferred to be introgressed from the blackberry-grass aphid S. fragariae (Walker)) was found only on cocksfoot. The third group of interrelated genotypes was found commonly on both hosts. Although most genotypes were found only once, some were much more numerous in the sample than expected from the frequency of the alleles they contained. This, and rapid temporal changes in genotypic composition of samples, indicates strong selective differences between genotypes and lineages. In the major genotypic group, the commonest genotypes were significantly more homozygous than were rare ones: thus these data may help to explain the frequent observation of homozygous excess in aphid allozymes. The genotype group showing S. avenae -like as well as S. fragariae -like alleles also carried S. fragariae -like mitochondrial DNA in at least 25/31 cases, indicating gender-asymmetrical hybridization.     

Grain aphid population structure: no effect of fungal infections in a 2-year field study in Denmark

1 Sitobion avenae (F.) is a serious pest in Danish cereal crops. To understand the population genetic structure, aphids were sampled in seven different winter wheat (Triticum sativum Lamarck) fields throughout Denmark. The aphids were genotyped with seven microsatellite markers. In total, 2075 aphids were collected and 1203 of these were genotyped. 2 The Danish S. avenae populations displayed very high genotypic diversity, high percentages of unique genotypes and low linkage disequilibria; this is likely to be a result of genetic recombination encompassed by their holocyclic lifestyle. The populations showed very limited differentiation and no sign of isolation by distance. Almost all the genetic variation was ascribed within the populations rather than between populations, probably due to a high migration rate at approximate 10% per generation. 3 Seasonal changes in clonal diversity and distribution of asexual summer generations of S. avenae within the infestation period in a single winter wheat field were followed over two consecutive years by weekly sampling from 60 plots each of 20 × 20 m. Clonal diversity was high in all samples with no dominant clonal lineages and no significant difference in the genotypic diversity between weeks or between years. However, a temporal genetic differentiation effect, throughout the infestation, suggests that selective factors or high temporal migration play an important role in shaping the genetic structure S. avenae. 4 Analyses of fungal infected and uninfected aphids were performed to test whether some clonal linage were more often infected by fungi from the Entomophthorales under field conditions. In total, 54 progeny from aphids with Entomophthorales were genotyped and compared with 422 uninfected aphid genotypes. The Entomophthorales‐infected aphid genotypes did not cluster out together, suggesting that these fungal pathogens did not affect the population differentiation or clonal distribution of S. avenae in a Danish agroecosystem. 5 Our findings indicate that S. avenae populations can be controlled using conservation biological control     

Genetic structure and clonal diversity of an introduced pest in Chile, the cereal aphid Sitobion avenae

In Chile, the aphid Sitobion avenae is of recent introduction, lives on cultivated and wild Poaceae, and is thought to reproduce by permanent parthenogenesis. In order to study the genetic variability and population structure of this species, five microsatellite loci were typed from individual aphids collected from different cultivated and wild host plants, from different geographical zones, and years. Chilean populations showed a high degree of heterozygosity and a low genetic variability across regions and years, with four predominant genotypes representing nearly 90% of the sample. This pattern of low clonal diversity and high heterozygosity was interpreted as the result of recent founder events from a few asexually reproducing genotypes. Most geographical and temporal variation observed in the genetic composition resulted from fluctuations of a few predominant clones. In addition, comparisons of the genotypes found in Chile with those described in earlier surveys of S. avenae populations in Western Europe led us to identify 'superclones' with large geographical distribution and high ecological success, and to make a preliminary exploration of the putative origin(s) of S. avenae individuals introduced to Chile.     

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.     

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.     

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.     

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 (∼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.     

Limited genetic exchanges between populations of an insect pest living on uncultivated and related cultivated host plants

Habitats in agroecosystems are ephemeral, and are characterized by frequent disturbances forcing pest species to successively colonize various hosts belonging either to the cultivated or to the uncultivated part of the agricultural landscape. The role of wild habitats as reservoirs or refuges for the aphid Sitobion avenae that colonize cultivated fields was assessed by investigating the genetic structure of populations collected on both cereal crops (wheat, barley and oat) and uncultivated hosts (Yorkshire fog, cocksfoot, bulbous oatgrass and tall oatgrass) in western France.Classical genetic analyses and Bayesian clustering algorithms indicate that genetic differentiation is high between populations collected on uncultivated hosts and on crops, revealing a relatively limited gene flow between the uncultivated margins and the cultivated part of the agroecosystem. A closer genetic relatedness was observed between populations living on plants belonging to the same tribe (Triticeae, Poeae and Aveneae tribes) where aphid genotypes appeared not to be specialized on a single host, but rather using a group of related plant species. Causes of this ecological differentiation and its implications for integrated pest management of S. avenae as cereals pest are discussed.     

Discovery of English grain aphid (Hemiptera: Aphididae) biotypes in China

The English grain aphid, Sitobion avenae (F.) (Hemiptera: Aphididae), is an important pest insect of wheat, Triticum aestivum (L.), in China. Grain aphid biotypes are necessary to breed aphid-resistant wheat varieties; however, none have currently been identified. Here, we describe a method to identify grain aphid biotypes and survey the aphid biotype variation in the wheat growth area of China. Clones of S. avenae were collected from 11 locations in China and used to establish culture populations. These populations were then used to assess the resistance of 12 wheat varieties. Based on resistance responses, seven differential hosts were selected to identify the biotype of S. avenae: Amigo, 'Fengchan No. 3', Zhong 4 wumang, JP1, L1, 885479-2, and 'Xiaobaidongmai'. S. avenae was ultimately classified into five biotypes: EGA I, EGA II, EGA III, EGA IV, and EGA V. These methods provide a mechanism to detect the variation and evolution of grain aphids in different wheat-growing locations and also allow for selection of appropriate aphid-resistant germplasm for wheat breeding of commercial wheat cultivars.     

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.     

Migration and genetic structure of the grain aphid (Sitobion avenae) in Britain related to climate and clonal fluctuation as revealed using microsatellites

Genetic structuring of populations reflects the interaction of genetic drift, mutation, migration and selection, with influences from life history. Aphids are interesting in this regard as they have the potential for unusually high levels of dispersal and natural selection, which typically counter each other. In the present study, winged grain aphids Sitobion avenae (F.) were collected in four 12.2-m high suction traps along a north-south transect in Britain in order to eliminate sampling bias from plant hosts (cereals and grasses; Poaceae), it being known that these insects show host adaptation demonstrable using molecular markers. Samples were analysed at four polymorphic microsatellite loci over two consecutive years. Population allele frequencies were similar nationally during the two years, although clonal diversity varied greatly between sites and years. In the first sampling year following a harsh winter, diversity was found to display a latitudinal clinal trend: the proportion of unique clones (genotypes) increased with latitude. However, this pattern was less apparent the following year, after a milder winter. Nonetheless, overall FST analysis showed that there was little spatial genetic structuring in either sampling year. These data support the view that the insect is highly migratory and also support a theoretical model and previous data suggesting that the reproductive mode is clinal in S. avenae. This appears to be because natural selection (reduced reproductive success of asexual genotypes under cold conditions) is sufficiently powerful to overcome the homogenizing effects of strong migration. There was no clear evidence for isolation by distance for the genetic data obtained. The data are compared with similar data from other aphid species and other insects. Only by the collection of such data sets can an accurate picture be built up relating genetic variability to flight behaviour, including migratory ambit in this group of insects since, due to their small size and rapid dilution in the air, other marking approaches are impracticable over large geographical distances.     

Genetic interactions influence host preference and performance in a plant-insect system

Phytophagous insects generally feed on a restricted range of host plants, using a number of different sensory and behavioural mechanisms to locate and recognize their host plants. Phloem-feeding aphids have been shown to exhibit genetic variation for host preference of different plant species and genetic variation within a plant species can also have an effect on aphid preference and acceptance. It is known that genotypic interactions between barley genotypes and Sitobion avenae aphid genotypes influence aphid fitness, but it is unknown if these different aphid genotypes exhibit active host choice (preference) for the different barley genotypes. Active host choice by aphid genotypes for particular plant genotypes would lead to assortative association (non-random association) between the different aphid and plant genotypes. The performance of each aphid genotype on the plant genotypes also has the ability to enhance these interactions, especially if the aphid genotypes choose the plant genotype that also infers the greatest fitness. In this study, we demonstrate that different aphid genotypes exhibit differential preference and performance for different barley genotypes. Three out of four aphid genotypes exhibited preference for (or against) particular barley genotypes that were not concordant with differences in their reproductive rate on the specific barley genotype. This suggests active host choice of aphids is the primary mechanism for the observed pattern of non-random associations between aphid and barley genotypes. In a community context, such genetic associations between the aphids and barley can lead to population-level changes within the aphid species. These interactions may also have evolutionary effects on the surrounding interacting community, especially in ecosystems of limited species and genetic diversity.     

The effects of reproductive specialization on energy costs and fitness genetic variances in cyclical and obligate parthenogenetic aphids

Organisms with coexisting sexual and asexual populations are ideal models for studying the consequences of either reproductive mode on the quantitative genetic architecture of life-history traits. In the aphid Rhopalosiphum padi, lineages differing in their sex investment coexist but all share a common parthenogenetic phase. Here, we studied multiple genotypes of R. padi specialized either for sexual and asexual reproduction and compared their genetic variation in fitness during the parthenogenetic phase. Specifically, we estimated maintenance costs as standard metabolic rate (SMR), together with fitness (measured as the intrinsic rate of increase and the net reproductive rate). We found that genetic variation (in terms of broad-sense heritability) in fitness was higher in asexual genotypes compared with sexual genotypes. Also, we found that asexual genotypes exhibited several positive genetic correlations indicating that body mass, whole-animal SMR, and apterous individuals production are contributing to fitness. Hence, it appears that in asexual genotypes, energy is fully allocated to maximize the production of parthenogenetic individuals, the simplest possible form of aphid repertoire of life-histories strategies.     

Isozyme variation under different modes of reproduction in two clonal winter annuals, Sedum rosulato-bulbosum and Sedum bulbiferum (Crassulaceae)

We studied isozyme variation in two annual species that produce bulbils, Sedum rosulato-bulbosum , which includes both sexually reproducing plants and obligate clonal plants that result from triploidy (fertile and sterile S. rosulato-bulbosum , respectively), and an obligate clonal plant, Sedum bulbiferum , to examine the relationship between reproductive mode and isozyme variation. The sterile S. rosulato-bulbosum population exhibited no genotypic variation, but showed high genetic variation (gene diversity, H _e  = 0.60) because five of the six loci that we analyzed were heterozygous. Almost all ramets of S. bulbiferum across 20 populations shared an identical isozyme phenotype, although we could not identify the genetic basis of the phenotype. In contrast, fertile S. rosulato-bulbosum exhibited genotypic variation across the species, but comprised genotypically uniform and polymorphic populations whose genotypic variations correlated positively with the genetic variations within the populations ( H _e at the genet level per population ranged from 0.08 to 0.37). Genetic drift and habitat conditions inhibiting seedling recruitment may have caused this among-population variation. The results for sterile and fertile S. rosulato-bulbosum suggest that exclusive clonal reproduction causes low genotypic variation, but maintains genetic variation within individuals. Factors that affect the maintenance of genetic variation in these plants are discussed on the basis of these findings.     

Aphid genotypes vary in their response to the presence of fungal endosymbionts in host plants

Genetic variation for fitness‐relevant traits may be maintained in natural populations by fitness differences that depend on environmental conditions. For herbivores, plant quality and variation in chemical plant defences can maintain genetic variation in performance. Apart from plant secondary compounds, symbiosis between plants and endosymbiotic fungi (endophytes) can produce herbivore‐toxic compounds. We show that there is significant variation among aphid genotypes in response to endophytes by comparing life‐history traits of 37 clones of the bird cherry‐oat aphid Rhopalosiphum padi feeding on endophyte‐free and endophyte‐infected tall fescue Lolium arundinaceum. Clonal variation for life‐history traits was large, and most clones performed better on endophyte‐free plants. However, the clones differed in the relative performance across the two environments, resulting in significant genotype × environment interactions for all reproductive traits. These findings suggest that natural variation in prevalence of endophyte infection can contribute to the maintenance of genetic diversity in aphid populations.     

Introgressive hybridization and morphological transgression in the contact zone between two Mediterranean Solea species

Hybrid zones provide natural experiments where new combinations of genotypes and phenotypes are produced. Studying the reshuffling of genotypes and remodeling of phenotypes in these zones is of particular interest to document the building of reproductive isolation and the possible emergence of transgressive phenotypes that can be a source of evolutionary novelties. Here, we specifically investigate the morphological variation patterns associated with introgressive hybridization between two species of sole, Solea senegalensis and Solea aegyptiaca. The relationship between genetic composition at nuclear loci and individual body shape variation was studied in four populations sampled across the hybrid zone located in northern Tunisia. A strong correlation between genetic and phenotypic variation was observed among all individuals but not within populations, including the two most admixed ones. Morphological convergence between parental species was observed close to the contact zone. Nevertheless, the samples taken closest to the hybrid zone also displayed deviant segregation of genotypes and phenotypes, as well as transgressive phenotypes. In these samples, deviant body shape variation could be partly attributed to a reduced condition index, and the distorted genetic composition was most likely due to missing allelic combinations. These results were interpreted as an indication of hybrid breakdown, which likely contributes to postmating reproductive isolation between the two species.     

三种玉米蚜虫种群的生态位分析

对重庆市郊区1998～1999年春玉米上3种蚜虫种群的数量动态、生态位宽度和生态位重叠进行了系统的分析研究．结果表明,发生在春玉米上的玉米蚜(Rhopalosiphum maidis),禾谷缢管蚜(Rhopalosiphum padi)和麦长管蚜(Sitobion avenae)3种蚜虫混合种群的种群数量消长呈现出5月中下旬和6月下旬两个高峰;在生态位宽度上,Livens生态位宽度指数(Bi)分析表明,麦长管蚜表现出时间上的高度分化,玉米蚜表现出空间上的高度聚集特性,而禾谷缢管蚜的分化并不明显;在生态位重叠上,玉米蚜和禾谷缢管蚜的HturlbeIt生态位重叠指数(Lij)最高,玉米蚜和麦长管蚜次之,禾谷缢管蚜和麦长管蚜最小．在春玉米上发生的3种蚜虫中,玉米蚜具有较强的竞争优势．     

Population genetic consequences of extreme variation in sexual and clonal reproduction in an aquatic plant

Most plants combine sexual reproduction with asexual clonal reproduction in varying degrees, yet the genetic consequences of reproductive variation remain poorly understood. The aquatic plant Butomus umbellatus exhibits striking reproductive variation related to ploidy. Diploids produce abundant viable seed whereas triploids are sexually sterile. Diploids also produce hundreds of tiny clonal bulbils, whereas triploids exhibit only limited clonal multiplication through rhizome fragmentation. We investigated whether this marked difference in reproductive strategy influences the diversity of genotypes within populations and their movement between populations by performing two large-scale population surveys (n = 58 populations) and assaying genotypic variation using random amplified polymorphic DNA (RAPDs). Contrary to expectations, sexually fertile populations did not exhibit higher genotypic diversity than sterile populations. For each cytotype, we detected one very common and widespread genotype. This would only occur with a very low probability (< 10-7) under regular sexual recombination. Compatibility analysis also indicated that the pattern of genotypic variation largely conformed to that expected with predominant clonal reproduction. The potential for recombination in diploids is not realized, possibly because seeds are outcompeted by bulbils for safe sites during establishment. We also failed to find evidence for more extensive movement of fertile than sterile genotypes. Aside from the few widespread genotypes, most were restricted to single populations. Genotypes in fertile populations were very strongly differentiated from those in sterile populations, suggesting that new triploids have not arisen during the colonization of North America. The colonization of North America involves two distinct forms of B. umbellatus that, despite striking reproductive differences, exhibit largely clonal population genetic structures.