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Parthenogenesis is the main mode of reproduction of aphids. Their populations are therefore composed of clones whose frequency distribution varies in space and time. Previous population genetic studies on aphids have highlighted the existence of highly abundant clones (‘super-clones’), distributed over large geographic areas and persisting over time. Whether the abundance of ‘super-clones’ results from their ecological success or from stochastic forces, such as drift and migration, is an open question. Here, we looked for the existence of clines in clonal frequency along a climatic gradient in the cereal aphidRhopalosiphum padi(Linnaeus, 1758) and examined the possible influence of geographical distance and environmental variables in the buildup and maintenance of such clonal clines. We investigated the spatial distribution of the commonest genotypes ofR. padiby sampling populations along an east–west transect in maize fields in the northern half of France in both spring and late summer. Individual aphids were genotyped at several polymorphic loci, allowing the assessment of frequency distributions of multilocus genotypes (MLGs) across the cropping season. We found several MLGs showing longitudinal clines in their frequency distribution in both spring and summer. In particular, two dominant asexual genotypes ofR. padishowed inverted geographical clines, which could suggest divergent adaptations to environmental conditions. We concluded that while the distribution of some ‘super-clones’ ofR. padiseems most likely driven by the action of migration and genetic drift, selection could be also involved in the establishment of longitudinal clines of others.

International audience; 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. a! venae 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.

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.

A descriptive epizootiological model is proposed and parameterized to predict the dynamics of the host-pathogen system “Cereal aphid — entomophthoralean fungusErynia neoaphidis”. The model partitions the host population into distinct classes corresponding to fungal development stages. The dynamics of each class density are described by a set of non-linear ordinary differential equations with the reciprocal of estimated parameter giving average transit or developmental times. The model is compared to the biological data. The experiment was performed under controlled conditions of 20°C and RH >95% using the cereal aphidSitobion avenae as the host. The stability of the model was examined and indicated periodic oscillations, i.e. epizootic intervals separated by longer enzootic periods. In addition, Lotka-Volterra mass action assumption for transmission rate was tested using the data.

International audience

The development and reproduction of 60 clones of Sitobion avenae (Fabricius), collected in the Rennes Basin, were compared on a resistant Triticum monococcum (Linnaeus) line (Tm44) and a susceptible Triticum aestivum (Linnaeus) cultivar (Arminda). All clones had lower larval survival and mean fecundity when reared on Tm44 in comparison with Arminda. They all performed equally well on Arminda whereas there was a marked and continuously distributed variation in performance parameters among clones maintained on Tm44. The plant species, from which clones originated, significantly affected aphid performances on resistant Tm44. A more detailed experiment was carried out with four clones, shown to differ in their level of fitness on Tm44. Their performances were compared on resistant (Tm44 and Tm46) and susceptible (Tm47 and Arminda) wheat genotypes. On the basis of larval development time and intrinsic rate of natural increase (rm), two types of response were distinguished among the four clones: clones Sa1 and Sa39 appeared to be less affected by Tm44 and Tm46 resistance than Sar2 and Sa48. Moreover, Tm46 was much less resistant to clone Sal than was Tm44. These results are discussed in relation to the origins of the variation in performance of S. avenae on resistant wheat, and their implications in plant breeding for resistance to S. avenae.

Summary The incidence and distribution of the three principal isolates of barley yellow dwarf virus (PAV, RPV and MAV) are described in winter cereal crops, cereal (stubble) regrowth and grasses from 11 sites in western France and south-west England during 1987 and 1988. Isolates were identified by indirect ‘sandwich’ ELISA using the monoclonal antibodies MAC91, MAC92 and MAFF2. More virus infection occurred in all localities and in most of the plant communities sampled, with the exception of perennial grass leys, in 1987 than in 1988. All three isolates were widespread. MAV was associated more with sites further north and PAV more with those further south. The geographical distribution of RPV was less variable. Underlying these trends, the relative abundance of isolates differed considerably between habitats. RPV always predominated in perennial grass leys and MAV in most cereal crops, although in the latter MAV was less prevalent in 1987 than in 1988. The greatest regional difference was found in stubble regrowth where PAV predominated in France but MAV predominated in England. Grasses from field margins (only sampled in England) were mainly infected by MAV and RPV. The implications of these findings for the epidemiology of BYDV are discussed, especially the roles of different host plant communities or habitats in the annual infection cycle of small-grain cereals.