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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 aphid Rhopalosiphum 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 of R. padi by 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 of R. padi showed inverted geographical clines, which could suggest divergent adaptations to environmental conditions. We concluded that while the distribution of some 'super-clones' of R. padi seems most likely driven by the action of migration and genetic drift, selection could be also involved in the establishment of longitudinal clines of others.

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.