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.

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. Heredity (2005) 95, 24-33.

Flolocyclic individuals of the aphid Rhopalosiphum padi reproduce by cyclical parthenogenesis, while anholocyclic individuals are obligate parthenogens. Although admixed on the secondary hosts in summer, holocyclic and anholocyclic populations occur separately on the primary and secondary hosts during winter and spring. In this study, we compared the genetic diversity and population structure of holocyclic and anholocyclic populations collected in spring in the northern half of France. We also analysed the genetic composition of summer populations of R. padi on its secondary hosts. In spring, holocyclic populations were in Hardy-Weinberg equilibrium at individual loci and had a relatively high genotypic diversity. Conversely, anholocyclic populations deviated from Hardy-Weinberg equilibrium and often consisted of a single clone. Moreover, these populations showed very low mean heterozygosities compared with holocyclic populations. Analysis of summer populations suggested that in regions with cold winters, summer populations were largely recruited from holocyclic clones, whereas in areas with mild winters, summer populations were mainly derived from anholocyclic clones. These results permit an assessment of the geographical distribution of the two modes of reproduction on a large scale. The reasons for the diminished heterozygosity of anholocyclic populations are also discussed in relation to the mechanisms which may induce transitions to asexuality in aphids.