Patterns of genetic differention between populations of the specialized herbivore Macrosiphoniella tanacetaria (Homoptera, Aphididae).

For herbivorous insects, studies of isolation by distance (IBD) are available for large spatial scales, whereas studies over small geographic distances are relatively rare, in particular for species where population turnover is high. In this study, we investigated IBD and population genetic structure in the aphid Macrosiphoniella tanacetaria, a specialist herbivore of tansy (Tanacetum vulgare). Owing to clonal growth, an individual plant (genet) has one to many shoots (ramets), which can host aphid colonies. Both at the level of ramets and genets, aphid persistence is short, in the order of weeks. Sampling of 17 populations was performed on a logarithmic scale, along the Saale River in Germany in June 2001, with distances between populations ranging from 1 m to 170 km. For the six microsatellites used, allelic and genotypic variability within aphid populations was high, and deviations from Hardy-Weinberg equilibrium and linkage disequilibrium were frequent. Most pairs of populations were significantly differentiated but there was no pattern of IBD. However, including into the analysis four additional populations from Alsace, France, collected at distances of, on average 470 km, resulted in a weak but significant IBD. Aphids are passive dispersers that are known to occasionally disperse over large distances, even though most dispersal is likely to occur over a small spatial scale. We suggest that for the host-specific M. tanacetaria, patterns of genetic variation among populations are, at an ecologically meaningful scale, governed by colonization/extinction dynamics and genetic drift rather than by a drift-dispersal equilibrium.