origin and genetic diversity of western european populations of the potato cyst nematode globodera pallida inferred from mitochondrial sequences and microsatellite loci native to south america the potato cyst nematode globodera pallida is one of the principal pests of andean potato crops and is also an important global pest following its introduction to europe africa north america asia and oceania building on earlier work showing a clear south to north phylogeographic pattern in peruvian populations we have been able to identify the origin of western european populations with high accuracy they are all derived from a single restricted area in the extreme south of peru located between the north shore of the lake titicaca and cusco only four cytochrome b haplotypes are found in western europe one of them being also found in some populations of this area of southern peru the allelic richness at seven microsatellite loci observed in the western european populations although only onethird of that observed in this part of southern peru is comparable to the allelic richness observed in the northern region of peru this result could be explained by the fact that most of the genetic variability observed at the scale of a field or even of a region is already observed at the scale of a single plant within a field thus even introduction via a single infected potato plant could result in the relatively high genetic variability observed in western europe this finding has important consequences for the control of this pest and the development of quarantine measures