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 one-third 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.

Amplified fragment length polymorphism (AFLP) was used to obtain information on the within-species genetic variability of the tobacco cyst-nematode (TCN) complex. AFLP was found to be well suited to this type of study. The current classification of TCN was confirmed. Results indicate that the Globodera tabacum solanacearum group, believed to be restricted to the U.S.A., also occurs in Mexico. The within-species variability of TCN is considerable. Populations from Mexico may form a new subgroup. AFLP group-specific markers were identified for two of the TCN subgroups: Globodera tabacum tabacum and Globodera tabacum solanacearum.

Two Algerian populations of Heterodera avenae originating from Oued Smar in a sub-humid coastal plain and from Tiaret in an inland semi-arid plain were subjected to different temperature treatments and juvenile emergence evaluated. These populations were compared with two populations, Fr1 and Fr4 from southern and northen France, respectively, which represent two ecotypes with respect to hatching cycles in response to temperature. The two Algerian populations hatched over a wide range of constant temperatures (3 to 25°C) but differed in the times at which juveniles emerged from the cysts. Simulation of seasonal temperature variations (from summer to autumn or winter and, conversely, from winter to spring or summer) demonstrated diapause in these two populations, induced by higher temperature (20 and 25°C) treatments and broken subsequently by lower temperatures (3 and 7°C). Both Algerian populations of H. avenae represent the Mediterranean ecotype, with biological variations that may represent regional adaptations to more or less severe climatic conditions.

Eight populations of Heterodera avenae, H. lipjevi and H. latipons were tested in Petri dishes with six cereal differentials of the International Test Assortment and a set of resistant genotypes of wheats. Ratios of white females/juveniles inoculated were equivalent to those obtained previously in tubes and pots and con rmed the validity of the miniaturised technique used to study the virulence/resistance relationships of these nematodes and cereals. The populations were differentiated by virulence to barley differentials and a new group of pathotypes of H. avenae originating from Algeria was established. Differentiation among H. avenae populations from North Africa and West Asia was clearly demonstrated by their (a)virulence to the gene Cre1 in Triticum aestivum cv. Loros. As previously observed, cultivated and wild oats were poor hosts for Mediterranean populations of H. avenae, but contrasting host responses were observed for a physiological winter type of oat. In addition to describing the virulence status of H. lipjevi and H. latipons populations to barley, oat and wheat differentials, these results provide information for the introduction of resistance to barley and wheat breeding programmes.

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