Population genetic structure of the sugar beet cyst nematode Heterodera schachtii: a gonochoristic and amphimictic species with highly inbred but weakly differentiated populations.

The sugar beet cyst nematode Heterodera schachtii is a soil-dwelling phytoparasitic nematode that feeds on beet roots. It is an important pest in most sugar beet growing areas, and better knowledge of its genetic variability is an important step to preserve the durability of resistant sugar beet varieties. The population genetic structure of this species in northern France was studied using five microsatellite markers. A hierarchical sampling design was used to investigate spatial structuring at the scale of the region, the field and the plant. Multilocus genotypes were obtained for single individual second-stage larvae, using only one individual per cyst in order to avoid the analysis of closely allied individuals (larvae from the same cyst share at least the same mother). A consistent trend of heterozygote deficit at all loci was observed at all spatial scales. Heterozygote deficit at the level of individual plants argues against its generation through a Wahlund effect. Inbreeding could be due to very limited active dispersal of larvae in the soil, favouring mating between siblings, such as larvae emerging from the same cyst. Such behaviour could have important consequences for the evolution of virulence in increasing the production of homozygous virulent individuals. Moreover, an analysis of molecular variance (amova) reveals that only 1.6% of the genetic variability is observed among regions, 3.7% among fields of the same region and 94.6% within fields. The very low level of genetic differentiation among fields is also indicated by low values of FST (</= 0.105) even for fields 150 km apart. We suggest that long-distance gene flows occur due to passive transport of cysts by human activities, water or wind. As such mechanisms of gene flow would involve cysts and not larvae, the strong signals of inbreeding observed at the local scale should not be disrupted. To our knowledge, this study constitutes the first investigation of the genetic structure of a phytoparasitic nematode based on neutral codominant genetic markers scored in single individuals.