Abstract
Entomopathogenic nematodes from Steinernema}genus are symbiotically associated with Xenorhabdus bacteria. Their life cycle comprises two phases~: a free stage in the soil, where bacteria are carried inside nematodes' gut and a parasitic stage in the insect, where both partners are separated and multiply in parallel. For nematodes, benefits are clear during the parasitic stage while costs prevail during the free stage. We first tried to measure the balance between costs and benefits in these symbioses to identify selective pressures acting on nematode's symbiotic investment. We showed that nematodes endure costs to the association in free stage in terms of survival, these costs increasing with bacterial load. On the other side, nematodes benefit from the symbiosis in proportion of bacterial load in parasitic stage in terms of reproduction. These two antagonistic effects lead to a trade-off between nematodes' survival and reproduction which is mediated by their bacterial symbionts. Thus, depending on the environment, these correlations could challenge association stability. It could also impact the way the specificity of these associations evolves. In a second part, we addressed the question of symbiosis specificity in natura as well as in laboratory. Field samples confirmed the already-known constancy of association between a nematode species and a bacteria species all over the world. Experimental re-associations between two nematode species and foreign bacteria showed differences between retention and benefit specificities as well as inter-specific variability. Thus the one-to-one association between nematode and bacteria species may not be as strict as previously thought.