Résumé
Recolonisation of protected areas by new host species, and their parasites, or the translocation of individuals (accidentally or intentionally) to new locations may induce new host-parasite associations. Parasites are usually found to be less well-adapted and more virulent to newly colonized host species. Such new host-parasite associations may represent threats to the survival of the host populations. Our study compared the reaction of a naive host facing a new parasite with the reaction of a host in a population already associated with that parasite. We collected 305 Pomatoschistus microps (Krøyer, 1838), second intermediate host of the digenean Labratrema minimus (Stossich, 1887), from 4 regions around Europe. The fish were experimentally exposed to the parasite strain endemic to one of these regions. The initial step consisted of evaluating the genetic variation among the different fish populations. The genetic results, based on isozyme electrophoresis, revealed a significant differentiation among the populations studied. The second step determined both the quantitative and qualitative success of infestation of the different host populations. Our results show that there is no quantitative difference between sympatric host-parasite success and allopatric success. However, at the ultrastructural level, sympatric infection appears to be more successful. The results are discussed in terms of the local adaptation of host-parasite associations and the consequences to biological conservation.