Hybrid Zones and Host-Parasite Relationships: Effect on the Evolution of Parasitic Specificity

In southern France, Diplozoon gracile (Monogenea, Polyopisthocotylea), parasitizes four sympatric cyprinids. One of these host species, Barbus meridionalis, naturally hybridizes with another species of barbel, Barbus barbus, which is never parasitized by D. gracile under natural conditions. This hybridization has previously been studied and described as an introgression of B. barbus by B. meridionalis. The hybrids are parasitized by D. gracile, and parasite prevalence increases in proportion to the introgression rate, i.e., the percentage of B. meridionalis genes. The causes for this preferential distribution of the parasite in the hybrid population are analysed on the basis of ecological and ethological differences between the two parent species.