Hybridization is a common phenomenon in fish species. There are two hypotheses explaining parasitism in hybridizing animals. 1) Co-adaptation between host genotype and parasite genotype. The genes of MHC are predicted to co-evolve in associations with specific parasites. 2) The super-optimal individual MHC diversity in hybrids is associated with high parasite load because T-cell clone repertoire is reduced due to high number of different MHC variants in hybrids resulting from the interspecies crosses. However, ecological aspects (spatial and trophic position of fish) may also determine parasite load in hybrids. The pattern of parasite infection in coexisting parental-hybrid complex using non-congener and congener model fish will be analyzed using the MHC IIB polymorphism. The effect of interspecies hybridization on host specificity will be tested experimentally. The multiple hybridizations may lead to asexual species forms associated with polyploidy. We will investigate whether asexual triploids (or the most common triploid MHC clone) are the target of parasite selection in gibel carp.