Our study presents direct comparisons of selected basicmeasures of immunity between two parental species and their hybrids. Šimková et al. (2013) showed that common carp × gibel carp hybrids harbor more parasite species than their parental species, but that the intensity of infection is lower in hybridswhen compared to parental species, suggesting that the hybrids harbor favorable genotypes (i.e. hybrid genotypes are less susceptible to the parasite species specific to each parental species). The immunemechanisms specific to parental species were proposed as potential mechanisms limiting the intensity of parasite infection in gibel carp × common carp hybrids. Our study suggested a temporal increase in all immune parameters linked to seasonal changes inwater temperature and reproductive investment in spring as previously showed by Buchtíková et al. (2011).
In addition, our study revealed an intermediate leukocyte count and intermediate complement activity in hybrids when compared to parental species. This phenomenon in hybrids was especially pronounced in spring, when compared to the lower values of these parameters in gibel carp and higher values in common carp. However, in spite of the intermediate character of the leukocyte count and complement activity in hybrids, both parameters in hybrids were still more similar to common carp when compared to gibel carp (thiswas especially evident in autumn). The fact that complement activity in hybrids did not follow seasonal changes, in contrast to the clear seasonal differences reported in both common carp and gibel carp, may suggest low hybrid susceptibility to pathogens, as suggested by Šimková et al. (2013). However, this is not in linewith the hypothesis of Buchtíková et al. (2011), who proposed that complement activity in common carp in spring decreases as a result of immunosuppression by steroid hormones.