Our study presents direct comparisons of selected basic measures 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 hybrids when 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 immune mechanisms 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 in water 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 (this was 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 line with 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.