In our study, the general trend for GSI in hybrids was to be intermediate between common carp and gibel carp. In spring, the gonads of common carp and gibel carp were highly different, suggesting some differences in reproduction aspects. Generally, the reproduction times of both species overlap. However, there are two aspects that could explain the observed pattern of GSI and the presence of F1 hybrids with a maternal origin in gibel carp. In spring, even though the males of both gibel carp and common carp start to spawn in the same time period and the overlapping spawning period of both species covers several months (until summer), the females of gibel carp start to spawn before common carp. In addition,we showed that at the same age, common carp attain a greater body size than gibel carp. Under breeding pond conditions, females of common carp tend to spawn with larger common carp males,while smaller gibel carp females may spawnwith larger common carp (resulting in hybridization) or smaller gibel carp males (unpublished data).
Thus, unidirectional genetic introgression generating F1 hybrids of gibel carp maternal origin in breeding ponds probably results from reproductive behavior. Nevertheless, our crossbreeding experiments demonstrated successful bidirectional hybridization, although the viability of hybrids with gibel carp in the maternal position was greater than that of hybrids of common carp maternal origin (Šimková et al., unpublished). This may support the hypothesis of Haynes et al.(2012) suggesting biased gene flowin favor ofmale common carpmating with female goldfish (C. auratus), which could allow advantageous alleles e.g. resistance to the koi herpes virus in hybrids