Genetic evidence for gonochoristic reproduction in gynogenetic silver crucian carp (Carassius auratus gibelio bloch) as revealed by RAPD assays

Sex evolution has been a debating focus in evolutionary genetics. In lower vertebrates of reptiles, amphibians, and fish, a species or a bioform repro duces either sexually or asexually but never both. A few species were found to consist of all females in fish. These all-female species can propagate by asexual reproduction modes, such as gynogenesis and hybridogenesis. Howe ver, the coexistence of sexuality and asexuality in a single species was re cently noted only in a cyprinid fish silver crucian carp, Carassius auratus gibelio. This fish had been demonstrated to be capable of gynogenesis stim ulated by sperm from other related species. Surprisingly, natural populatio ns of this fish consist of a minor but significant portion (approx. 20%) of males. As different clones with specific phenotypic and genetic characteri stics have been found, and RAPD markers specific to each clone have recentl y been identified, this fish offers many advantages for analyzing whether o r not genetic recombination occurs between different clones. In this study, artificial propagation was performed in clone F and clone D. Ovulated eggs from clone F were divided into two parts and respectively inseminated with sperm from a clone D male and from a red common carp (Cyprinus carpio) mal e. The control clone D individuals were selected from gynogenetic offspring of clone D activated by sperm of red common carp. The phenotype and sex ra tio in the experimental groups were also observed. Using RAPD molecular mar kers, which allow for reliable discrimination and genetic analysis of diffe rent clones, we have revealed direct molecular evidence for gonochoristic r eproduction in the gynogenetic silver crucian carp and confirmed a previous hypothesis that the silver crucian carp might reproduce both gynogenetical ly and gonochoristically. Therefore, we conclude that the silver crucian ca rp possesses two reproductive modes, i.e., gynogenetic and gonochoristic re production. The response mechanism of two reproductive development modes ma y be the first discovery in vertebrates. Additionally, we discuss the evolu tionary implication between gynogenetic and gonochoristic reproduction mode s and the contribution of the minor proportion of males to genetic flexibil ity in the gynogenetic silver crucian carp.