AbstractThe gynogenetic diploid hybrid clone line (GDH) derived from red crucian carp (♀ RCC) × common carp (♂ CC) possesses the unusual reproductive trait of producing unreduced diploid eggs. To identify the mechanism underlying this phenomenon, we examined the structure, in vivo developmental process and in vitro dynamic development of the GDH gonad. In summary, compared with RCC and CC, GDH showed certain special straits. First, a high frequency (84.7%) of germ cell fusion occurred in gonadal tissue culture in vitro as observed by time-lapse microscopy. Second, microstructural and ultrastructural observation showed numerous binucleated and multinucleated germ cells in the gonad, providing evidence of germ cell fusion in vivo. By contrast, in the diploid RCC and CC ovaries, neither cell fusion nor multinucleated cells were observed during the development of gonads. Third, the ovary of GDH remained at stage I for 10 months, whereas those of RCC and CC remained at that stage for 2 months, indicating that the GDH germ cells underwent abnormal development before meiosis. This report is the first to demonstrate that cell fusion facilitates the formation of unreduced gametes in vertebrates, which is a valuable finding for both evolutionary biology and reproductive biology.