Koi ×Goldfish Hybrid Females Produce Triploid Progeny when Backcrossed to Koi Males

Abstract Hybrids of koi (an ornamental variant of the common carp Cyprinus carpio) and goldfish Carassius auratus auratus were produced by artificial spawning. All 3-year-old F1 hybrid males examined were sterile, whereas some F1 hybrid females were fertile and produced eggs after hormonal injection. Backcross progeny were obtained by using intact koi sperm to inseminate eggs from F1 hybrid females; gynogenetic progeny were obtained by inseminating eggs from F1 hybrid females with koi sperm that was genetically inactivated by ultraviolet irradiation. Flow cytometric analysis of DNA content indicated that the backcross progeny were triploid, while the gynogenetic progeny, pure koi, pure goldfish, and F1 hybrids were all diploid. The triploidy of backcross progeny obtained without application of any treatment to the eggs demonstrates that the koi × goldfish hybrid females produce diploid eggs. Received November 11, 2011; accepted February 18, 2012