Hybridization of common carp and goldfish was first investigated by a Japanese scientist, Matsui Yoshiichi in 1918, which inspired numerous lines of research conducted in Japan, China, Russia, Hungary and Poland. First generation of hybrid males were described as sterile. Nina Cherfas in Russia then discovered in 1980 that hybrids of common carp and goldfish are somatic diploids, but produced diploid (2n) gametes. This phenomenon, premeiotic endoreduplication, provides a direct opportunity for implementing sterility in backcross progeny (triploids) and may serve as an ideal candidate for exploring hybrid growth and metabolic (resistance to hypoxia) advantages as well as biocontrol.
We tested 6 males of first generation Koi (female) x Carassius auratus (male) hybrid (H) and obtained motile sperm following hormonal stimulation, although sperm density was low and highly variable compared to koi sperm. We then carried out single spawning where eggs were divided to obtain control purebred koi (K), koi subjected to cold shock (1-2oC of 45- or 90-min duration) (CS), and koi female backcross with H males. We confirmed through these experiments, Gomelsky’s conclusion that fertile H males do occur, though rarely. Fertile H males produced diploid sperm, and when crossed with K females, the resulting progeny were 100% triploid, characterized by high viability and excellent, uniform growth. CS 45 min fish were all triploids; however, they had very variable individual growth rate, while the CS 90 min group suffered high mortality and consisted of both triploid and diploid individuals.