Effect of heat-shock duration and zygote age on tetraploid induction for triploid broodstock production in platinum koi (

The koi fish (Cyprinus rubrofuscus) is an ornamental freshwater species valued for its vibrant body patterns, colors, and high market price. Despite extensive breeding efforts, the demand for koi fry remains unmet. Advances in biotechnology, such as chromosome manipulation, offer a promising solution to increase production yields. This study evaluated the effectiveness of tetraploid induction in koi through heat shock treatment, testing different zygote ages and shock durations. The highest proportion of tetraploids (86.6%) was achieved with a heat shock of 39 °C for 2.5 minutes. When considering zygote age, heat shock applied at 31 minutes post fertilization (mpf) produced 66.7% tetraploids, while treatment at 27 mpf yielded a comparatively higher proportion than other age groups. Tetraploidy was confirmed through cytogenetic analysis, ensuring accurate identification of induced individuals. These findings provide a practical protocol for koi breeders to produce tetraploid broodstock, which can be used for triploid production or to enhance genetic resources in ornamental fish aquaculture.