Telophase of the first cleavage is the key stage for optimally inducing mitotic gynogenesis in rice field eel (Monopterus albus)

Abstract

The rice field eel, Monopterus albus, is an important freshwater fish cultured in China. In this study, mitotic gynogenesis was induced in the rice field eel using heat shock (40.5 ± 0.5 °C for 3.5 min) on eggs activated by ultraviolet (UV) - irradiated sperm of common carp (Cyprinus carpio). Initially, the first embryonic cell cycle was identified using histological analysis. The optimal initiation time for heat shock treatment was then determined by analyzing both the diploid and normal hatching rates. Ploidy was confirmed using flow cytometry and karyotype analyses. Embryos began to enter prophase at 70 mpf (min post-fertilization) and the first cleavage was completed at 125 mpf. Both haploid and hybrid embryos died before hatching. Heat shock treatment before metaphase (75 mpf) and at telophase (100 mpf) can induce chromosome doubling and produce viable individuals. For eggs treated before metaphase, heat shock treatment at 60 mpf, corresponding to the stage of pronuclear fusion, had the highest diploid rate (33.26 ± 4.4%) and normal hatching rate (3.94 ± 0.12%). For eggs treated at telophase (100 mpf), the diploid rate was 100% and the highest normal hatching rate was 26.77%. Therefore, the optimal time to apply heat shock treatment to induce mitotic gynogenesis in the rice field eel was at telophase during the first mitosis process. This technology could potentially accelerate the development of a new breed of rice field eel.