The application of meiotic cold shock treatment (MCST) to inhibit meiosis II of fertilized egg can simultaneously induce androgenesis and polyploidization. The reason is that, after the failure of meiosis II, the second polar body and the female nucleus form a meiotic complex, which may be retained in the fertilized egg to form a polyploid, or extruded out the fertilized egg to form an androgenetic individual. However, androgenesis or polyploidization? The induced trends for different MCST conditions are not yet known. The crossing between red crucian carp (2n, female) x tetraploid carp (4n, male) is a suitable object to study this problem, because both the androgenetic diploids and the hybrid tetraploids induced by MCST can develop normally. In this study, three gradient variables were set, namely post-fertilization time (0, 5, 10 min, the time before MCST after fertilization), shock temperature (0, 4, 6, 8 degrees C) and shock time (20, 40, 60 min). We found that the shorter the post-fertilization time, the lower the shock temperature and the longer the shock time, the greater the likelihood of meiosis failure, leading to the production of diploids or tetraploids. When the post-fertilization time is 5 min, the shock temperature was 4 degrees C and the shock time was 60 min, 100 % diploids can be detected in surviving individuals. Importantly, we found that the longer the post-fertilization time, the higher the shock temperature and the longer the shock time, the higher the proportion of diploids in these individuals with meiosis failure. Conversely, the higher the proportion of tetraploids. We speculate that under MSCT, the outward progression of meiotic complex is not completely stop, and the higher the temperature, the faster it progresses. Therefore, we think that prolonged postfertilization time, increased shock temperature, and prolonged shock time can all increase the outward distance of meiotic complex before the end of treatment. The more the outward distance, the easier it is for the meiotic complex to be extruded from the fertilized egg (inducing androgenesis). Conversely, the less the outward distance, the easier it is for the meiotic complex to remain in the fertilized egg (inducing polyploidization). This study may provide new insights into androgenesis, polyploidy and gynogenesis breeding in fish.
