Production of viable diploid mitogynogenetic Oreochromis aureus using the cold shock and its optimization through definition of cleavage time

Production of viable diploid mitogynogenetic tilapias was achieved using Oreochromis aureus eggs collected from a third generation of meiogynogenetic females, fertilized with UV-irradiated sperm of red tilapia. Diploidy was restored by cold shock treatment (1 h at 11°C) starting at the zygotic age of 77 min. Mitogynogens could be identified at embryo and juvenile ages on the basis of their body pigmentation, which clearly differentiated them from normal hybrid controls which showed the presence of paternally inherited red and yellow chromatophores. Because of asynchronization in cell divisions, assessed during the first, second and third cell division periods, maximal increment cleavage times (MICT) were defined within these periods. Intervals between MICT's define a cell cycle length of exactly 28 min between these cleavages performed at 28.2±0.2°C and of 75 min between the second meiotic and first mitotic divisions. The optimal cleavage blocking time (OCBT), which leads to diploidization when cold shock is applied, could be defined as occurring 2 min before MICT following shocking of embryo groups at different intervals of time before and after MICT. Because MICT is separated by only 2 min from OCBT, it is suggested that furrow appearance, namely the contractile ring formation, is directly influenced by cold shock rather than the spindle chromosome separation mechanism.