Chilling sensitivity in zebrafish (Brachydanio rerio) oocytes is related to lipid phase transition

Oocytes of zebrafish were used to study chilling sensitivity and membrane lipid phase transitions in tropical fish. The oocytes were divided into two groups: small (without yolk, <0.1mm) and large (with yolk, >0.1mm). After exposure of the oocytes to different temperatures (25, 22, 19, 16, 12, 8, 0, -8+0.5 degree C) for 15 minutes, the integrity of their membranes was determined by carboxyfluorescein diacetate (CFDA) staining. At 16 and 12 degree C, damage was maximum (membrane integrity decreased by 50%) for small and large sizes, respectively. Lipid phase transition (LPT), which was evaluated using Fourier transform infrared (FTIR) microscopy, indicated phase transitions at the same temperatures at which damage was maximal (between 22 and 12 degree C).In another series of experiments, the chilling sensitivity of oocytes taken from zebrafish which had been held at 16 degree C for different periods of time (0, 15, 30, 60 minutes) was determined as described above. In small oocytes membrane integrity decreased after 15 minutes, and in large oocytes integrity decreased after 30 minutes. Chilling sensitivity was also measured in oocytes from zebrafish that had been held at 16 degree C for 30 minutes and then rewarmed to 28 degree C for 2 hours. Despite this recovery period, the integrity of the oocytes remained low. We suggest that chilling sensitivity in zebrafish oocytes is related to lipid phase transition of their membranes and starts at 10 degree C below the physiological temperature