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

Oocytes of zebrafish were used to study chilling sensitivity and membrane l ipid phase transitions in tropical fish. The oocytes were divided into two groups: small (without yolk, <0.1mm) and large (with yolk, >0.1mm). After e xposure of the oocytes to different temperatures (25, 22, 19, 16, 12, 8, 0, -8+/-0.5 degrees C) for 15 minutes, the integrity of their membranes was d etermined by carboxyfluorescein diacetate (cFDA) staining. At 16 and 12 deg rees C, damage was maximum (membrane integrity decreased by 50%) for small and large sizes, respectively. Lipid phase transition (LPT), which was eval uated using Fourier transform infrared (FTIR) microscopy, indicated phase t ransitions at the same temperatures at which damage was maximal (between 22 and 12 degrees C). In another series of experiments, the chilling sensitiv ity of oocytes taken from zebrafish which had been held at 16 degrees C for different periods of time (0, 15, 30, 60 minutes) was determined as descri bed above. In small oocytes membrane integrity decreased after 15 minutes, and in large oocytes integrity decreased after 30 minutes. Chilling sensiti vity was also measured in oocytes from zebrafish that had been held at 16 d egrees C for 30 minutes and then rewarmed to 28 degrees C for 2 hours. Desp ite this recovery period, the integrity of the oocytes remained low. We sug gest that chilling sensitivity in zebrafish oocytes is related to lipid pha se transition of their membranes and starts at 10 degrees C below the physi ological temperature.