Monotreme Cell-Cycles and the Evolution of Homeothermy

Abstract

We have studied the effects of temperature on the phases of the cell cycle in cells derived from the monotreme mammals, platypus and echidna, which have the unusually low body temperature of 32�C. We report here that M phase and the cycle time conform to expectations, but in the case of cycle time this is due to different effects of high and low temperatures on GI, G2 and S phases. The finding that the G2 and S phases apparently have an inverse linear relationship with temperature up to 37�C (the upper lethal temperature) suggests that the low body temperature of the monotremes is not primitive, but rather has been the result of a lowering of the body temperature during their evolutionary history.