Cell cycle oscillators. Temperature compensation of the circadian rhythm of cell division in Euglena

The effects of different constant temperatures ranging from 16 degrees to 32 degrees C on the free-running, circadian rhythm of cell division were examined in axenic, photoautotrophic batch cultures of the unicellular algal flagellate Euglena gracilis Klebs. A comparative study was undertaken on the wild-type (Z strain) and a diuron-(DCMU)-resistant (ZR) strain. Although the overall growth rate (g) of both strains was rather dependent on temperature, lengthening increasingly at temperatures both higher and lower than the optimum range (about 23 degrees-29 degrees C), the free-running period (tau) of the oscillator hypothesized to underlie the overt rhythmicity in the cell division cycle (CDC) was found to be temperature-compensated over at least a 10 degrees C range. The degree of temperature compensation was most striking in the Z strain (Q10 = 1.05) over the permissive temperature interval of 22 degrees-32 degrees C, where periodic growth could occur. This Z strain had a slightly faster growth rate and displayed a higher degree of synchrony than that observed in the ZR strain, whose circadian clock was not as well compensated (Q10 = 1.23) over the permissive temperature interval of 18 degrees-28 degrees C. These results imply that the CDC is regulated by a circadian oscillator sharing the same features as those generating the many other overt biochemical and physiological circadian periodicities that have been documented for Euglena.