Effects of senescence on somatic cell physiology in the green alga Volvox carteri

Abstract

In every generation resulting from asexual reproduction, the somatic cells of the green alga Volvox carteri undergo synchronous senescence and cell death. Although the somatic cells of the asexual and sexual (female) generations of V. carteri f. weismannia ceased growth 72–96 h after the cells had stopped dividing, the somatic cells in the sexual spheroid remained alive approx. 96 h longer than comparable cells in the asexual spheroid. Measurement of several environmental factors demonstrated that senescence was not caused by extrinsic factors, such as changes in the light period, temperature, or culture medium. If these factors (light, temperature) were purposely altered, the timing of viability decline changed. Other changes occurring with senescence included a decreased rate of [ 14C ]bicarbonate assimilation after 96 h and a rapid decline in total soluble cell protein. The addition of the protein synthesis inhibitor, cycloheximide, prolonged the onset of senescence and delayed the decline in somatic cell viability. By adding cycloheximide at different times during senescence, we were able to show that the drug had less effect after 96 h than it did when added at 72 h. These results rule out extrinsic events governing senescence and are consistent with our idea that senescence in V. carteri is due to an intrinsic, sequential genetic program.