ELECTRON MICROSCOPY OF A CONTRACTILE‐VACUOLE MUTANT OF CHLAMYDOMONAS MOEWUSII (CHLOROPHYTA) DEFECTIVE IN THE LATE STAGES OF DIASTOLE 1

Electron microscopy of a “vacuole‐less” mutant of Chlamydomonas moewusii Gerloff revealed the presence of small anterior vacuoles. These vacuoles behaved like contractile vacuoles in wild‐type cells, but they were apparently unable to complete diastole and discharge their contents. When wild‐type and mutant cells were incubated in hypertonic medium, small coated vacuoles persisted in the region where contractile vacuoles form. When these cells were transferred to hypotonic medium, the vacuoles appeared to fill and fuse to form larger vacuoles Shortly after the appearance of full expanded contractile vacuoles, collapsed vacuoles were observed in wild‐type cells suggesting the completion of diastole and the onset of systole. In mutant cells, the initial steps of filling and fusion to form larger vacuoles apparent interactions of vacuoles with the plasma membrane were not observed. New contractile vacuoles accumulated around the nucleus. When fusion of the contractile vacuole with the plasma membrane was blocked by EGTA, a similar accumulation of large vacuoles occurred. Our observations suggest that the contractile‐vacuole mutant of C. Moewusii produces vacuoles which can accumulate excess water as part of the mechanism of osmoregulation but which cannot complete diastole.