Abstract
Abstract Formation of asexual cysts can be induced in Gonyaulax polyedra by melatonin at 15 °C, or by its metabolite, 5‐methoxytryptamine, at 15 or 20 °C. When cells were exposed to a temperature of 10 °C, which holds the circadian oscillator(s), they were almost insensitive to melatonin, and the cyst‐inducing capacity of 5‐methoxy‐tryptamine was reduced to about 1/3. This desensitization was seen in various lighting conditions (LL, LD 16:8; LD 12:12; LD 11:13; DD). The reduction of cyst formation by decreased temperature was not due to a general incompetence of cells to encyst, because (a) low temperatures can lead to non‐photoperiodic cyst induction, and (b) both the MAO inhibitor tranylcypromine and the electroneutral Na+/H+ antiporter monensin lead to encystment in almost the entire cell population. After a 22‐h passage through the aperiodic state with a subsequent step‐up to 15 or 20 °C, the responsiveness to melatonin or 5‐methoxytryptamine, respectively, was approximately normal. Our results indicate that cyst formation via the short‐day/melatonin pathway requires an operating circadian oscillator.
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