SUMMARY: Gametes of lose their mating competence during the light periods of a 24 h light/dark (LD) cycle, and reacquire it in the dark periods. A diurnal rhythm in sexual agglutinability of the flagella underlies these fluctuations. Under constant environmental conditions the oscillation persists as an endogenous circadian rhythm. Under natural environmental conditions the rhythm is cooperatively driven by endogenous and exogenous signals. Exogenous control includes a decline in agglutinability upon illumination, which can be countered by Diuron, and a rise in agglutinability upon lowering of the temperature. In LD, pronounced fluctuations of the external pH are found that can be blocked by Diuron. The pH rhythm does not interfere with the agglutinability rhythm. The control of other rhythms in is discussed. As sexual agglutinability is due to well-characterized agglutinins located on the flagellar membrane, this biological rhythm lends itself to further exploration at the molecular level.


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