CIRCADIAN-RHYTHMS IN 3 UNICELLULAR ORGANISMS - THE PECULARITIES OF THE ORGANISMS, THE EVIDENCE BROUGHT ON RHYTHMS AND THEIR SPECIFIC PRACTICAL PROBLEMS - OUTLINE OF RECENT HYPOTHESES

Three unicellular organisms are considered: Gonyaulax, Euglena and Ace tabularia. They differ in morphology, in some specific macromolecules and in cell cycle. Their temporal morphology is reviewed. Circadian rh ythms (CR) in single cells were described in Gonyaulax and Acetabulari a. Besides common features, the three organisms, owing to some species -specific peculiarities, contributed to basic knowledge of the CR mech anism and functioning since the sixties. In particular, the CR depende ncy - or not - on transcription and translation. Light and temperature effects are reviewed as well as the occurrence of a state of arrhythm icity. The question of intercellular communication is discussed. Hormo nal effects and their relationships with CR have been approached by tw o groups, for Gonyaulax and for Acetabularia. They suggested interesti ng hypotheses related to evolution. Euglena turned out to be a good mo del system for the study of CR as related to the cell cycle. In all th ree species, CR dissociation has been described. Photosynthesis and en ergy metabolism are compared and discussed. Interestingly, the three P rotocysts display a photosynthesis CR post-transcriptionally regulated . The general theories developed by different authors are summarized a nd our alternative view is developed. Finally, the previously hypothet ized role of CR in evolution is briefly presented.