On the Mechanism of Temperature Independence in a Biological Clock

Diurnal rhythms which persist under constant conditions have been found in a variety of plants and animals (see Bunning;(1) Pittendrigh (2)). Most of the evidence points to the conclusion that diurnal rhythmicity is endogenous and results from oscillations of physiological processes of one sort or another. An unusual feature of many rhythms, however, is their temperature independence. In instances where study has been directed toward the temperature characteristics (Wahl;(3) Brown and Webb;(4) Pittendrigh;(5) Bruce and Pittendrigh;(6) Buihnemann(7)), it has been found that the period, or duration of a cycle, is unaffected or only slightly affected by temperature changes. The way in which this is achieved in organisms which are otherwise markedly temperature-sensitive is not known. It is possible that some uncontrolled environmental variable might be involved, i.e., that there is an exogenous factor which plays a role in determining the period in rhythmic organisms. Another possibility is that temperature compensation is incorporated as a part of the rhythmic mechanism. In either case, it may be noted that temperature independence constitutes support for the idea that a diurnal rhythm could be related to an accurate biological timing device, or clock. If, in fact, the rhythms are related to a chronometer mechanism, the significance of temperature independence is clear.