582 PHOTOPERIODISM IN INVERTEBRATES 



tion may be heterogeneous with respect to the mitotic cycle, with no 

 evident change in clock activity. It is therefore not possible to identify 

 the clock with processes involved in the mitotic cycle, although it is 

 clear that the mitotic cycle may be clock controlled. 



From what we know of clock systems on the one hand, and from 

 our knowledge of nuclear metabolism on the other, it seems that a de- 

 tailed study of the ribose nucleic acid (RNA) metabolism in clock 

 systems would be of value. Its known metaboHc role suggests it as a 

 likely candidate for a "master" oscillator, and a model for the way in 

 which it might oscillate is not difficult to envision. From the studies of 

 Mazia (1956) we know that RNA is formed in the nucleus and moves 

 unidirectionally into the cytoplasm. Specific enzymes, formed in re- 

 sponse to RNA, could decrease the level of substrates specific for new 

 RNA synthesis, thereby decreasing the rate of RNA production. Via 

 an appropriate turnover of the components involved, a rhythmic RNA 

 production could occur, giving rise to a rhythmic aspect to all RNA- 

 coupled systems, which we might measure as overt physiological 

 rhythms. Such a system has the possibility of controlhng many diverse 

 cellular reactions, and at the same time the feedback in the system 

 might be rather specific. 



This model is essentially a prey-predator type, in the general class 

 considered by Lotka (1920). In the simple case that he considers, the 

 period is inversely proportional to the square root of the product of 

 the velocity constants. All that would be necessary to expect tempera- 

 ture independence is that the period be, instead, a function of the ratio 

 of the velocity constants. Additional assumptions concerning the nature 

 of the mechanism might result in this. Since very little is known con- 

 cerning the chemical events involved in a clock system under constant 

 conditions, a great variety of specific models is possible. 



The need for biochemical information is evident; what does RNA 

 metabolism look like in such a system, for example? We certainly are 

 not suggesting that RNA is the only component which could be sup- 

 posed to function as the "master" oscillator. We merely point to it as 

 a likely example, in order to illustrate the fact that additional informa- 

 tion concerning the nature of the key compounds involved in the clock 

 system will be of indispensable value in understanding the mechanism. 



