56 PHYSIOLOGICAL TRIGGERS 



beam of light one can turn on growth in a group of cells, while unilluminated 

 neighboring cells are prevented from growing. 



The data just considered amplify the respiration studies and reveal system- 

 atic shifts from a cytochrome oxidase system to a non-cytochrome oxidase 

 system during development. The loss or inactivation of the carbon monoxide- 

 sensitive cytochrome oxidase system at the time of pupation is compensated 

 by the development or activation of a carbon monoxide- and cyanide-stable 

 respiratory system capable of underwriting the maintenance requirements of 

 the diapausing insect. The identity and intracellular location of the terminal 

 oxidase in the diapausing pupa is still uncertain. The most likely candidates 

 are an autoxidizable flavoprotein transferring electrons from reduced pyridine 

 nucleotides to oxygen or an autoxidizable cytochrome of the b type (35, 24). 



The reappearance of a functioning cytochrome oxidase system at the outset 

 of development is necessary for the continuation of development because this 

 enzyme system plays an obligatory role in the energetics of the insect's develop- 

 ment. The absence of all but a trace of a complete cytochrome oxidase system 

 in the diapausing pupa therefore assumes special signiticance. Since the presence 

 and function of this system appear to be prerequisite for adult development, its 

 virtual absence in the dormant pupa can in itself account for the developmental 

 standstill of diapause. These facts persuade us that the reactivation of the cyto- 

 chrome oxidase system is among the biochemical changes set in motion by PGH, 

 changes which couple endocrine action to the termination of the pupal diapause. 



How does PGH reactivate the cytochrome oxidase system in the pupal tis- 

 sues? Recently Shappirio (38) has demonstrated by sensitive spectro-photo- 

 metric techniques that the absence of a functional cytochrome oxidase system 

 within the epidermis of the diapausing pupa is due to the virtual absence within 

 the diapausing cells of cytochromes b and c, a finding which narrows down the 

 defective cytochrome oxidase system of the diapausing pupa to the absence of 

 these two molecules (though of course other biochemical defects, as yet unde- 

 fined, may exist). In some way PGH effects the resynthesis of cytochromes 

 b and c. 



It appears unlikely that we shall get answers to this problem until extensive 

 studies are made with the pure hormone on isolated tissues, cell fractions and 

 enzyme preparations. This objective is now at hand, since Karlson and Buten- 

 andt at the Max Planck Institute in Tubingen have recently isolated and 

 crystallized PGH (8, 20). The hormone has the elementary formula Ci,sH:io04 

 and its spectrum suggests that it may be an unsaturated ketone. Injection of 

 this material to a final concentration of 10 parts per million induces development 

 in isolated pupal abdomens of Gecropia (53, 20). The material is still available 

 only in microgram quantities, but already attempts have been made to demon- 

 strate an in vitro effect (20-22). Thus far, like most attempts with other hor- 

 mones, they have failed. However, it is clear that it is from this point that 

 future work will proceed. 



