OXIDATIVE ENZYMES AND THE INJURY METABOLISM OF DIAPAUSING CECROPIA SILKWORMS*

Abstract

The remarkable control of insect growth by the hormone ecdyson is readily apparent in the events that transform a diapausing Cecropia pupa into a developing adult moth. The pupal diapause, as shown by Williams,' is a state of developmental arrest enforced by the failure of the insect's endocrine system to supply the hormonal stimulus required for growth and development. In essence, what the individual diapausing tissues require is ecdyson, secreted by the prothoracic glands. The latter become inactive after pupation and remain so for many months; but when the prothoracic glands recover their secretory function under neuroendocrine stimulation from the brain, the individual tissues once again begin to grow, and commence a sequence of rnorphogenetic changes that culminates in three weeks' time with the emergence of the adult moth.' One of the principal tasks of present-day insect endocrinology is to seek an understanding of how ecdyson is able to exert its control over the growth process. In molecular terms, one avenue of approach to the problem lies in elucidating the biochemical changes that occur a t the termination of diapause in response to the growth-promoting action of ecdyson. The properties of the diapausing insect itself are likewise of interest for understanding the physiological and biochemical adaptations of cells to life in a state of arrested developmen t. Both these matters were discussed by Williams2s3 about ten years ago in papers that called attention to the suitability of diapausing and developing silkmoths as subjects for examining biochemical mechanisms in insect growth and metamorphosis. At the time of these papers i t was already clear that alterations in oxidative enzymes a t the onset and termination of diapause had an important bearing on the probIems just mentioned. In particular, and of special interest for the present paper, was the evidence that a synthesis of oxidative enzymes within the pupal tissues represented one type of change promoted by ecdyson in favoring growth. This matter has subsequently been explored in detail in a number of publication^.^-* In the present paper I propose to review recent progress in our understanding of respiratory metabolism and respiratory enzymes in relation to the diapause and development of the Cecropia silkworm. After a brief rCsum6 of the metabolic and enzymatic changes during the course of metamorphosis, two principal topics will be considered: (1) the peculiar properties of respiratory metabolism in diapausing tissues and recent changes in our ideas as to their physiological basis; and (2) the close parallel between respiratory changes normally associated with the termination of diapause and those that follow * The work described in this article was supported in part by research grants from the Faculty Research Fund of the Horace H. Rackham School of Graduate Studies, University of Michigan, Ann Arbor, Mich., and by Research Grant RG-6101 from the National Institutes of Health, Public Health Service, Bethesda, Md.