Dynamics of Ecdysone Secretion and Action in the Fleshfly Sarcophaga Peregrina

Abstract

1. Ecdysone is in a highly dynamic state after its injection or its secretion by the ring-gland of Sarcophaga peregrina. Hormonal activity is rapidly destroyed by an inactivating mechanism which is present in the tissues but not in the blood. 2. Inactivation is blocked by low temperatures or anaerobic conditions-a finding that implicates chemical and, more particularly, oxidative reactions. The mechanism in question could be demonstrated in larval fragments but not in crude or fractionated homogenates. 3. When injected into mature larvae, 1 µg of α-ecdysone loses 50% of its activity in 1 hour and 98% in 8 hours. Lower doses show even briefer "half-lives." 4. The rapid inactivation of ecdysone can account for its low titer in both the blood and tissues. Thus at the "critical period" for puparium formation, the entire larva contains only 2.5 nanograms, corresponding to only 7% of a Sarcophaga unit. 5. The evidence points to the accumulation, not of the hormone itself, but the covert biochemical and biophysical effects of the hormone. The covert effects undergo spatial and temporal summation within the target organs and finally discharge the overt developmental response. 6. The role of the blood is to serve, not as a reservoir, but as a pipeline through which ecdysone flows from the ring-gland to its sites of action and swift inactivation.

Overt and Covert Effects of Endogenous and Exogenous Ecdysone in Puparium Formation of Flies

“…This had already been previously proved by Ohtaki7 and Ohtaki et al 3 The nature of the mechanism of this inhibition is not known. We have, however, now shown that larvae pupariate even under wet conditions upon injection of eedysone.…”

Counteraction by 20‐hydroxyecdysone of the effect of juvenile hormone on phosphorylation of ribosomal protein S6

“…However, induction of the phosphorylation of S6 under these conditions depended upon the concentration of 20-hydroxyecdysone and 10m6 M 20-hydroxyecdysone was much less effective, as shown in lane 3. The densitometric scanning of the band of S6, shown in fig.2b, showed that restoration of phosphorylation by 10e6 M 20-hydroxyecdysone was only 30% of that by lo-' M 20-hydoxyecdysone, which is a physiological concentration to induce metamorphosis of this insect [12]. Values are averages for three fat bodies Therefore, a higher concentration of the hormone seemed to be rather inhibitory.…”

The role of 20-hydroxyecdysone in the CNS metamorphosis in flesh fly (Neobellieria bullata) larvae (Diptera: Sarcophagidae)

“…It should be emphasized that the effect was caused by relatively large doses of the hormone that exceeded the endogenous levels by several orders of magnitude. Small amounts of ecdysteroids cause gradual accumulation of "covert effects" of the hormones on target tissues (Ohtaki et al, 1968), which in turn cause progressive switching on of the genes responsible for individual episodes of the pre-metamorphosis and metamorphosis development. This can explain why a rather small elevation of the hormonal titre -the "commitment peak" -can trigger such a dramatic change of behaviour at the end of feeding stage, when the tissues are already sensitive to the hormone.…”

“…If a flesh fly larva does not find a dry place, or if it is experimentally forced to remain in a wet one, the formation of puparium is postponed or abolished (Ohtaki, 1966). Injection of 20-hydroxyecdysone (20E) into such wet-treated larvae restores development and the larva forms puparium (Ohtaki et al, 1968). This suggests that the switch from preferences for wet to preferences for dry is associated with hormonal action of the ecdysteroid.…”