Faculté des sciences

Apolar conjugates of ecdysteroids are not used as a storage form of molting hormone in the argasid tick Ornithodoros moubata

Connat, Jean-Louis ; Dotson, Ellen Marie ; Diehl, Peter A.

In: Archives of Insect Biochemistry and Physiology, 1988, vol. 9, no. 3, p. 221-235

Fifth (last) instar nymphs of the tick Ornithodoros moubata convert ingested 20-hydroxyecdysone (20E) to apolar conjugates AP2, which are then converted to the more polar conjugates AP1. Only small quantities of free hormone were transferred to the hemolymph and the carcass within the first 2 days after the blood meal. The proportion of radiolabel in these two compartments was highest at... Plus

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    Summary
    Fifth (last) instar nymphs of the tick Ornithodoros moubata convert ingested 20-hydroxyecdysone (20E) to apolar conjugates AP2, which are then converted to the more polar conjugates AP1. Only small quantities of free hormone were transferred to the hemolymph and the carcass within the first 2 days after the blood meal. The proportion of radiolabel in these two compartments was highest at the time of the endogenous ecdysteroid peak; however, no traces of free [3H]20E were detected. The conversion probably occurs principally in the intestinal cells. Eleven days after ingestion, 84% of the radiolabel is located in the digestive tract, mainly in the form of AP1 conjugates. AP1 obtained in second instar nymphs fed with [3H]ecdysone ([3H]E) remain stable throughout the following nymphal instars. The ecdysteroid moiety of AP1 remained unchanged. The hydrolysis, although not complete, always yielded a peak comigrating with the reference E but never 20E or any other clearly distinct peaks that may have corresponded to metabolites of 20E. Less label per individual was present in adults, but its nature remained the same, viz., AP1 mainly located in the digestive tract. In females, 2.5% of the label was transferred to the progeny during the first ovipositional cycle.
    Apolar products (mainly AP2) that accumulated in eggs of females injected with [3H]E or [3H]20E during vitellogenesis remained unchanged during the whole embryonic development. During the molting cycle of larvae, there was only a slight conversion of AP2 to AP1, but esterase hydrolysis of these products released the same percentages of E and 20E as in the freshly laid eggs.
    We conclude that in this tick species apolar conjugates of ecdysteroids are inactivation metabolites that are not reutilized during the development of the animal. These metabolites are mainly retained in the tick, probably because of its peculiar blocked midgut. Several studies have shown that in other arthropod species (ticks, spiders, and insects), these apolar metabolites are excreted in the feces.