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

Abstract

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.

Metabolism of [3H]‐ecdysone in embryos and larvae of the tick Ornithodoros moubata

“…An esterase-labile substance increased at the appearance of each ES peak [9]. The agreement between our previous results demonstrating conjugation of the endogenous ES produced during embryonic development and the present data, indicating that the apolar pathway is particularly efficient at times when endogenous free ES levels must be low, and the fact that these conjugates are not used as a hormonal source during development [13] lead us to conclude that AP represent products of a mechanism of detoxification of ecdysteroids.…”

“…rnicropllis embryos metabolize the radiolabelled AP to ecdysteroid-26-oic acids [22] which correspond to inactivation products in insects [23]. On the other hand, in 0. rnoubufa embryos, the radiolabelled AP are not hydrolysed during embryonic or larval development [13]. On the contrary, they are endogenously produced during embryonic and larval development as shown by RIA analysis of extracts before and after esterase hydrolysis throughout embryonic and larval development.…”

“…This detoxification mechanism is present and active in all the developmental stanes of 0. rnaubatlz from vounn ovocvtes to nvmphs 1181 and adults 110,131. [13,24] ES in the diet. Localization of this enzymatic system has not been completely defined but it is present in numerous tick organs: ovaries [lo], Malpighian tubules, intestine, and dorsal and ventral carcass (Vuilleme, unpublished results).…”

Identification and Characterization of Genes Involved in Ecdysteroid Esterification Pathway Contributing to the High 20-Hydroxyecdysone Resistance of Helicoverpa armigera

“…It is an interesting question why domesticated silkworm cannot protect themselves against 20E through the same enzymes as the cotton bollworm. In addition, ecdysteroid-22-acylesters are also found in the ovaries and eggs of some insects, and the hormone derivates are served as storage forms for embryogenesis (Connat et al, 1988;Whiting and Dinan, 1989). Thus, more studies are needed to survey the novel function of the ecdysteroid esterification pathway during insect developmental process in the future.…”