Correlations between integument structure and ecdysteroid titers in fifth-stage nymphs of the tick, Ornithodoros moubata (Murray, 1877; sensu Walton, 1962)

Isolation and expression of the retinoid X receptor from last instar nymphs and adult females of the soft tick Ornithodoros moubata (Acari: Argasidae)

Retinoid X receptors (RXR) exist broadly from invertebrates to vertebrates, and play essential roles in physiological processes of these organisms. In arthropods, RXRs form a complex with the ecdysteroid receptor (EcR) and ecdysteroids to mediate the regulation of ecdysis and reproduction. Compared to EcR, RXR and its homologue ultraspiracle (USP) are much less well understood. Therefore, we identified RXR of the soft tick Ornithodoros moubata (OmRXR) and used real-time PCR to examine the expression of OmRXR. This is the first report of RXR from a soft tick. OmRXR showed higher homology to hard tick, crustacean and vertebrate RXRs than insect RXRs and USPs. OmRXR expression was observed during molting in the last instar nymphs coinciding with EcR expression and increases in ecdysteroid titers. Tick vitellogenesis normally occurs soon after engorgement and OmRXR expression coinciding with EcR expression and ecdysteroid titers in engorged females occurred before vitellogenin (Vg) synthesis and egg maturation. The ecdysteroid/EcR/RXR complex appears to be important in the regulation of molting and vitellogenesis of soft ticks.

Cloning and expression of the ecdysteroid receptor during ecdysis and reproduction in females of the soft tick, Ornithodoros moubata (Acari: Argasidae)

Molecular mechanisms of ecdysteroid regulation in development and reproduction have been thoroughly investigated in Diptera and Lepidoptera, but few studies report the molecular actions of ecdysteroids in hemimetabolous insects and more primitive arthropods. Ecdysteroids appear to be the main hormones regulating development and vitellogenesis in ticks. An ecdysteroid receptor that showed high homology with EcRs of other arthropods was isolated from Ornithodoros moubata (OmEcRA). OmEcR expression patterns coincided with ecdysteroid titres in the haemolymph during moulting and vitellogenesis and differed between mated and virgin females. Therefore, OmEcR appears to mediate the regulation of moulting and vitellogenesis by ecdysteroids in O. moubata females as seen in other arthropods.

Ecdysteroid hormone titer and its relationship to vitellogenesis in the soft tick, Ornithodoros moubata (Acari: Argasidae)

A blood meal is required for reproduction in most argasid female ticks. The blood meal appears to stimulate an organ in the posterior end to produce a fat body stimulating factor (FSF), which is thought to be an ecdysteroid, to induce vitellogenin (Vg) synthesis. In this study, the relationship of vitellogenesis and ecdysteroids was investigated by measuring Vg and ecdysteroid titers while observing oocyte development and oviposition in mated and virgin females. Oviposition occurred from day 10 after engorgement in mated females and continued up to 40-50 days, whereas egg maturation and oviposition did not occur in virgin females. Vg titers in the hemolymph peaked on day 6 after engorgement and subsequently declined in mated females. Interestingly, Vg synthesis occurred and ovarian development progressed to the development of early vitellogenic oocytes in virgin females but oocyte maturation and oviposition did not occur. Topical application of ecdysteroids induced oviposition in fed virgin females indicating that ecdysteroids may induce oviposition. Concentrations of ecdysteroids for 20 days after engorgement revealed several peaks in mated female whole body extracts, but no peaks in virgin female extracts. In the hemolymph of only mated females, ecdysteroid titers showed two peaks that followed the early peak of ecdysteroids in the whole body on day 4 and 6 after engorgement. In addition, ecdysteroids in the reproductive tissues increased with the development of the ovary in mated females and this increase coincided with the latter peaks of the whole body. These observations indicate that physiological elevation of ecdysteroids accelerate Vg synthesis, and may induce egg maturation and stimulate oviposition in fed mated Ornithodoros moubata females.

Hormonal control of tick development and reproduction

Ecdysteroids (moulting hormones), juvenoids and neuropeptides in ticks are reviewed but, by far, the emphasis is on the former since this class of hormones has been the subject of most investigations. In immature stages of ticks, ecdysteroids have been shown to regulate moulting and to terminate larval diapause. Although there is a paucity of information on the molecular action of ecdysteroids in ticks, their action appears to be via a heterodimeric ecdysone/ultraspiracle receptor, as in insects. The role of ecdysteroids in sperm maturation in adult males is considered. In females, ecdysteroids function in the regulation of salivary glands, of production of sex pheromones and of oogenesis and oviposition. There is evidence for ecdysteroid production in the integument and pathways of hormone inactivation are similar to those in insects. Ecdysteroids also function in embryogenesis. Although evidence for the occurrence and functioning of juvenile hormones in ticks has been contradictory, in recent thorough work it has not been possible to detect known juvenile hormones in ticks, nor to demonstrate effects of extracts on insects. Factors (neuropeptides) from the synganglion affect physiological processes and limited immunocytochemical studies are reviewed. Sigificantly, a G-protein-coupled receptor has been cloned, expressed, and specifically responds to myokinins.

Effect of methoprene and 20-hydroxyecdysone on vitellogenin production in cultured fat bodies and backless explants from unfed female Dermacentor variabilis

The effect of 20-hydroxyecdysone (20HE) and the juvenile hormone analogue methoprene (JHA) on vitellogenin (Vg) production in fat body organ cultures and backless explants of unfed female Dermacentor variabilis was measured. An indirect double antibody enzyme linked immunosorbent assay (ELISA) was developed using a monoclonal antibody that recognized a 98 kDa subunit of Vg and a Vg specific polyclonal antibody made against vitellin (Vn). Peak Vg titers in culture medium from fat body cultures treated with 0.1 &mgr;M 20HE or 1 &mgr;M 20HE were 24 ng/ml and 20 ng/ml respectively. In culture medium from backless explants treated with 0.1 &mgr;M 20HE or 1 &mgr;M 20HE, peak Vg titers were 36 ng/ml and 26 ng/ml, respectively. JHA produced only a slight increase in Vg titers that was statistically different from Vg titers produced by 20HE but was not statistically different from hormone-free controls. These results support the conclusion that Vg production in fat body trophocytes of D. variabilis is regulated by 20HE.

Ecdysteroid titre and metabolism and cuticle deposition during embryogenesis of the ixodid tick Amblyomma hebraeum (Koch)

Three embryonic cuticles are formed before larval cuticle deposition during embryonic development of Amblyomma hebraeum. The quantity of radioimmunoassay-positive material varied between 50 and 200 pg ecdysone equivalents per mg, but no significant peaks were detected. Maternally incorporated [3H]-20-hydroxyecdysone and [3H]-ecdysone contained in freshly laid eggs appear to be conjugated to C-22 fatty acid esters and 3 alpha epimers of those esters, and, thus, appear doubly inactivated. In addition, ecdysone is converted to an unknown product called 2'. The role of these maternally derived ecdysteroids is unknown.

Cuticle deposition and ecdysteroid titers during embryonic and larval development of the argasid tick Ornithodoros moubata (Murray, 1877, sensu Walton, 1962) (Ixodoidea:Argasidae)

Timing of embryonic and larval molts at the ultrastructural level and presence of ecdysteroids (ES) during embryonic and larval development of the argasid tick Ornithodoros moubata were studied. Embryonic "cuticles" A, B, and C were deposited 24-30 hr, 48-56 hr, and 6 days after oviposition, respectively. Deposition of the larval cuticulin layer started on Day 8 of embryonic development and procuticle deposition continued after hatching until apolysis of the larval cuticle 40 hr posthatch. Plaques of cuticulin formed on tips of microvilli 48-56 hr after hatching and procuticle was deposited until after ecdysis. Radioimmunoassay (RIA) was used to determine the ES titer in methanolic extracts of various ages of embryos and larvae. No peaks of RIA-positive material were detected during deposition of envelopes A, B, and C. However two peaks of ES were observed during embryonic development, one which coincided with the shortening of the germ band and a second which coincided with the deposition of the larval epicuticle on Day 8. During larval development, a peak of ES was observed on Day 3 (48-56 hr posthatch) and was correlated with nymphal epicuticle deposition. HPLC-RIA revealed that these last two peaks consisted mainly of 20-hydroxyecdysone together with a small quantity of ecdysone. Conjugated RIA positive material was present in freshly laid eggs and an augmentation of this esterase hydrolysable material was noted at the appearance of each ES peak. Thus the embryos did not appear to be hydrolyzing the maternal apolar conjugates to release ES during embryonic development; on the contrary, they appeared to be conjugating the newly synthesized hormones.

Epidermis as the source of ecdysone in an argasid tick

Various tissues excised from nymphs of the tick Ornithodoros parkeri at the time of epicuticle deposition were incubated in vitro. The medium from the incubation of salivary glands, coxal glands, synganglion, testis, midgut, and fat body associated with tracheal trunk showed little or no ecdysteroid immunoreactivity. Only medium from incubated integument contained ecdysteroids. The following evidence indicated that epidermal cells are the source of ecdysone: (i) when dorsal and/or ventral integuments were incubated separately, both produced ecdysteroid immunoreactive material during the course of incubation. As compared with the ecdysteroid content in the integument before incubation, the amount of ecdysteroids produced after a 24-h incubation increased 4- to 7-fold; (ii) enzymatic hydrolysis showed that neither highly polar ecdysteroid conjugates nor apolar conjugates were stored in the integument; (iii) histological and scanning electron microscope observations demonstrated that these excised integuments consisted of newly deposited epicuticle and epidermis as well as some fat body cells; (iv) HPLC RIA showed that the integument with associated fat body produced ecdysone and 20-hydroxyecdysone, while the integument produced only ecdysone after removing fat body. Presumably, ecdysone secreted by epidermis was converted into 20-hydroxyecdysone by fat body.

Identification of ecdysone 22-long-chain fatty acyl esters in newly laid eggs of the cattle tick Boophilus microplus

The five major apolar ecdysone esters present in newly laid eggs of the cattle tick Boophilus microplus have been purified by h.p.l.c. The quantities of the apolar esters present in the eggs were increased by administration of ecdysone to the mature females. G.c.-m.s. analysis, as their methyl esters, of the fatty acids released from the apolar ecdysone derivatives by alkali, coupled with positive-ion fast-atom-bombardment m.s. of the intact ecdysone esters, showed that the compounds consisted of a series of fatty acyl esters of ecdysone. The position of esterification of the ecdysone was established by p.m.r. spectroscopy. The combined data show that the novel apolar derivatives of ecdysone consist of the 22-palmitate, -palmitoleate, -stearate, -oleate, and -linoleate esters respectively. Confirmation was obtained by comparison with synthetic ecdysone 22-palmitate. The significance of the ecdysone fatty acyl esters as a possible source of free hormone during embryogenesis is discussed.

Metabolism of ecdysteroids during the vitellogenesis of the tick Ornithodoros moubata (Ixodoidea, Argasidae): accumulation of apolar metabolites in the eggs

The fate of injected [3H]ecdysone [( 3H]E) and 20-hydroxy-[3H]ecdysone [( 3H]20E) has been investigated in the female tick Ornithodoros moubata (Murray, 1877; sensu Walton, 1962). When injected into fed mated vitellogenic females, [3H]E is converted into [3H]20E and two apolar classes of metabolites, AP1 and AP2. Injected [3H]20E is directly converted into AP1 and AP2. AP2 is incorporated into the ovaries in a high proportion and at the end of the vitellogenic cycle represents about 25% of the total label recovered from the animal. The fate of labeled hormones injected into virgin females which perform an abortive vitellogenic cycle is quite similar. However, the ovaries incorporated less of the AP2 products. Ovaries of mated females cultured in vitro in the presence of [3H]E are able to produce [3H]20E and AP2. AP2 is incorporated, while [3H]20E is mainly found in the medium. Ovaries of virgin females presented a slower rate of transformation and of incorporation of the label. Labeled AP2 is recovered in freshly laid eggs and AP1 in the females after oviposition. AP1 and AP2 can produce [3H]20E, [3H]E, and other minor polar peaks when submitted to hydrolysis by esterase. It is concluded that the female O. moubata possesses a special enzymatic mechanism for transformation of ecdysteroids into apolar products and for selective incorporation of AP2 into the ovaries. These products are present in the freshly laid eggs and could play a role during embryogenesis.