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.