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Vitellogenin (Vg) synthesis and oocyte development appear to be induced by ecdysteroids in the soft tick Ornithodoros moubata. Vg transcription in insects has been shown to be mediated by two nuclear receptors, an ecdysteroid receptor (EcR) and a retinoid X receptor (RXR). EcR and RXR expression in female ticks increase after engorgement. However, there is little information about EcR and RXR expression in tissues as related to reproduction. Therefore, the sites of EcR and RXR expression during vitellogenesis were determined. Expression levels of EcR and RXR were determined for salivary glands, midgut, ovary, anterior reproductive tract, fat body, muscle, and cuticle of mated females by reverse transcriptase RT-PCR and real-time PCR. Ovaries showed constant expression of EcR and RXR throughout the adult female stage. The fat body showed higher expression of EcR and RXR on one day after engorgement. The fat body of female ticks has been reported to be the organ of Vg synthesis. Therefore, increases in EcR and RXR of the fat body support the hypotheses that the site of Vg synthesis is the fat body and ecdysteroids regulate this synthesis in O. moubata. EcR and RXR in the ovary and anterior reproductive tract may also play roles in the regulation of oocyte development. [ABSTRACT FROM AUTHOR]

During insect oogenesis, the oocyte acquires nutrients and genetic determinants to support embryonic development (previtellogenesis and vitellogenesis) and subsequently becomes surrounded by a protective eggshell (choriogenesis). In many insects, ecdysteroids are synthesized during ovarian growth which is often followed by the accumulation of ecdysteroid conjugates into the eggs. The exact role of the ecdysteroids during oogenesis remains largely unclarified although functions as paracrine or autocrine regulators to signal the progression of follicle development or the resumption of meiosis in the oocyte have been proposed. In the silkmoth, Bombyx mori, although ecdysteroids are synthesized by the ovarian follicles, progression of follicle development towards choriogenesis requires down-regulation in ecdysteroid signaling. Using the regulation of silkmoth oogenesis by 20-hydroxyecdysone as a starting point, this review discusses the physiological roles of ecdysteroids and the function of the ecdysone regulatory pathway during insect oogenesis. [ABSTRACT FROM AUTHOR]