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Analysis of the vitellogenin gene from the soft tick Ornithodoros moubata
Female ticks require a blood meal for vitellogenesis to occur. Vitellogenin (Vg), a precursor of yolk protein, is essential for egg development and Vg synthesis appears to be regulated by ecdysteroids in ticks. To better understand the regulation of Vg synthesis in ticks, the Vg gene was identified from Orinthodoros moubata. OmVg is composed of 5,502 bp encoding a 1,834 aa protein with Vg specific characteristics. OmVg gene showed the highest homology with the hard tick Dermacentor variabilis and was included in the same clade of a phylogenetic tree as the hard tick and crustacean Vg genes.
OmVg gene expression was observed in females but not in nymphal stages and during molting. Both mated and virgin females showed OmVg expression approximately 3 days after engorgement. However, as time passed, mated females showed significant increases in OmVg expression whereas virgin females didn’t. OmVg expression is thought to be regulated by ecdysteroids functioning through a complex with an ecdysteroid receptor (EcR) and retinoid X receptor (RXR). EcR and RXR expression increased in both mated and virgin females soon after engorgement. However, ecdysteroid titer only increased in mated females indicating a high titer of ecdysteroids in mated females up-regulates OmVg expression. The OmVg gene was expressed in the fat body and midgut of O. moubata. Further studies are underway to determine other factors that may explain differences between mated and virgin O. moubata females. Understanding the regulation of reproduction in ticks may lead to the development of better mechanisms for controlling ticks and preventing the spread of tick vectored diseases.
