Isolation and expression of the retinoid X receptor from last instar nymphs and adult females of the soft tick Ornithodoros moubata (Acari: Argasidae) Mari Horigane a , Kazumasa Ogihara a , Yoshiro Nakajima b , DeMar Taylor a, * a Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8572, Japan b Institute for Biological Resources and Functions, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8566, Japan Received 13 August 2007; revised 20 December 2007; accepted 29 January 2008 Available online 10 February 2008 Abstract 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 ecdyster- oid/EcR/RXR complex appears to be important in the regulation of molting and vitellogenesis of soft ticks. Ó 2008 Elsevier Inc. All rights reserved. Keywords: RXR; EcR; Nuclear receptor; Ornithodoros moubata; Ecdysteroid; Vitellogenesis; Ecdysis; Molting; Development; Tick 1. Introduction Molting and reproduction are characteristic phenomena regulated by endocrine mechanisms and these processes are regulated by ecdysteroids and juvenile hormone (JH) in numerous arthropods. Despite earlier studies indicating JH is important in the regulation of reproduction and development in ticks (reviewed by Sonenshine, 1991; Rees, 2004), more recent studies were unable to detect JH in ticks (Neese et al., 2000) or show a role for JH in the regulation of tick reproduction (reviewed by Taylor and Chinzei, 2002; Rees, 2004). To date ecdysteroids are the only hor- mones that have been identified in ticks. Generally, ecdys- teroids are released from endocrine tissues such as the prothoracic gland or ovary and induce various events in target tissues. Hematophagous arthropods require blood feeding for growth and reproduction and these events are also regulated by ecdysteroids (Raikhel and Dhadialla, 1992). After blood feeding, ecdysteroids are released into the hemolymph and regulate gene transcription in various tissues. Ecdysteroids directly and indirectly regulate gene expression as switches through the ecdysteroid receptor. In many insects, the functional ecdysteroid receptor is a heterodimer of the ecdysteroid receptor (EcR) and ultrasp- iracle (USP) (Henrich et al., 1990; Yao et al., 1992, 1993). The ecdysteroid/EcR/USP complex binds to ecdysteroid response elements (EcRE) in the gene transcriptional region of target genes and regulates gene transcription (Riddihough and Pelham, 1987; Yao et al., 1992, 1993; Thomas et al., 1993; Hall and Thummel, 1998). USP is a 0016-6480/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.ygcen.2008.01.021 * Corresponding author. Fax: +81 29 853 4806. E-mail address: taylor@sakura.cc.tsukuba.ac.jp (D. Taylor). www.elsevier.com/locate/ygcen Available online at www.sciencedirect.com General and Comparative Endocrinology 156 (2008) 298–311