## 3 Discussion  
Here, the complete VgR sequence was obtained from _B. dorsalis_. This was the first report of VgR from a _Tephritidae_ to date. As expected, _Bd_VgR harbored several typical functional motifs of VgRs and the LDLR superfamily, namely, ligand binding domains, EGF-precursor domains, transmembrane domain and cytoplasmic tail domain, and showed similarity to the sequences (VgRs and YPR) in insect, particularly to those sequences in Diptera, perhaps due to the evolutionary conservation in their ligands (Figure 1). Although those motifs are conserved in insect, there were some special traits in _Bd_VgR sequence. Similar to the VgR sequences in Diptera and Blattaria, _Bd_VgR also had a typical 13 cysteine-rich LDLR\({}_{\rm A}\) repeats with classical arrangement, which was five and eight repeats in the first and second LBD, respectively [26; 27; 28]. However, the number of LDLR\({}_{\rm A}\) repeats and arrangement are quite different from other insect orders and species. For example, there are four- and seven-LDLR\({}_{\rm A}\) repeats in Lepidoptera, four/two and eight repeats in Hymenoptera, single eight-repeats in Coleoptera (_Tribolium castaneum_), and four and eight repeats in ticks [5; 29] and vertebrates [9]. In addition, both of the EGF-precursor domains contained a calcium-binding site. However, no O-link sugar domain appeared in the C-terminal of _Bd_VgR sequence, although it was supposed to play a role in improving VgR stability and regulating the signal pathway [2] and very conserved in Blattaria [26; 28] and present in _A. aegypti_[19], _D. melanogaster_[21], _M. domestica_ and other insects. Besides, only one LL motif was found in the cytoplasmic tail of _Bd_VgR, and no NPXY motif was found as described in the fire ant [30] and cockroaches [26; 27; 28]. Figure 5: The ovary diameter and expression levels of _BdVgR_, _Bdyp1_ and _Bdyp2_ 72 h after injected with dsRNA. (**A**) The diameters of ovaries; (**B**) The mRNA level of _BdVgR_; (**C**) The mRNA level of _Bdyp1_; (**D**) The mRNA level of _Bdyp2_. The bars represent the mean \(\pm\) SE. The asterisks indicate significant difference with _t_-test, * \(p<0.05\) or **\(p<0.01\). Unlike its ligands, which are widely detected in the fat body, ovary, head and thorax in the female adult [25], the _BdVgR_ mRNA were exclusively expressed in ovary and no traces were detected in the other tissues or segments (Figure 3). The result is along with the reports in other insect and vertebrates, such as _D. melanogaster_[21], _A. aegypti_[19], _Bombyx mori_[31], _Antheraea pernyi_[32], _Spodoptera liutura_[33], _Periplaneta americana_[28], _Leucophaea maderae_[26], _Solenopsis invicta_[30], _Thunnus thynnus_[34] and _Oncorhynchus chusmykiss_[35], consistent with its specific distribution and conserved role for egg development. However, recent studies point out that the VgR is no longer exclusively located in ovaries of female adults: the fat body [32], hypopharyngeal glands [36], head and even embryos [37] are found to be the extraovarian tissues or segment that express _VgR_; this may due to the alternative functions of their ligands in food storage [37], immunity [38] and resistance [39]. The relationship between expression level of VgR and ovarian development has been extensively studied in a variety of insects, such as, cockroach [26; 27; 28], fire ant [30], armyworm [33] and vertebrates, like shrimp [9], trout [35] and chicken [40]. The developmental-specific analysis indicated that the _BdVgR_ shared a similar mRNA expression pattern with other insects, which expressed at all ovarian developmental stages, extremely low in the very beginning of previtellogenic stage, increased dramatically before fully vittellogenic period. To be more exact, the transcript pattern of _BdVgR_ was up-regulated with the process of sexual maturation (Day 1-7), but quick declined when the ovaries fully developed and oviposition initiated (Day 9-10) (Figure 4), in coordination with the expressions of _Bdvp1_ and _Bdvp2_.