# De Novo Transcriptome Identifies Olfactory Genes in _Diachassimomorpha Ionigicaudata_ (Ashmead)
## 1 Introduction  
_Diachassimomorpha Ionigicaudata_ (Ashmead, _D. longicaudata_) is a solitary species of parasitoid wasp of several fruit fly species and has been introduced to many countries as a biological control agent. Its host, _Bactrocera dorsalis_ [PERSON], can attack many fruit species and some other plants, such as Caricaceae, Moraceae, Myrtaceae, Rosaceae, and Solaneaceae [1]. It is said that the female Ashmeadinsects can detect the fly larvae by sound in rotting fruit and that the attractant could be the fungal fermentation products rather than the chemical substances produced by the fly larvae [2; 3]. [PERSON] and colleagues reported that the presence of fly larvae was essential for the orientation of wasps [4]. Further, it was proposed that cues from the fruit can be used by the wasps directly and that the presence of the host can enhance the attraction towards a patch [5]. Interestingly, chemical compounds produced by the larvae can be detected by wasps to locate the host [6; 7]. Once the female parasitoid is on the fruit, a specific chemical compound released by some Pephritidae species can be used to enhance the host search [7]. However, much is unknown about the chemosensory system of parasitoid wasps in response to their hosts. Animals can use the chemosensory system to detect and discriminate chemical cues in the environment [8]. The chemical sensors of insects are mainly from the antennae system, which is a highly specific and extremely sensitive chemical detector, and olfactory proteins in the antennae can be used by the insects to detect very low-abundance odorants from thousands of odors and further to guide their behaviors, such as forage, mate hunting, host plant location, shelter, and selection of spawning sites [9]. Since the first odorant-binding protein (OBP) was identified in _Antheraea polyphemus_[10], research on insect OBP has become a hot spot in the field of entomology. However, OBP-related studies have mainly been demonstrated in some important pests [11], and very few have been reported in parasitoid wasps, which are very important natural enemies of the pests. The chemoreception of insects involves three important events: (i) The uptake of signal molecules from the external environment; (ii) transport (diffusion) through the sensory hair; and (iii) interaction with the chemoreceptor, which in turn activates the cascade of events leading to spike activity in sensory neurons [12]. Some important protein families have been reported to participate in these events, such as OBP, sensory neuron membrane protein (SNMP), chemosensory protein (CSP), odorant receptor (OR), gustatory receptor (GR), and odorant ionotropic receptor (IR) [12]. Insect OBPs are small globular proteins (\(\sim\)135 to 220 amino acids) and are characterized by a specific domain that comprises six \(\alpha\)-helices joined by three disulphide bonds [12]. They can be categorized into two subgroups: Pheromone-binding proteins, which are mainly distributed in the male antenna, and general OBP (GOBP), which can be found in multiple tissues of male and female insects and function in the recognition of odorants from plants and other animals [13]. There are about 300 OBPs in the NCBI database and not many studies have been demonstrated to identify the OBPs in parasitoid wasps. [PERSON] and colleagues used transcriptome sequencing and identified 1 CSP, 21 OBPs, 53 ORs, 29 IRs, and 4 SNMPs in _Bactrocers minax_[14], an oligophagous tephritid insect whose host selection, and oviposition behavior largely depend on the perception of chemical cues. [PERSON] et al. reported Sgua-OBP1 and Sgua-OBP2 in _Scleroderma guani_ (Hymenoptera: Bethylidae) and NvitOBP in _Nasonia vitripennis_[15]. [PERSON] identified 10 OBPs in _Microplitis mediator_ (Haliday) [16]. [PERSON] et al. identified 25 OBPs, 80 ORs, 10 IRs, 11 CSP, 1 SNMPs, and 17 GRs in adult male and female _Chouioia cunea_ antennae [17]. However, little is known about the olfactory proteins in \(D\). _longicaudata_.