Expression and evolutionary divergence of the non-conventional olfactory receptor in four species of fig wasp associated with one species of fig

BMC Evolutionary Biology Expression and evolutionary divergence of the non-conventional olfactory receptor in four species of fig wasp associated with one species of fig Bin Lu 0 2 5 Nina Wang 2 Jinhua Xiao 1 Yongyu Xu 2 Robert W Murphy 3 4 Dawei Huang 1 2 0 Chengdu Institute of Biology, Chinese Academy of Sciences , Chengdu, Sichuan 610041 , PR China 1 Institute of Zoology, Chinese Academy of Sciences , Beijing 100101 , PR China 2 College of Plant Protection, Shandong Agricultural University , Tai'an, Shandong 271018 , PR China 3 State Key Laboratory of Genetic Resources and Evolution, Kunming Institute of Zoology, the Chinese Academy of Sciences , Kunming 650223 , PR China 4 Department of Natural History, Royal Ontario Museum , 100 Queen's Park, Toronto, ON M5S 2C6 , Canada 5 Graduate School of the Chinese Academy of Sciences , Beijing 100049 , PR China Background: The interactions of fig wasps and their host figs provide a model for investigating co-evolution. Fig wasps have specialized morphological characters and lifestyles thought to be adaptations to living in the fig's syconium. Although these aspects of natural history are well documented, the genetic mechanism(s) underlying these changes remain(s) unknown. Fig wasp olfaction is the key to host-specificity. The Or83b gene class, an unusual member of olfactory receptor family, plays a critical role in enabling the function of conventional olfactory receptors. Four Or83b orthologous genes from one pollinator (PFW) (Ceratosolen solmsi) and three non-pollinator fig wasps (NPFWs) (Apocrypta bakeri, Philotrypesis pilosa and Philotrypesis sp.) associated with one species of fig (Ficus hispida) can be used to better understand the molecular mechanism underlying the fig wasp's adaptation to its host. We made a comparison of spatial tissue-specific expression patterns and substitution rates of one orthologous gene in these fig wasps and sought evidence for selection pressures. Results: A newly identified Or83b orthologous gene was named Or2. Expressions of Or2 were restricted to the heads of all wingless male fig wasps, which usually live in the dark cavity of a fig throughout their life cycle. However, expressions were widely detected in the antennae, legs and abdomens of all female fig wasps that fly from one fig to another for oviposition, and secondarily pollination. Weak expression was also observed in the thorax of PFWs. Compared with NPFWs, the Or2 gene in C. solmsi had an elevated rate of substitutions and lower codon usage. Analyses using Tajima's D, Fu and Li's D* and F* tests indicated a non-neutral pattern of nucleotide variation in all fig wasps. Unlike in NPFWs, this non-neutral pattern was also observed for synonymous sites of Or2 within PFWs. Conclusion: The sex- and species-specific expression patterns of Or2 genes detected beyond the known primary olfactory tissues indicates the location of cryptic olfactory inputs. The specialized ecological niche of these wasps explains the unique habits and adaptive evolution of Or2 genes. The Or2 gene in C. solmsi is evolving very rapidly. Negative deviation from the neutral model of evolution reflects possible selection pressures acting on Or2 sequences of fig wasp, particularly on PFWs who are more host-specific to figs. - Background The interactions of flowers and insect pollinators are the classic examples of co-evolution. The intimate relationships and specialization between figs and fig wasps are among the best studied cases [1-3]. Pollinating fig wasps (PFWs) usually exclusively pollinate species of figs [2,4-7], although recent discoveries indicate that the species-specific association is much less specific than has been thought previously [8-13]. The pollinators completely depend on figs for their life cycles [14-17]. In addition to PFWs, several non-pollinating species of fig wasps (NPFWs) also exploit figs [18]. Evidently, non-pollinators are less specific to a given host species than pollinators [13,18-23]. The host-specificity behaviour of insects relies heavily on olfaction [24], which has evolved to a level of extreme sensitivity and specificity [25]. This highly discriminative sensory mode is facilitated by an odour-activated seventransmembrane-domain G protein-coupled receptor (GPCR) that signals cascades [26]. Fig wasps are no exception. The mechanism underlying the host-specificity of fig wasps is almost certainly based on volatile chemicals released by the fig [27-29]. Fig wasps should be able to distinguish the particular odours emitted by hosts from other volatile compounds [30-32]. Different species of wasps appear to have unique volatile profiles which could account for host-specificity [28]. Such chemical constraints likely reduce host switching. Currently, investigations on adaptive changes in fig wasps are limited to morphological characters associated with an ecological niche [14,33-36]. For example, a pollinator must enter the syconium to deposit eggs. Thus, their antennae are easily broken to ease crawling through the ostiole. Unlike PFWs, most NPFWs cannot enter syconia. They usually possess a long ovipositor that is inserted through the syconial wall for egg laying [37-39]. Furthermore, both eyes and wings in most adult male PFWs exhibit highly vestigial traits, a correlate of living in the dark fig cavity all their life [40,41]. Unlike these attributes, very little is known about genetic changes in fig wasp's olfactory system. As members of the GPCR superfamily, olfactory receptors (ORs) for odorous compounds play the critical role in the olfactory process [42]. The process consists of several linked systems ranging from stereo chemical recognition to the generation of an odour code in the brain. A characteristic trait of conventional odour ligand-binding OR types is the tremendous diversity of their sequences, often exhibiting only ~20% identities to each other [43-45]. A highly conserved, non-conventional member in the insect OR family is known as Or83b [46]. Orthologs have been identified from Drosophila melanogaster (DOr83b) [47-49], Anopheles gambiae (AgOr7) [50], Heliothis virescens (HvirR2) [51], Apis mellifera (AmelR2) [52] and others [50,53,54]. Apparently, Or83b does not directly respond to odorants but rather acts as a chaperone receptor to form heterodimers with other odorant and pheromone receptors, thus ensuring dendritic localization [55-57]. While spatial expression patterns of conventional ORs are restricted in small subpopulations of olfactory sensory neurons (OSNs), Or83b is co-expressed with conventional ORs in most, if not all, neurons [52,55]. Tissue-specific expression patterns of putative ORs have been observed in ecologically distinct species. For example, in the hematophagous mosquitoes An. gambiae and Aedes aegypti, Or83b orthologs are expressed in the antennae, legs and proboscis (i.e., general gustatory organs). As such, Or83b orthologs may be involved in locating human hosts for blood feedi (...truncated)