| Chemoreception is a crucial biological process that is essential for the survival and reproduction of insects, including food detection, avoiding predators, finding hosts, oviposition, copulation, and etc. There are two major chemosensory mechanisms through olfaction and taste, in which chemical signals are detected by one of the large sized multigene families which encode the odorant-binding proteins (OBPs), chemosensory proteins (CSPs), olfactory receptors (ORs), gustatory receptors (GRs), and ionotropic receptors (IRs). The function of OBPs and CSPs is the first step in odor detection.The system of figs (Ficus:Moraceae) and their pollinating fig wasps (Agaonidae: Hymenoptera) is widely regarded as a model for the study of coevolved mutualism. Pollinating fig wasps only develop in the closed and dark galls, formed from the fig ovaries, within the syconium, and one wasp lives in one’s own gall. The mated female wasps will fly away from their natal figs to search for a suitable fig to fulfill their spawning and pollination. Like in other insects, the chemoreception also plays a key role in the wasps’ life history, particularly in finding their specific host fig and mating. The high degree of host specificity is presented by a phenomenon that a female is only attracted by its specific fig tree species through the volatile chemicals released by the figs when they are ready to be pollinated. The within-tree fruiting synchrony of figs and the short life-spans of the wasps mean that the female wasps must usually leave their natal trees in order to find figs that are suitable for oviposition, and the generations can be extended. So the precise seeking and identification for figs is crucial for the female wasps. For the male wasps, they usually do not leave the dark fig in which they develop. When the wasps mature, the male wasps always come out of the galls earlier than the females, emerge into the syconium, find the gall with a female in, and mate with her. Since there are many others wasps (such as non-pollinating wasps) in the same fig, how do the male wasps exclude interference from other wasps and find their mating females resources most effectively in the dark figs? Clearly, the process by which the male wasps perceive the pheromone released by the female wasps of the same species is the key step.There is no doubt that the OBPs and CSPs play a crucial role in Ceratosolen solmsi. We identified the OBPs and CSPs genes in the fig pollinator specifically parasitized in Ficus hispida, and analyzed genes evolution and genes expression differences between female and male wasps among the various developmental stages and the body parts using real-time PCR. The results were as follows.Two OBPs members and three CSPs members were coloned and were verified in the genome of C. solmsi. Seven CsolOBPs were identified from the C. solmsi genome. The ColOBPs repertoire was one of the smallest reported among insects because they lived in monotonous habitats and need not lots of OBPs to perceive stimuli outside. We suggested that these insects which habitats were relatively concealed and monotonous had fewer OBPs compared with insects lived in open habitats. Through the conserved cysteines-pattern analysis the OBPs in C. solmsi may be still evolving. Phylogenetic analyses on the OBPs in hymenopteran insects showed that two groups of orthologous sequences were shared across Hymenoptera:CsolOBP1-NvitOBP02-AmelOBP10-SinvOBP10and CsolOBP2-NvitOBP79-AmelOBP1-SinvOBP1. Integrating the results of ligand binding assays, motifs order and phylogenetic analysis, we therefore speculated that the OBPs containing the motifs order of3-2-1-4and3-2-1can bind to general odorant, and CsolOBPl and CsolOBP2in C. solmsi, which contained motifs order of2-1-4, may combine some pheromone. The conserved cysteines-pattern in C. solmsi CSPs was Cl-X6-C2-X18-19-C3-X2-C4. There were conserved motifs in ColCSPs. And in the order Hymenopera, the phylogeny of CSPs was variable.The relative transcript levels of the OBPs and CSPs genes in the fig wasp C. solmsi showed that:1) Expression comparisons in females indicated that six of the seven OBPs genes (except for CsloOBP6) had the highest expression levels when the females emerged from the galls, especially CsolOBP4and CsolOBP5, both of which were also the only two genes that presented higher expression in females than in males in this stage. The results suggested that CsolOBPs, especially CsolOBP4and CsolOBPJ, seemed to play vital functions in the process of host searching and locating by combining the volatile compounds emitted by the figs when they were receptive.2) Expression comparisons in females indicated that three CSPs genes had the highest expression levels when the females emerged from the figs. The results suggested that CSPs seemed to play vital functions in the process which the females need to feel the tactile signal to determine to spawn when they reached the surface of figs.3) In the expression comparisons of the OBPs in different developmental stages of male fig wasps, we detected that five OBPs genes were expressed dramatically higher when the mature males were in the process of mating or mates searching (emerged out from the galls into the fig syconium) than in pupal stage, among which, three genes (CsolOBP1, CsolOBP2and CsolOBP5) were extremely elevated with more than190folds. These interesting results indicated that these three genes highly expressed in mated males seemed to function to combine pheromones released from females.4) Expression comparisons in males indicated that CSPs genes had the highest expression levels when the males emerged from the galls. The results suggested that CSPs seemed to play vital functions in the process which the males need to feel the tactile signal to determine to mate with the females still in the figs when they crawled on the figs.5) The males emerged from the syconium and the others remaining in figs were in the same developmental stage, but the differences of OBPs expressions between them were that the levels were high when males emerged from figs. This might be related to that the male wasps in C. solmsi may have primary and simple social divisions and this hypothesis need further experimental verifications.The relative transcript levels of OBPs and CSPs in different parts showed that:1) Expression comparisons in the parts of females emerged from the figs indicated that OBPs genes were expressed in the female head, thorax and abdomen, and most OBPs genes had the highest expression levels in the female head, especially CsolOBP4and CsolOBP5.2) Expression comparisons in the parts of females emerged from the figs indicated that CSPs genes expressed in the female head, thorax and abdomen, and CSPs genes had the higher expression levels in the female head and thorax. The results suggested the CsolCSPs were expressed in the sensilla of females head and thorax.3) Expression comparisons in the parts of males emerged from the figs indicated that OBPs genes were expressed in the male head, thorax and abdomen, and all OBPs genes had the highest expression levels in the male head, especially CsolOBPl, CsolOBP2and CsolOBP5.4) Expression comparisons in the parts of males emerged from the figs indicated that CSPs genes expressed in the male head, thorax and abdomen, and CsolCSP1and CsolCSP4had the higher expression levels in the male head and thorax. |
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