Gene Cloning, Evolution And Specific Expression Pattern Of The Non-conventional Olfactory Receptors In Four Fig Wasp Species Associated With Ficus Hispida

The interaction between fig pollinators(Hymenopter, Chalcidoidea, Agaonidae) and their host figs(Ficus spp., Moraceae) provides one of the best cases for coevolution researches. Fig species are usually exclusively pollinated by a unique species of pollinating fig wasp (counterexamples increasing). In addition to pollinators, there are also several to tens of non-pollinating fig wasp species inhabiting within a fig.Chemical volatiles produced by receptive figs. These volatiles are thought to be the means by which the wasps detect a suitable host and fig wasps should be able to distinguish the particular odors emitted by hosts from other volatile compounds. Different pollinating species appear to have unique volatile profiles which could account for host-specificity. Host-specificity is still expected in non-pollinators. Chemical constraints could reduce the likelihood of host switches. It is thus clear that olfaction plays a critical role in this mutualism.The molecular units constituting the basis for the detection of animal- or plant-derived odors are the olfactory receptors (ORs), which are expressed in olfactory sensory neurons (OSNs). ORs belong to members of the G protein coupled receptor (GPCR) superfamily, cell surface proteins with a conserved structure of seven transmembrane domains. Furthermore, while each mammalian OSN expresses a single OR that determines its functional specificity, insect OSNs express a conventional ligand-binding OR together with Or83b orthologous gene, a highly conserved non-conventional member of the insect OR family. Recent studies suggested that Or83b don't directly respond to odorants but act as a chaperone receptor to form heterodimers with other ORs and ensure their appropriate dendritic localization and function.Here we have, for the first time, obtained 4 full lengh coding region of Or83b orthologous genes encoding non-conventional olfactory receptors from one pollinator(Ceratosolen solmsi marchali) and three non-pollinators (Apocrypta bakeri, Philotrypesis pilosa and Philotrypesis sp.) fig wasp species associated with Ficus hispida, respectively named CsmOr2, AbOr2, PpOr2 and PsOr2. The higher rate of synonymous substitution (Ks) and lower codon usage of this olfactory receptor in pollinator were detected in comparison with non-pollinators. Synonymous mutations of this olfactory receptor in pollinator may be subject to week selection at the translation level relative to non-pollinators, though the host-specificity of pollinator is more obligate than non-pollinators'. RT-PCR experiments demonstrated that this type receptors are expressed in antennae, abdomens and legs of adult female wasps but only expressed in head of adult male wasps. It may be necessary for adult female wasps in host searching and ovipositing process. In the comfortable environment of fig, adult male wasps don't need to have sensitive sense of smell. In addition, the lower expression level was exclusively detected in the thorax of adult female of pollinator, indicating it may play a role in pollinator ovipositing in the dark fig cavity. These specific expression patterns of Or83b orthologous genes may imply the adaptive evolution of fig wasps with special ecological niche of fig...