Abstract
Odorant binding proteins (OBPs) play critical roles in chemical communication of insects, as they recognize and transport environmental chemical signals to receptors. The diving beetle Cybister japonicus Sharp shows a remarkable sexual dimorphism. The foreleg tarsi of males are equipped with large suction cups, believed to help holding the female during underwater courtship and mating. Here, we identified two OBPs highly and specifically expressed in male tarsi, suggesting important functions of these structures in chemical communication. The first protein, CjapOBP1, exhibits the 6 conserved cysteines motif of classic OBPs, while the second, CjapOBP2, contains only four cysteines and can be assigned to the sub-class of C-minus OBPs. Both proteins were expressed in a bacterial system and the purified recombinant proteins were used to for antibodies preparation. Western Blot analysis showed that CjapOBP1 is predominantly expressed in male tarsi and could be also detected in antennae and palpi of both sexes, while CjapOBP2, besides male tarsi, is also present in testis. Ligand-binding experiments showed a good binding affinity between CjapOBP1, CjapOBP2 and citral and coniferyl aldehyde, respectively. These results support a possible function of these two OBPs in the male foreleg tarsi of diving beetles in chemical communication.
Highlights
Chemoreception in insects is mediated by membrane-bound receptors[1,2,3,4,5] and soluble proteins, named as odorant-binding proteins[6,7,8] and chemosensory proteins[9,10,11,12]
The peculiar structure of male tarsi in water beetles has been reported[52], we first undertook a morphological investigation to describe in detail such organ in the diving beetle C. japonicus Sharp using scanning electron microscopy (SEM)
The prolegs of males are equipped with large suction cups, which are absent in females (Fig. 1A,B)
Summary
Chemoreception in insects is mediated by membrane-bound receptors[1,2,3,4,5] and soluble proteins, named as odorant-binding proteins[6,7,8] and chemosensory proteins[9,10,11,12]. Functional studies with receptors expressed in heterologous systems have reported that the presence of the appropriate OBPs increases the sensitivity and selectivity of the receptors to pheromones[30,31,32,33] Their specific function in sensory organs of insects and in particular the interplay between OBPs and olfactory receptors remain largely unknown. OBPs are abundantly expressed in secretory glands and cells that often produce species specific pheromones In such organs, OBPs are believed to help release of such semiochemicals into the environment. The three basal segments of the pro-tarsi in males are usually equipped with various combinations of small and large suction cups These cups are suggested to play roles in climbing, swimming and catching preys, as well as reproduction[50,51,52]. Males use specialized proleg tarsi to adhere the slippery females elytra[50]
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