Gustatory organs of Drosophila melanogaster: fine structure and expressionof the putative odorant-binding protein PBPRP2

In Drosophila, as in most insects, gustation is mediated by sensory hairs l ocated on the external and internal parts of the proboscis and on the legs and wings. We describe in detail the organization and ultrastructure of the gustatory sensilla on the labellum and legs and the distribution of PBPRP2 , a putative odorant-binding protein, in the gustatory organs of Drosophila . The labellum carries two kinds of sensilla: taste bristles and taste pegs . The former have the typical morphology of gustatory sensilla and can be f urther subdivided into three morphological subtypes, each with a stereotype d distribution and innervation. Taste pegs have a unique morphology and are innervated by two receptor cells: one mechanoreceptor and the other a puta tive chemoreceptor cell. PBPRP2 is abundantly expressed in all adult gustat ory organs on labellum, legs, and wings and in the internal taste organs on the proboscis. In contrast to olfactory organs, where PBPRP2 is expressed in the epidermis, this protein is absent from the epidermis of labial palps and legs. In the taste bristles of the labellum and legs, PBPRP2 is locali zed in the crescent-shaped lumen of the sensilla, and not in the lumen wher e the dendrites of the gustatory neurons are found, making a function in st imulus transport unlikely in these sensilla. In contrast, PBPRP2 in peg sen silla is expressed in the inner sensillum-lymph cavity and is in contact wi th the dendrites. Thus, PBPRP2 could be involved as a carrier for hydrophob ic ligands, e.g., bitter tastants, in these sensilla.