Structural differences in the drone olfactory system of two phylogenetically distant Apis species, A-florea and A-mellifera

Male insects that are attracted by sex pheromones to find their female mate s over long distances have specialized olfactory subsystems. Morphologicall y, these subsystems an characterized by a large number of receptor neurons sensitive to components of the female's pheromones and hypertrophied glomer ular subunits ('macroglomeruli' or 'macroglomerular complexes') in the ante nnal lobes, in which the axons of the receptor neurons converge. The olfact ory subsystems are adapted for an increased sensitivity to perceive minute amounts of pheromones. In Apis mellifera. drones have 18,600 olfactory pore plate sensilla per antenna, each equipped with receptor neurons sensitive t o the queen's se?; pheromone, and four voluminous macroglomeruli (MG1-MG4) in the antennal lobes. In contrast, we show that drones of the phylogenetic ally distant species, Apis florea, have only 1,200 poreplate sensilla per a ntenna and only tno macroglomeruli in their antennal lobes. These macroglom eruli are homologous in anatomical position to the two most prominent macro glomeruli in A. mellifera, the MG1 and MG2, but they are much smaller in si ze. The morphological and anatomical differences described here suggest maj or modifications in the ser;pheromone processing subsystem of both species: (1) less pheromone sensitivity in A. florea and (2) a more complex sex-phe romone processing and thus a more complex sex-pheromone communication in A. mellifera. Research in honey bee sex-pheromone communication dates back to the 1960s, when Gary (1962) demonstrated that in Apis mellifera the queen' s mandibular gland secretion and especially its main component, 9-ODA (9-ke to-2(E)-decenoic acid), is highly attractive to drones on their nuptial fli ght. Later, cross-species attraction experiments showed that other honey be e species, Apis florea? A. cerana, and A. dorsata probably also use the que en's mandibular gland secretion as a mating attractant (Butler ct al. 1967: Sanasi et nl. 1971). Besides its function in mating behavior, the queen's mandibular gland secretion is the main pheromone regulating queen-worker in teractions (Free 1987). In this context, several studies have demonstrated the behavioral significance of single components (Slessor et al. 1988) and differences in the composition of the secretion between Apis species (Plett ner ct al. 1996, 1997; Keeling et al. 2000). Regarding the interspecific di fferences in the queen's signal, the question arises whether this variation is reflected in the olfactory system of drones and workers of the various species.