European Journal of Neuroscience, Vol. 10, pp. 2964–2974, 1998 © European Neuroscience Association Odour coding is bilaterally symmetrical in the antennal lobes of honeybees (Apis mellifera) C. Giovanni Galizia, Karl Na ¨gler, Bert Ho ¨ lldobler 1 and Randolf Menzel Institut fu ¨ r Neurobiologie, Fachbereich Biologie, Freie Universita ¨ t Berlin, D-14195 Berlin, Germany 1 Theodor Boveri Institut der Universita ¨ t, Lehrstuhl fu ¨r Verhaltensphysiologie und Soziobiologie der Universita ¨ t (Zoologie II), Am Hubland, D-97074 Wu ¨ rzburg, Germany Keywords: development, insect, olfactory glomeruli, optical recording, statistical analysis Abstract The primary olfactory neuropil, the antennal lobe (AL) in insects, is organized in glomeruli. Glomerular activity patterns are believed to represent the across-fibre pattern of the olfactory code. These patterns depend on an organized innervation from the afferent receptor cells, and interconnections of local interneurons. It is unclear how the complex organization of the AL is achieved ontogenetically. In this study, we measured the functional activity patterns elicited by stimulation with odours in the right and the left AL of the same honeybee (Apis mellifera) using optical imaging of the calcium-sensitive dye calcium green. We show here that these patterns are bilaterally symmetrical (n = 25 bees). This symmetry holds true for all odours tested, irrespective of their role as pheromones or as environmental odours, or whether they were pure substances or complex blends (n = 13 odours). Therefore, we exclude that activity dependent mechanisms local to one AL determine the functional glomerular activity. This identity is genetically predetermined. Alternatively, if activity dependent processes are involved, bilateral connections would have to shape symmetry, or, temporal constraints could lead to identical patterns on both sides due to their common history of odour exposure. Introduction Information processing in the nervous system relies on a very complex interaction between different neuron types. The connections that underlie the computational architecture of the brain are partly genetic- ally programmed, partly established in development via processes such as activity dependent plasticity, and partly remain plastic during the entire life span. Also activity dependent wiring can lead to highly stereotyped functional maps. For example, in the mammalian lateral geniculate nucleus (LGN), afferents from both eyes sort into separate layers. This sorting is accomplished by local synaptic interactions in the LGN, and is driven by spontaneous activity in the ganglion cells of the retinae (Wong et al., 1993; Wong & Oakley, 1997). Nevertheless, the resultant layering is highly predictable, and bilaterally symmetrical. Further up the visual pathway, in mammals with binocular vision, information from the left and right eye is organized in ocular dominance columns, the layout of which is arranged in alternating stripes. The spacing of these stripes is uniform, while the exact pattern which they form is variable (Anderson et al., 1988). In fact, modelling approaches rely on local, randomly distributed inhomogen- eities to trigger the segregation into left-innervated and right-innerv- ated areas for each particular map (Miller et al., 1989; Miller, 1994; Obermayer et al., 1995). In other words, local, activity dependent synaptic interactions lead to functional maps with predetermined overall properties, but local variabilities. Therefore, these maps are not bilaterally symmetrical. Nevertheless, in reversed occlusion experiments in cats an intrinsic cortical component of the columnar layout was found (Go ¨decke & Bonhoeffer, 1996). The mechanisms involved in the interaction between genetically Received 9 February 1998, revised 20 April 1998, accepted 21 April 1998 determined and/or activity modulated connections in sensory systems can be well studied in the olfactory system and particularly precisely in that of the bee brain. Olfactory receptor neurons project to the olfactory antennal lobe (AL) in insects or the olfactory bulb in mammals. These structures are organized in glomeruli, which are believed to be the functional units in the first stage of central olfactory processing. The glomerular activity patterns resulting from these projections have been shown to be odour specific (Joerges et al., 1997; Friedrich & Korsching, 1997). With the exception of locusts (Hansson et al., 1996), single receptor neurons of adult animals always branch in a single glomerulus (Boeckh & Tolbert, 1993). Also, the projection patterns of receptor neurons all expressing the same putative receptor protein have been shown to converge on a particular glomerulus in mammals (Ressler et al., 1994; Vassar et al., 1994). Consequently, it is the innervation pattern of receptor neurons that creates a spatial map of the odour world accessible to the individual animal (Mombaerts et al., 1996). However, it is still unclear how this map is formed in development. Two extremes are possible: each glomerulus could be predetermined for one (or more) particular (i.e. genetically predetermined) receptor cell family, or an activity dependent sorting mechanism could account for the specific innerva- tion pattern. An example of a genetically predetermined glomerular map is seen in the macroglomerular complex devoted to sexual pheromone processing in male lepidoptera (Hansson et al., 1992; Hildebrand, 1996). The separation between the main olfactory bulb and the vomeronasal organ in mammals is also genetically determined (Dulac