Only one out of five mushroom body lobes is sufficient for correct olfactory learning and memory. A. Pascual , T. Préat. Institut de Neurobiologie Alfred Fessard. CNRS, Gif-sur-Yvette, FRANCE.

   
   Mushroom bodies (MBs) are the main centre for olfactory learning and memory in insects. In Drosophila, each MB is made up of 2500 intrinsic neurons (Kenyons cells), wich receive olfactory inputs preferentially at the calyx. These neurons send axon projections to the anterior region of the brain, where they bifurcate to form five lobes: two dorsal lobes, a and a', and three medial lobes, b, b' and g. The a and b lobes originate from a division of the same neurons, as do a' and b' lobes. Mushroom body-less flies show normal sensory behaviour but are deficient in olfactory learning (de Belle and Heisenberg, 1994). Recently, the olfactory learning defect of the rutabaga mutant was rescued by expressing the rut gene in the MB using GAL4 enhancer trap lines (Zars et al., 2000). Here we take advantage of a MB anatomical mutant to characterize the smallest set of MB structures necessary for olfactory learning and memory. alpha lobes absent (ala) was described as a mutant lacking a lobes (Boquet et al., 2000). More detailed analysis revealed that in fact homozygous ala flies completely lack a and a' or b and b' lobes in a random pattern. In a mutant contex, the a/b and a'/b' neurons project normally in the peduncle, but they do not branch at the division point to form the dorsal (a and a') and medial lobes (b and b'). Instead, all fibres choose the same medial or dorsal direction. In contrast, g lobes appear normal. We have analyzed the olfactory associated memory of this mutant. Surprisingly ala flies displayed normal learning and memory, even in the absence of a and a' lobes or b and b' lobes on both sides of the brain. This result demonstrates that MB g lobes are sufficient for a correct olfactory learning and memory.