# Division of Molecular Genetics, Glasgow University, Glasgow G11 5JS, Scotland, % Max-Planck-Institut für Biologische Kybernetik, D-72076 Tübingen, Germany,
Drosophila mushroom bodies (MBs) are a paired neuropil in the protocerebrum, each consisting of ~ 2500 parallel Kenyon cell (KC) fibres derived from four neuroblasts (MBNbs)1. A prevailing concept of the MB is that of functionally isomorphic intrinsic KC arrays. Recently, P[GAL4] enhancer-trap technology2 has been used to visualise KCs in flies, revealing novel internal MB architecture3. The immunohistochemical technique employed allows visualisation of even small numbers of KC fibres at high resolution. In our selection of P[GAL4] lines, many expression patterns identify a four-fold symmetrical arrangement of MB internal substructure. This may reflect the pattern of MB development, with each element being derived from a single MBNb. Here we examine this hypothesis by combining the P[GAL4] enhancer-trap system with chemical (HU) ablation4. (see abstracts by O’Dell et al. de Belle et al.).
MB ablation by feeding HU to newly hatched larvae is not 100% effective4. MBNbs give rise to ~ 40-300 KCs during embryonic development1,5. Moreover, at the level of the light microscope we can recognise reduced MBs in a small proportion of flies (giving a mean of 0.7% of the normal calyx volume)4. These visible KC subsets are likely MBNb progeny which survive HU treatment. In this report we employ b-galactosidase expression patterns in a range of HU-treated P[GAL4] lines to identify and describe the embryonic KC fibres in the g lobe born prior to HU-treatment. Furthermore, we show that progeny KCs derived from each MBNb contribute to single symmetrical elements in the calyx and peduncle rather than to each element equally. Although most of the symmetrical divisions in the calyx and peduncle were continuous with patterns in the a and b lobes, partial ablations in g lobe-staining lines showed that a substantial proportion of the g lobe is formed post-embryonically from each MBNb. MB output neurons from the g-lobe were identified in some lines. These appeared to be unaffected by HU treatment and lack of MBs.
One lateral neuroblast (LNb) in each hemisphere of the brain has a temporal pattern of proliferation which is similar to that of the four MBNbs1. LNb proximity to the antennal lobe (AL) and a 32% AL volume reduction in HU-treated flies suggested that LNbs likely contribute to intrinsic elements of this olfactory neuropil4. A closer look into some HU-treated AL staining P[GAL4] lines revealed a complete absence of AL local interneurons. The antennal glomerular tract (AGT), which provides AL input to the MB calyces, also appeared quite normal in our study. However, different AGT fibre composition and projection patterns have been observed by another group6 (see abstract by Heimbeck et al.).