Organisms and organs come in sizes and shapes. With size, science has no problems, but how to quantify shape? How similar are two birds or two brains? This problem is particularly pressing in cases like brains where structure reflects function. The problem is not new, but satisfying solutions have yet to be worked out. For brain anatomy, no general methodology for a statistically secured quantitative description is available. Using the small brain of the fly Drosophila melanogaster, we have explored a new approach combining immunohistochemistry, high-resolution 3D confocal microscopy, and advanced graphics computing. For a genetic model organism such as Drosophila, a quantitative assessment of brain structure is particularly rewarding, since it allows for the identification of genetic variants with subtle brain structure phenotypes and, even more importantly, the organization of the wealth of gene expression patterns in the brain into a genetic atlas linking molecular and organismic gene function. We now provide a representative standard for the brain of D. melanogaster wild-type with means and variances for several aspects of its shape. Its application to volumetry, mutants, and gene expression patterns is demonstrated.
A freely walking single fly (Drosophila melanogaster) can be conditioned to avoid one side of a small test chamber if the chamber is heated whenever the fly enters this side. In a subsequent memory test without heat it keeps avoiding the heat-associated side. The memory mutants dunce and rutabaga successfully avoid the heated side but show no avoidance in the memory test. Wildtype flies can be trained to successively avoid alternating sides in a reversal conditioning experiment. Every single fly shows strong avoidance and a positive memory score. The new conditioning apparatus has several advantages: (1) Statistically significant learning scores can be obtained for individual flies. (2) Learning scores are obtained fully automatically without interference of the experimenter. (3) The procedure is fast, robust and requires little handling. Therefore the apparatus is suitable for largescale mutant screening. (4) Animals are not attached to a hook and thus can easily be used for breeding.
Brains are organized by the developmental processes generating them. The embryonic neurogenic phase of Drosophila melanogaster has been studied in detail at the genetic, cellular and molecular level. In contrast, much of what is known of postembryonic brain development has been gathered by neuroanatomical and gene expression studies. The molecular mechanisms underlying cellular diversity and structural organisation in the adult brain, such as the establishment of the correct neuroblast number, the spatial and temporal control of neuroblast proliferation, cell fate determination, and the generation of the precise pattern of neuronal connectivity, are largely unknown. In a screen for viable mutations affecting adult central brain structures, we isolated the mushroom bodies tiny (mbt) gene of Drosophila, which encodes a protein related to p21-activated kinase (PAK). We show that mutations in mbt primarily interfere with the generation or survival of the intrinsic cells (Kenyon cells) of the mushroom body, a paired neuropil structure in the adult brain involved in learning and memory.
A big step in the neurobiology of Drosophila would be to establish a standard for brain anatomy to which to relate morphological, developmental and genetic data. We propose that only an average brain and its variance would be a biologically meaningful reference and have developed an averaging procedure. Here, we present a brief outline of this method and apply it to the optic lobes of Drosophila melanogaster wild-type Canton S. Whole adult brains are stained with a fluorescent neuropil marker and scanned with the confocal microscope. The resulting three-dimensional data sets are automatically aligned into a common coordinate system and intensity averages calculated. We use effect-size maps for the fast detection of differences between averages. For morphometric analysis, neuropil structures are labelled and superimposed to give a three-dimensional probabilistic map. In the present study, the method was applied to 66 optic lobes. We found their size, shape and position to be highly conserved between animals. Similarity was even higher between left and right optic lobes of the same animal. Sex differences were more pronounced. Female optic lobes were 6% larger than those of males. This value corresponds well with the higher number of ommatidia in females. As females have their additional ommatidia dorsally and ventrally, the additional neuropil in the medulla, lobula and lobula plate, accordingly, was found preferentially at these locations. For males, additional neuropil was found only at the posterior margin of the lobula. This finding supports the notion of male-specific neural processing in the lobula as described for muscid and calliphorid flies.
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