| Summary: | In order to produce a functional nervous system, it is essential that neurons project synaptic terminals into particular regions of the developing nervous system, so as to make connections with appropriate pre- and postsynaptic partners. While axon targeting has been studied extensively, much less is known about how the postsynaptic dendrites grow and branch. To study dendrite morphogenesis, a mosaic loss of function screen was developed and carried out in Drosophila melanogaster for chromosomal regions required for motor neuron dendrite development. Specifically, the Mosaic Analysis with a Repressible Cell Marker (MARCM) method was modified so that individual motor neurons can be made homozygous for a defined genomic aberration in an otherwise wild-type (heterozygous) background and visualised during early larval stages. In addition, 85 defined chromosomal deficiencies were recombined individually onto FRT-carrying chromosomes to be screened for genes involved in dendrite morphogenesis. These recombinant FRT-deficiency chromosomes provided a coverage of ~64.3% of chromosome 2 (3864 annotated genes). After analysis of ~35% of chromosome 2 (2092 annotated genes), 814 central nervous systems and 414 neurons, five genomic regions were identified that had a dendritic phenotype when absent. One of these regions, 2R:23D2;23E, uncovered by FRT40A Df(2L)S2590, was studied in more detail. Overlapping deficiencies were screened to define more precisely the region of chromosome 2 where the loss of gene(s) caused a dendritic phenotype in motor neurons. Of the seven candidate genes identified in this region, only one was shown to have expression in the embryonic central nervous system – the as yet uncharacterised gene CG34393. Based on sequence comparisons, CG34393 is predicted to encode a putative Ras guanyl exchange factor. Expression data from putative homologues in other species and Drosophila genes that are expressed in a similar developmental time course suggest that CG34393 may be involved in synapse development.
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