A multi-lineage based analysis of neural morphogenesis in Drosophila melanogaster

The Drosophila nervous system offers a unique opportunity to study the molecular mechanisms that control brain development. A stereotyped pattern of cell divisions combined with a manageable number of cell types provides a tractable model system. In this study, we analyze brain development at the level of the neural lineage, an anatomical unit unique to the insect nervous system. The current literature in Drosophila neurobiology restricts analysis to one or two popular lineages of the Drosophila brain. Here, we take a multi-lineage approach to understand how glia-neuron interaction and polarity protein localization establish secondary lineage morphology.;In the first part of this study (Chapter 3) we visualize cortex and neuropile glia in the third instar brain using the glia-specific driver nirvana2 -GAL4 while simultaneously labeling all secondary axon tracts (SATs). We then correlate the level of glia-SAT association with phenotypes observed upon glial elimination. We find that the level of glia-SAT interaction positively correlates with SAT pathway guidance, growth, and fasciculation.;In the second study (Chapter 4), the function of polarity protein Bazooka is examined in four distinct test lineages using lineage-specific drivers and loss-of-function clones. We find that Bazooka is globally expressed within SAT axons. Ectopic Bazooka:GFP protein accumulates in a lineage specific manner, including accumulation in the growth cone as well as continuous expression throughout the entire tract. In lineages that localize Bazooka to the growth cone, such as BAla1, Bazooka is intrinsically required for axon pathway choice without effecting axon or dendrite specification. In other lineages, bazooka loss-of-function clones result in ectopic axon extensions and excessive branching along the SAT. We observe similar phenotypes with manipulation of Rac1 and Par6, which suggests that Bazooka acts in concert with Rac1 and Par6 to control SAT growth and development.;Taken together, these studies support a multi-lineage approach when using the Drosophila brain as a model system. A negative finding in one lineage does not discount the requirement of a given cell or gene for all neurons. The comparison among lineage phenotypes after genetic manipulation is key in this work.