The Role of LOZENGE in Drosophila melanogaster Photoreceptor Axon Extension and Synaptogenesis
Bayer School of Natural and Environmental Sciences
John A. Pollock
David J. Lampe
lozenge, photoreceptor, visual system
The Drosophila melanogaster visual system is structured so that neural connections reflect not only eye structure, but also the specific points of view of individual, light-sensing photoreceptor neurons. Such a complex network is created in response to numerous extracellular cues and signal transduction pathways, the coordination of which is crucial to proper visual system formation. Eye development begins in the third instar larva, when a morphogenetic furrow traverses the eye primordial and initiates the sequential recruitment of cells within each of the 750-800 ommatidia that will form the adult eye. As each ommatidium is completed, its photoreceptor neurons mature and begin to extend their axons through the optic stalk and into the optic lobe of the brain in response to both internal and external signals. Upon arrival, additional signals guide R1-6 axons to the lamina and R7/R8 axons to the medulla; a third set of signals then regulates the synaptogenesis of photoreceptor neurons with their proper targets.
Among the pleiotropic effects of lozenge mutation is an eye phenotype in which post-precluster ommatidial recruitment and differentiation occurs abnormally, and photoreceptor axons exhibit both fasciculation and extension defects. Although previous research has established the roles played by LOZENGE during cell recruitment, it has not yet examined the role of LOZENGE as photoreceptors mature and grow in preparation for synaptogenesis.
Consequently, in an attempt to identify potential LOZENGE targets during photoreceptor axon extension and targeting, I selected 26 genes whose products are involved in these processes, and analyzed their transcript levels in both wild type and lozenge mutant backgrounds; transcripts produced independently of LOZENGE were not expected to change, while those falling under direct or indirect LOZENGE activity would most likely change in response to manipulation of LOZENGE levels. In a separate analysis, potential LOZENGE and LOZENGE/ETS binding sites were identified within every selected target gene, preventing the elimination of any target genes before PCR analysis. Primer analysis and immunohistochemistry/microscopy have confirmed primer specificity and have provided confidence that larvae used for transcript analysis were correctly chosen.
Following real-time PCR analysis of transcript levels and subsequent data analysis, several genes appeared to display altered transcript levels in an lzmr1 mutant background, including gilgamesh, Ptp69D, and Insulin Receptor and hedgehog levels also showed a degree of change, and these gene products are respectively involved in pathways involving Gilgamesh and Abelson TK. These genes provide a foundation for further experimentation.
Information compiled for Dephrin, Runt, and brakeless and for misshapen suggests their involvement in LOZENGE activity as well, although definitive transcript analyses were not obtained.
Meyers, J. (2004). The Role of LOZENGE in Drosophila melanogaster Photoreceptor Axon Extension and Synaptogenesis (Master's thesis, Duquesne University). Retrieved from https://dsc.duq.edu/etd/1681