Control of photoreceptor axon target choice by transcriptional repression of Runt

Abstract

Drosophila photoreceptor neurons (R cells) project their axons to one of two layers in the optic lobe, the lamina or the medulla. The transcription factor Runt (Run) is normally expressed in the two inner R cells (R7 and R8) that project their axons to the medulla. Here we examine the relationship between Run and the ubiquitously expressed nuclear protein Brakeless (Bks), which has previously been shown to be important for axon termination in the lamina. We report that Bks represses Run in two of the outer R cells: R2 and R5. Expression of Run in R2 and R5 causes axonal mistargeting of all six outer R cells (R1–R6) to the inappropriate layer, without altering expression of cell-specific developmental markers.

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Figure 1: Runt is misexpressed in bks mutant eye discs causing axonal mistargeting.
Figure 2: Run expression in R2 and R5 does not alter developmental markers in the eye disc.
Figure 3: Run misexpression in R2 and R5 causes axonal mistargeting to the medulla without affecting ommatidial organization in adult animals.
Figure 4: Mistargeting of R1–R6 axons to the medulla is not due to a disruption of the lamina target region.

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Acknowledgements

We are grateful to A. Brand, R. Carthew, B. Dickson, P. Gergen, S. L. Zipursky, N. Perrimon, K. Saigo, R. Stocker, G. Rubin, K. Matthews and the Bloomington Stock Center for fly stocks and antibodies. We thank T. Clandinin, C-H. Lee, T. Herman and members of the Banerjee laboratory for thoughtful suggestions and comments. We thank L. Zipursky for helpful discussions and for supporting part of this work in his laboratory. We thank Y. Ito and his collaborators and Y. Groner and his collaborators, for communicating results prior to publication. The Rough and Dac antibodies were obtained from the University of Iowa Developmental Studies Hybridoma Bank, developed under the auspices of the National Institute of Child Health & Human Development. This work was supported by a National Institutes of Health grant (U.B.), U.S.P.H.S. National Research Service Awards (J.K. and J.C.) and the Medical Research Council (I.S.).

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Correspondence to Utpal Banerjee.

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