Abstract
The coordinated regulation of cell fate and cell survival is crucial for normal pattern formation in developing organisms. In Drosophila compound eye development, crystalline arrays of hexagonal ommatidia are established by precise assembly of diverse cell types, including the photoreceptor cells, cone cells and interommatidial (IOM) pigment cells. The molecular basis for controlling the number of cone and IOM pigment cells during ommatidial pattern formation is not well understood. Here we present evidence that BarH1 and BarH2 homeobox genes are essential for eye patterning by inhibiting excess cone cell differentiation and promoting programmed death of IOM cells. Specifically, we show that loss of Bar from the undifferentiated retinal precursor cells leads to ectopic expression of Prospero and dPax2, two transcription factors essential for cone cell specification, resulting in excess cone cell differentiation. We also show that loss of Bar causes ectopic expression of the TGFβ homolog Decapentaplegic (Dpp) posterior to the morphogenetic furrow in the larval eye imaginal disc. The ectopic Dpp expression is not responsible for the formation of excess cone cells in Bar loss-of-function mutant eyes. Instead, it causes reduction in IOM cell death in the pupal stage by antagonizing the function of pro-apoptotic gene reaper. Taken together, this study suggests a novel regulatory mechanism in the control of developmental cell death in which the repression of Dpp by Bar in larval eye disc is essential for IOM cell death in pupal retina.
Highlights
Cell fate specification and pattern formation are major events in development
Bar is expressed in the nuclei of R1/R6 photoreceptors, undifferentiated cells posterior to the furrow in third instar larval eye disc (Fig. 1A, B) and the primary pigment cells in pupal eye (Fig. 1C)
Bar is required for achieving the proper level of IOM cell death in the pupal eye, providing a permissive condition for IOM
Summary
Cell fate specification and pattern formation are major events in development. Mutations that affect the eye morphology have been extensively utilized to identify specific gene functions in different steps of eye development such as retinal determination, axial patterning, and differentiation. Two Bar genes encoding similar homeodomain proteins, BarH1 and BarH2, exist in tandem repeat [2,3]. Both genes are expressed in the similar pattern in all tissues, and they are functionally redundant [3,4]. Bar gene functions during Drosophila eye development have been extensively studied using gain-of-function mutations, but our understanding of its loss-of-function is limited
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