Stochastic de-repression of Rhodopsins in single photoreceptors of the fly retina

Regulatory logic driving stable levels of defective proventriculus expression during terminal photoreceptor specification in flies

How differential levels of gene expression are controlled in post-mitotic neurons is poorly understood. In the Drosophila retina, expression of the transcription factor Defective Proventriculus (Dve) at distinct cell type-specific levels is required for terminal differentiation of color- and motion-detecting photoreceptors. Here, we find that the activities of two cis-regulatory enhancers are coordinated to drive dve expression in the fly eye. Three transcription factors act on these enhancers to determine cell-type specificity. Negative autoregulation by Dve maintains expression from each enhancer at distinct homeostatic levels. One enhancer acts as an inducible backup ('dark' shadow enhancer) that is normally repressed but becomes active in the absence of the other enhancer. Thus, two enhancers integrate combinatorial transcription factor input, feedback and redundancy to generate cell type-specific levels of dve expression and stable photoreceptor fate. This regulatory logic may represent a general paradigm for how precise levels of gene expression are established and maintained in post-mitotic neurons.

Lessons about terminal differentiation from the specification of color-detecting photoreceptors in the Drosophila retina

Metazoans require highly diverse collections of cell types to sense, interpret, and react to the environment. Developmental programs incorporate deterministic and stochastic strategies in different contexts or different combinations to establish this multitude of cell fates. Precise genetic dissection of the processes controlling terminal photoreceptor differentiation in the Drosophila retina has revealed complex regulatory mechanisms required to generate differences in gene expression and cell fate. In this review, I discuss how a gene regulatory network interprets stochastic and regional inputs to determine the specification of color-detecting photoreceptor subtypes in the Drosophila retina. These combinatorial gene regulatory mechanisms will likely be broadly applicable to nervous system development and cell fate specification in general.

Regional modulation of a stochastically expressed factor determines photoreceptor subtypes in the Drosophila retina

Stochastic mechanisms are sometimes utilized to diversify cell fates, especially in nervous systems. In the Drosophila retina, stochastic expression of the PAS-bHLH transcription factor Spineless (Ss) controls photoreceptor subtype choice. In one randomly distributed subset of R7 photoreceptors, Ss activates Rhodopsin4 (Rh4) and represses Rhodopsin3 (Rh3); counterparts lacking Ss express Rh3 and repress Rh4. In the dorsal third region of the retina, the Iroquois Complex transcription factors induce Rh3 in Rh4-expressing R7s. Here, we show that Ss levels are controlled in a binary on/off manner throughout the retina yet are attenuated in the dorsal third region to allow Rh3 coexpression with Rh4. Whereas the sensitivity of rh3 repression to differences in Ss levels generates stochastic and regionalized patterns, the robustness of rh4 activation ensures its stochastic expression throughout the retina. Our findings show how stochastic and regional inputs are integrated to control photoreceptor subtype specification in the Drosophila retina.

Dissection and immunohistochemistry of larval, pupal and adult Drosophila retinas

The compound eye of Drosophila melanogaster consists of about 750 ommatidia (unit eyes). Each ommatidium is composed of about 20 cells, including lens-secreting cone cells, pigment cells, a bristle cell and eight photoreceptors (PRs) R1-R8. The PRs have specialized microvillar structures, the rhabdomeres, which contain light-sensitive pigments, the Rhodopsins (Rhs). The rhabdomeres of six PRs (R1-R6) form a trapezoid and contain Rh1. The rhabdomeres of R7 and R8 are positioned in tandem in the center of the trapezoid and share the same path of light. R7 and R8 PRs stochastically express different combinations of Rhs in two main subtypes: In the 'p' subtype, Rh3 in pR7s is coupled with Rh5 in pR8s, whereas in the 'y' subtype, Rh4 in yR7s is associated with Rh6 in yR8s. Early specification of PRs and development of ommatidia begins in the larval eye-antennal imaginal disc, a monolayer of epithelial cells. A wave of differentiation sweeps across the disc and initiates the assembly of undifferentiated cells into ommatidia. The 'founder cell' R8 is specified first and recruits R1-6 and then R7. Subsequently, during pupal development, PR differentiation leads to extensive morphological changes, including rhabdomere formation, synaptogenesis and eventually rh expression. In this protocol, we describe methods for retinal dissections and immunohistochemistry at three defined periods of retina development, which can be applied to address a variety of questions concerning retinal formation and developmental pathways. Here, we use these methods to visualize the stepwise PR differentiation at the single-cell level in whole mount larval, midpupal and adult retinas (Figure 1).