# Hedgehog Signaling Coordinates Stochastic and Stereotyped Patterns in the Drosophila Eye

> **NIH NIH F32** · JOHNS HOPKINS UNIVERSITY · 2022 · $67,174

## Abstract

Project Summary
 Development of an organism requires both stereotyped and stochastic patterning. Stereotyped patterning
robustly generates nearly identical structures across individuals. In contrast, stochastic cell fate specification
produces randomized patterns that are unique to each individual. Stochastic fate decisions are required for the
development of many sensory organs, including visual and olfactory systems. Despite their importance, how the
molecular mechanisms controlling stereotyped and random patterns intersect within the same tissue has not
been addressed. This project aims to determine how gene regulatory mechanisms are tuned to generate highly
regular patterns and stochastic patterns in the same tissue using the Drosophila eye as a model.
 The Drosophila eye is composed of ~800 ommatidia in a near perfect array. Each ommatidium comprises
eight photoreceptors (R1-8) which develop in a predictable fashion. As photoreceptors are differentiating during
larval eye development, a wave of morphogenesis driven by Hedgehog (Hh) signaling drives the highly
reproducible structure of the eye. Underlying the uniform morphology of the fly eye is a random pattern of
photoreceptor subtypes. Two R7 photoreceptor subtypes are defined by expression of light-detecting Rhodopsin
proteins. Random patterning of these two R7 subtypes is controlled by stochastic ON/OFF expression of the
transcription factor, Spineless (Ss). SsON R7s express Rhodopsin 4 (Rh4), whereas SsOFF R7s express
Rhodopsin 3 (Rh3). ss is regulated by an interplay of transcription and chromatin regulation during larval eye
development.
 I found that Hh signaling plays a second role in eye development to regulate stochastic patterning. hh
mutants display a reduction in the percentage of SsON R7s. Cubitus Interruptus (Ci), an effector of Hh signaling,
binds at an eye specific enhancer in ss. This site overlaps with a binding site for Klumpfuss (Klu), a repressor of
ss, suggesting competitive binding and regulation.
 I hypothesize that Hh signaling is finely tuned to drive stereotyped eye patterning and induce ss
transcription in precursors to generate stochastic R7 subtype patterning. I will test this hypothesis by 1)
Determining how the Hh pathway regulates stochastic ss expression, 2) Describing how antagonism between
Ci and Klu regulates ss expression, and 3) Determining how Hh signaling and chromatin regulation are integrated
at the ss locus. Together, these experiments will provide the first analysis of coordination between stochastic
and stereotyped gene expression within a single tissue and will enhance our mechanistic understanding of
stochastic cell fate specification.

## Key facts

- **NIH application ID:** 10538065
- **Project number:** 1F32EY034378-01
- **Recipient organization:** JOHNS HOPKINS UNIVERSITY
- **Principal Investigator:** Alison J Ordway
- **Activity code:** F32 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2022
- **Award amount:** $67,174
- **Award type:** 1
- **Project period:** 2023-01-01 → 2025-12-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10538065

## Citation

> US National Institutes of Health, RePORTER application 10538065, Hedgehog Signaling Coordinates Stochastic and Stereotyped Patterns in the Drosophila Eye (1F32EY034378-01). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/10538065. Licensed CC0.

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