Homothorax controls a binary Rhodopsin switch in Drosophila ocelli

Area of Science:

• Neuroscience
• Evolutionary Biology
• Genetics

Background:

• Insect visual systems rely on photoreceptor neurons expressing opsins for light detection.
• Compound eyes and ocelli evolved from an ancestral organ, with opsin genes duplicating to confer distinct spectral sensitivities.
• Differential expression of Rhodopsin 1 (Rh1) and Rhodopsin 2 (Rh2) in fruit fly ocelli and compound eyes remains mechanistically unexplained.

Purpose of the Study:

• To investigate the molecular mechanisms controlling differential opsin expression in insect ocelli and compound eyes.
• To determine the role of Homothorax (Hth) in regulating Rhodopsin expression in Drosophila melanogaster visual organs.

Main Methods:

• Genetic analysis of Rh1 and Rh2 gene promoters.
• Molecular analysis of Homothorax (Hth) expression and function.
• Ectopic expression experiments of Hth in the fruit fly retina.

Main Results:

• Homothorax (Hth) is expressed in ocelli and regulates rhodopsin expression.
• Hth acts as a binary switch, promoting Rh2 and repressing Rh1 expression in ocelli.
• Ectopic Hth expression in the retina induces Rh2 expression in outer photoreceptors in a cell-autonomous manner.

Conclusions:

• Homothorax (Hth) plays a critical role in diversifying rhodopsin expression between ocelli and compound eyes.
• The evolution of distinct rhodopsins in ocelli and retinal photoreceptors is likely controlled by Hth acting on ancestral gene duplicates.