Crumbs and the Apical Spectrin Cytoskeleton Regulate R8 Cell Fate in theDrosophilaeye

The Hippo pathway is an important regulator of organ growth and cell fate. In the R8 photoreceptor cells of theDrosophila melanogastereye, the Hippo pathway controls the fate choice between one of two subtypes that express either the blue light-sensitive Rhodopsin 5 (Hippo inactive) or the green light-sensitive Rhodopsin 6 (Hippo active). The degree to which the mechanism of Hippo signal transduction and the proteins that mediate it are conserved in organ growth and R8 cell fate choice is currently unclear. Here, we identify Crumbs and the apical spectrin cytoskeleton as regulators of R8 cell fate. By contrast, other proteins that influence Hippo-dependent organ growth, such as the basolateral spectrin cytoskeleton and Ajuba, are dispensable for the R8 cell fate choice. Surprisingly, Crumbs promotes the Rhodopsin 5 cell fate, which is driven by Yorkie, rather than the Rhodopsin 6 cell fate, which is driven by Warts and the Hippo pathway, which contrasts with its impact on Hippo activity in organ growth. Furthermore, neither the apical spectrin cytoskeleton nor Crumbs regulate the Hippo pathway through mechanisms that have been reported to operate in growing organs. Together, these results show that only a subset of Hippo pathway proteins regulate the R8 binary cell fate decision and that aspects of Hippo pathway signalling appear to differ between growing organs and post-mitotic R8 cells.