Circuit mechanisms underlying chromatic encoding in Drosophila photoreceptors

Spectral information is commonly processed in the brain through generation of antagonistic responses to different wavelengths. In many species, these color opponent signals arise as early as photoreceptor terminals. Here, we measure the spectral tuning of photoreceptors in Drosophila. In addition to a previously described pathway comparing wavelengths at each point in space, we find a horizontal-cell-mediated pathway similar to that found in mammals. This pathway enables additional spectral comparisons through lateral inhibition, expanding the range of chromatic encoding in the fly. Together, these two pathways enable optimal decorrelation of photoreceptor signals. A biologically constrained model accounts for our findings and predicts a spatio-chromatic receptive field for fly photoreceptor outputs, with a color opponent center and broadband surround. This dual mechanism combines motifs of both an insect-specific visual circuit and an evolutionarily convergent circuit architecture, endowing flies with the unique ability to extract chromatic information at distinct spatial resolutions.

Rap1, AF6 and Pak4 cooperate with Par3 to promote ZA assembly and remodeling in the fly photoreceptor

The epithelial Zonula adherens (ZA) is a main adhesion compartment that enables organogenesis by allowing epithelial cells to assemble into sheets. How ZA assembly is regulated during epithelial cell morphogenesis is not fully understood. We show that during ZA morphogenesis, the function of the small GTPase Rap1 and the F-actin binding protein AF6/Cno are both linked to that of the P21-activated kinase Pak4/Mbt. We find that Rap1 and Mbt regulate each other’s localization at the ZA and cooperate in promoting ECadherin stabilization. During this process Cno regulates the recruitment of Baz at the ZA, a process that is also regulated by Arm phosphorylation by Mbt. Altogether, we propose that Rap1, Cno and Mbt regulate ZA morphogenesis by coordinating ECadherin stabilization and Baz recruitment and retention. In addition, our work uncovers a new link between two main oncogenes, Rap1 and Pak4/Mbt, in a model developing epithelial cell.