The wide diversity of eye designs present in arthropods makes them a unique group to study eye evolution. However, most the knowledge on the development and neural architecture of the visual system comes from few model organisms. My project aimed to contribute to the study of the diversity and evolution of the arthropod visual system by studying the eye of the crustacean Parhyale hawaiensis, focusing on its development, neuroarchitecture and function.
In this work, I demonstrate that Parhyale has an apposition compound eye, with each ommatidia being composed of 5 photoreceptors (R1-R5), in a 4+1 arrangement. It seems probable that the R1-R4 and R5 photoreceptors (PR) of Parhyale are related to the outer (R1-R6) and inner (R7 and R8) PR of insects, respectively. EM data showed that the rhabdoms of the R1-R4 PR cells are well aligned and do not rotate. This feature renders the PR intrinsically sensitive to specific directions of light polarization.
Two opsins, expressed exclusively in the retina, were found in Parhyale: Ph-Opsin1 (closely related to the long wavelength opsin of insects and crustaceans) and Ph-Opsin2 (related to the mid/short-wavelength opsins). Generation of transgenic lines using the upstream regulatory regions for each opsin showed that R1-R4 express Ph-Opsin1 while R5 expresses Ph-Opsin2. PR cells send long projections from the retina to the optic lobe which, unlike other arthropods, seems to be located away from the retina and closer to the central body. Live imaging of transgenic lines showed that both subsets of PR send projections to the lamina.
This work is pioneering the study of the visual system in Parhyale. It is the first time that genetic tools have been introduced to study the crustacean visual system. It makes Parhyale a powerful experimental system for in vivo studies of compound eye development and axonal targeting, a field currently dominated by studies in a single species of fruitfly.