Complex eye formation in the squid Doryteuthis pealeii and its evolutionary implications

Photoreception is a ubiquitous sensory ability found across the Metazoa, and photoreceptive organs are intricate and diverse in their structure. While the morphology of the compound eye in Drosophila and the single-chambered eye in vertebrates have elaborated independently, the amount of conservation within the “eye” gene regulatory network remains controversial with few taxa studied. To better understand the evolution of photoreceptive organs, we established the cephalopod, Doryteuthis pealeii, as a Lophotrochozoan model for eye development. Utilizing histological, transcriptomic and molecular assays we characterize eye formation in Doryteuthis pealeii. Through lineage tracing and gene expression analyses, we demonstrate that cells expressing Pax and Six genes incorporate into the lens, cornea and iris. Functional assays demonstrate that Notch signaling is required for photoreceptor cell formation and retina organization. This comparative approach places the canon of eye research in traditional models into perspective, highlighting complexity as a result of conserved or convergent mechanisms.