Kafetzis, G. | Search bioRxiv

bioRxiv2025

A median eye origin of the vertebrate retina explains its unique circuitry

The vertebrate retina is a uniquely complex and evolutionarily conserved structure among bilaterians, combining ciliary (rods and cones) and rhabdomeric (ganglion, amacrine, and horizontal) photoreceptor lineages within a multilayered circuit. This arrangement contrasts with the ancestral bilaterian cephalic pattern, where rhabdomeric photoreceptors dominate lateral eyes and ciliary photoreceptors are limited to unpigmented, non-visual median positions. We propose that the vertebrate retina evolved through the lateralization of a complex median photoreceptive organ already containing both photoreceptor types. This shift likely followed the loss of lateral rhabdomeric eyes in a burrowing, suspension-feeding deuterostome ancestor and the retention of a median eye. In the early chordates leading to vertebrates, this structure diversified into the pineal/parapineal complex and lateral retinas. Central to this transformation was the emergence of a bipolar cellular identity, linking ciliary and rhabdomeric circuits--an unusual feature in animal nervous systems. We suggest bipolar cells have dual evolutionary origins: Off bipolar cells from a ciliary effector lineage and rod- On bipolar cells from a chimeric sensory cell. This model explains key similarities between retina and pineal and supports a scenario in which vertebrate vision emerged by integrating and repurposing preexisting circuits. It reframes the retina not as a de novo innovation, but as a modified and lateralized, solution to sensory challenges faced by early chordates.

neuroscience↗

bioRxiv2022

Birds multiplex spectral and temporal visual information via retinal On- and Off-channels

Early retinal circuits divide incoming visual information into functionally opposite elementary signals: On and Off, transient and sustained, chromatic and achromatic. Together these signals can yield an efficient representation of the scene for transmission to the brain via the optic nerve. For example, primate On- and Off-parasol circuits are transient, while On- and Off-midget circuits are sustained. But this long-standing interpretation of retinal function is based on mammals, and it is unclear whether this functional arrangement is common to all vertebrates. Here we show that poultry chicks use a fundamentally different strategy to communicate information from the eye to the brain. Rather than using functionally opposite pairs of retinal output channels, chicks encode the polarity, timing, and spectral composition of visual stimuli in a highly correlated manner: fast achromatic information is encoded by Off-circuits, and slow chromatic information overwhelmingly by On-circuits. Moreover, most retinal output channels combine On- and Off-circuits to simultaneously encode, or multiplex, both achromatic and chromatic information. Our results from birds conform to evidence from fish, amphibians, and reptiles which retain the full ancestral complement of four spectral types of cone photoreceptors. By contrast, mammals lost two of these cones early in their evolution, and we posit that this loss drove a radical simplification and reorganisation of retinal circuits, while birds and many other extant non-mammalian lineages retain the ancestral strategy for retinal image processing. HIGHLIGHTSO_LIFirst large-scale survey of visual functions in an avian retina C_LIO_LIOff-circuits are fast and achromatic, On-circuits are slow and chromatic C_LIO_LIMost avian RGCs are OnOff and encode both types of information C_LIO_LIColour and greyscale information can be decoded based on the kinetics C_LI

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