Light-modulated stem cells in the camera-type eye of an annelid model for adult brain plasticity.

Camera-type eyes in vertebrates and cephalopods are striking examples of parallel evolution of a complex structure. While comparisons have focused on these two groups, camera-type eyes with likely high functionality are also found in other invertebrate phyla with simpler brains. Employing single-cell RNA sequencing, we identify neurogenic cells in the adult eyes and brain of the marine annelid worm Platynereis dumerilii. Distinct neural stem cells in the camera-type adult eyes, located at the edge of the cup-shaped retina, and adjacent to the glass body/lens, produce radial lines of cells, reminiscent of stem cells in ciliary marginal zones of vertebrate eyes exhibiting life-long growth. Normal proliferation in the eye depends on ambient light, a phenomenon that depends on the integrity of the photoreceptor gene c-opsin1, which is present in emerging rhabdomeric photoreceptors, and impacts on their differentiation. During reproductive maturation, proliferation in the eye as well as the entire brain sharply declines, while cells upregulate molecular characteristics of mammalian adult neural stem cell quiescence. Our data provide insights into the development and modulation of annelid head and brain cells, revealing similarities and differences to vertebrate eye development, neurogenesis and brain plasticity.