A new study, led by the University of Vienna and the Alfred Wegener Institute in Bremerhaven, shows how the eyes of adult marine bristleworms continue to grow throughout life – driven by a ring of neural stem cells reminiscent of vertebrate eyes. What's more, these stem cells respond to environmental light. The research, published in Nature Communications, offers new insight into the fundamental principles of eye evolution and the role of light in shaping the adult nervous system – even in organisms often considered rather simple.
When imagining creatures with complex eyes, most people think of mammals, birds, or maybe octopuses. Yet annelid worms from the sea – such as the bristleworm Platynereis dumerilii – also have eyes structured much like those of vertebrates and cephalopods, known as "camera-type" eyes, and some of them are capable of surprisingly high-resolution vision. But how do these invertebrate eyes continue to grow throughout adult life? An international research team from the University of Vienna, the Alfred Wegener Institute in Bremerhaven, and the University of Oldenburg has now taken a closer look at this question – and obtained intriguing new insights.
Different eyes, similar growth
Camera-type eyes in invertebrates and vertebrates are textbook examples of parallel evolution, thought to have arisen independently as similar solutions to the same biological challenge. To explore how such eyes continue to develop, the team examined adult eyes of Platynereis – a model system long used to study fundamental principles of photoreceptor function and brain evolution.
Using single-cell RNA sequencing, first author Nadja Milivojev from the Department of Neurosciences and Developmental Biology, University of Vienna, identified molecular markers characteristic of stem cells and mapped where these cells are located and active in the worm's retina. Her analyses revealed a distinct zone at the rim of the retina, densely packed with neural stem cells that actively divide when the adult eye is growing. "It was remarkable to find dividing cells at the edge of the worm's retina – the same place where some groups of vertebrates maintain their retinal stem cells for life-long eye growth," Milivojev says.
Indeed, this so-called "ciliary marginal zone" is thought to support continuous eye growth — a pattern now mirrored in the bristleworm retina. "In vertebrate examples of life-long growth as fishes and amphibians – such stem cells supply the eye with fresh retinal neurons while the animal develops," explains senior author Florian Raible, University of Vienna, whose lab has a long-standing track record in research on stem cell biology. "Remarkably, Nadja's work showed that bristleworm eyes can also add new photoreceptor cells and expand their size – a trait that has not been well studied outside the vertebrate lineage", Raible adds.
Light as a Regulator
Even more fascinating, the research team discovered that eye growth in adult worms is also regulated by light in their environment. Through detailed genetic and molecular analyses, they showed that this effect is mediated by a c-opsin, a light-sensitive molecule, also present in the rod and cone cells of vertebrate retinas. While previous studies had shown that worm eyes rely on a different family of opsin molecules, the new findings on the presence of a vertebrate-type c-opsin came as a major surprise. Milivojev and colleagues discovered that this light-sensitive molecule is present in early precursors of the worm's photoreceptor cells, suggesting that it acts as a molecular switch linking environmental light to stem cell activity. The discovery highlights that visual systems not only sense light, but that their development can be regulated by light as well.
Evolutionary echoes
The findings close a long-standing gap in understanding how invertebrate and vertebrate eyes grow and maintain themselves. The discovery that Platynereis eyes rely on a ring of neural stem cells brings biologists closer to understanding universal principles behind sensory organ evolution. It also raises new questions. Could other neural stem cells in the body also respond to environmental light? And how might artificial illumination disrupt such natural regulatory systems? The researchers hope that future work on the worm's stem cell machinery will help answer these questions – shedding light, quite literally, on how the nervous system adapts and regenerates. "Clearly, basic research to uncover the unexpected is essential to understand the biological complexity of life and the possible consequences of anthropgenic impacts," concludes senior author Kristin Tessmar-Raible, University of Vienna, Alfred Wegener Institute, University of Oldenburg.
Summary:
- The team, led by the University of Vienna and the Alfred-Wegener institute of marine research, examined adult marine bristle worms, which are highly relevant to unravel basic principles of eye and brain development and the non-visual effects of light.
- In the study, the scientists show how the eyes of Platynereisdumerilii continue to grow throughout their lives – driven by a ring of neural stem cells reminiscent of a similar ring in the eyes of those vertebrates that exhibit life-long growth.
- The results fill a long-standing gap in the understanding of how camera-type eyes of invertebrates and vertebrates grow and are maintained, and suggest that despite evolutionary divergence, common cellular strategies for growth and plasticity apply.
- The discovery that Platynereis eyes rely on a ring of neural stem cells brings biologists closer to understanding universal principles behind sensory organ evolution.
About the University of Vienna:
For over 650 years the University of Vienna has stood for education, research and innovation. Today, it is ranked among the top 100 and thus the top four per cent of all universities worldwide and is globally connected. With degree programmes covering over 180 disciplines, and more than 10,000 employees we are one of the largest academic institutions in Europe. Here, people from a broad spectrum of disciplines come together to carry out research at the highest level and develop solutions for current and future challenges. Its students and graduates develop reflected and sustainable solutions to complex challenges using innovative spirit and curiosity.
Original publication:
Nadja Milivojev, Federico Scaramuzza, Pedro Ozório Brum, Camila L. Velastegui Gamboa, Gabriele Andreatta, Florian Raible & Kristin Tessmar-Raible. Light-modulated stem cells in the camera-type eye of an annelid model for adult brain Plasticity. Nature Communications.
DOI: 10.1038/s41467-025-65631-0
