What is it about?
Light is essential for most life on Earth, and single-celled or small multicellular organisms were most likely first to develop the ability to respond to light. But now, we have identified interesting behavior in sea urchin larvae that may provide insights into the evolution of light-responsive tissues/organelles in macroscopic animals. In our study, we have revealed that sea urchin larvae reverse their swimming direction when exposed to strong photoirradiation (light) because of the impact of light on the neuron pathways that typically make them swim forward.
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Why is it important?
Light-response systems usually involve a combination of photoreceptors (cells in the retina that respond to light), nervous system components, and organs that respond to nerve impulses. These organs tend to be muscles in most macroscopic animals, and cilia (hair-like structures) play a role in microscopic aquatic organisms. The cilia-based response probably developed first, before being replaced by muscle-based responses during the evolution of deuterostomes, or more complex animals. However, ciliary responses are so subtle that they are difficult to identify. We thought that sea urchins have free-living planktonic larvae that mainly move using cilia rather than muscles, so they offer a rare opportunity to investigate the presence and mechanisms of cilia-based responses in deuterostomes.
Perspectives
Given that cilia are present on tissue cells and facilitate key functions in most organisms, including humans, the identification of this ciliary response in sea urchins may be key in understanding the mechanisms of human behavior or feelings in response to light. Photic sneeze reflex might be an interesting example. Revealing these signaling pathways in sea urchins thus sheds new light on the evolution and diversification of light-response systems.
Shunsuke Yaguchi
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This page is a summary of: Planktonic sea urchin larvae change their swimming direction in response to strong photoirradiation, PLoS Genetics, February 2022, PLOS,
DOI: 10.1371/journal.pgen.1010033.
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