Visual System of Marine Annelids Provides Insight Into the Evolution of Eyes



 The first neuronal map of marine annelids' visual system has been reconstructed by Max Plan C K Institute researchers, providing insight into eye evolution.




The marine bristle worm larvae, use light to orient themselves. These larvae use surface currents to spread out when they are young and swim toward the light. The older larvae escape the light by swimming to the sea floor, where they mature into adult worms. Researchers at the Max Institute for Developmental Biology in Tübingen have discovered that a variety of the eye's neuronal systems are connected to this shift in behavior in response to light. From the light stimulus' input to the behavioral response—the swimming larva's directional turning—the researchers have recreated the first neuronal map of a visual system.  The biologists are able to see how vision has developed through the use of this neuronal map. 


Marine invertebrate larvae frequently practice, or moving toward or, away from a light source. Numerous larvae undergo a transition from positive (movement toward light) to negative photo taxis during their development. The early larval stage is the only instance in which the underlying mechanism of photo taxis has been described to date. The larvae acquire additional eyes at a later stage in their development. The ability to switch between positive and negative photo taxis comes with these new eyes. According to Gáspár Jékely, the head of the research group "Neurobiology of Marine Zooplankton", "the larvae frequently display negative photo taxis and swim away from the light instead of only swimming towards the light"


The bristle worm's offspring have the simplest eyes on the planet for the first two days of their lives. On each side of the small head is a solitary photoreceptor cell along with one concealing color cell. Jékely and his colleagues from the European Molecular Biology Laboratory (E M B L) in Heidelberg made the discovery in 2008 that this photoreceptor cell is directly connected to the larval driving engine, a band of cilia in a collar below the head. Larvae move in a spiral pattern in response to light hitting the photoreceptor cell, always moving toward the stimulus. However, after three days of development, these straightforward larval eyes cease to mediate photo taxis. At this point, two pairs of more sophisticated eyes—the precursors of the adult eyes—appear on the upper side of the head. A pigment cup, a number of photoreceptor cells, and even a straightforward lens make up these brand-new eyes. In addition, a straightforward neuronal network that processes and transmits light stimuli emerges. The researchers in J E-K E-L Y group concentrated on this neuronal organization in a more subtle way, utilizing an electron-magnifying lens. They found 71 neurons connected by more than 1000 neuronal connections, or synapses, on a detailed map of the visual neuronal network of a 3-day-old larva. The researchers discovered that the larval body musculature is now receiving the light signal, as well as the cilia. In addition, the eyes on the two body sides are likewise associated at the neuronal level


A complete neuronal network of a straight forward visual system is described by a Tübingen-based group of developmental biologists. They also gained a deeper understanding of how eyes develop. The earliest evolutionary eyes were merely able to distinguish a bright field from a dark one, and they served as the foundation for more complex visual systems like the human eye 

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