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Abstract

The sensation of mechanical stimuli is a central function of all animal nervous systems. Mechanosensory systems are in charge of this function, using for that a set of specialized molecules and cells that transmit the signal to downstream circuits that initiate and guide a wide variety of behaviors, ranging from navigation, to social interactions. Despite the intensive study of mechanosensory systems in the main genetics models, a clear unified picture of these sensory systems is still lacking. Exploring mechanosensory systems in animals spread across the phylogeny may help reveal such common principles. To contribute towards this aim, the mechanosensory systems of the planktonic larva of the marine annelid Platynereis dumerilii are analysed in this work using genetics, circuit and behavioral approaches. At the behavioral level, a startle response elicited by mechanical stimuli is described in Platynereis larvae using high-speed recordings. This startle response is a fast and well-coordinated behavior involving the control of both the muscular and ciliary locomotor systems of the larva. The startle response is shown to be modulated according to the intensity and site of stimulation. Such responses have been observed in other planktonic organisms, but the mechanosensory cells responsible for initiating the response are not known. A group of penetrating uniciliated neurons in the Platynereis larva are shown by calcium imaging to respond to the mechanical stimuli eliciting the startle response. Their morphology is quite similar to putative mechanosensory cells found in other animals, thus suggesting a deep evolutionary conservation. It is not entirely understood what molecular and cellular mechanisms are required for transforming mechanical cues to cellular signals. Here it is shown that Platynereis has homologs to the main molecules that have been implicated in mechanotransduction. The ciliated hydrodynamic receptors identified in this study express PKD1-1 and PKD2-1, two members of the polycystin family that have been implicated in mechanotransduction in other animals. The CRISPR system is used to generate frame-shift mutations in these genes. The mutants no longer display the startle response upon mechanical stimulation, thus suggesting that PKD2-1 and PKD1-1 are essential for the transmission of the mechanical information to downstream circuitries. Startle behaviors generally have a role in avoiding, escaping or deterring predators. It is however not clear what specific adaptations are most useful to increase survival. Here, I used the mutants defective in the startle response to assess the survival value of this behavior. Competition experiments using a rheotactic planktonic predator showed that the mutants are predated more than their wildtype counterparts. These results show that seemingly simple behavioral adaptions can have a high adaptive value. Due to their relatively simplicity, startle responses such as the one described for Platynereis have been dissected at the circuit level. Here, the startle circuit of Platynereis larvae is reconstructed at the synapse level using serial transmission electron microscopy. The resulting circuit shows direct and indirect pathways that explain how ciliary bands and muscles are controlled in a coordinated and synchronous manner. A novel group of interneurons and motoneurons is described that provides candidates for further functional exploration of this circuit.