Expression of opsins, ultrastructure of eyes, and light-induced behaviour in tardigrades (water bears): Insights into the evolution of visual systems and photoreception in panarthropods

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The ability to perceive light is widely distributed among animals, but the evolution of this complex sensory capacity is not fully understood. One particular gap in our knowledge concerns the visual system of tardigrades (water bears). Despite their key phylogenetic position as one of the closest arthropod relatives, insufficient data are available on the ultrastructure of the photoreceptors and visual organs of tardigrades. Moreover, data on the expression of opsin genes, which have been identified recently in a tardigrade species, are completely missing, although these proteins play a key role in light reception and light-induced behaviour. The major goal of the planned project is therefore: (i) to analyse the ultrastructure of photoreceptors and to perform a 3D reconstruction of eyes in representatives of two major tardigrade subgroups, the Heterotardigrada and Eutardigrada, as these data will provide essential information on visual properties of the tardigrade eye; (ii) to immunolocalise all opsin proteins known from tardigrades in order to clarify which of these proteins are involved in vision and which ones are non-visual molecules; and (iii) to study the phototactic/photokinetic behaviour in the tardigrade Hypsibius dujardini and to examine the spectral sensitivity of this species to clarify to which wavelengths of light it responds. The new data will help us to understand the visual and photosensory system of tardigrades and to shed light on its evolution since the divergence of the three major panarthropod groups: Arthropoda, Onychophora, and Tardigrada.