Evolution of opsins and phototransduction in arthropods

Vision is a sensory system that has evolved in many animals including arthropods. There is however, a tremendous variety of visual capabilities among arthropods. Some crustaceans have eyes with many receptor cell types of different spectral sensitivities giving them excellent colour vision. Other arthropods have altogether lost eyes. The eyes of arthropods probably evolved from some kind of simple pigmented eye-cup as can be deduced by comparing the eyes of onychophorans or velvet worms, the sister group of arthropods, with the eyes of arthropods. Colour vision has likely evolved within the arthropod lineage. A prerequisite for colour vision is the presence of multiple opsins. Rhodopsins are composed of an opsin protein and a light sensitive chromophore retinal that upon absorption of light undergoes a conformational change that induce a second messenger cascade leading to phototransduction. Colour vision in spiders has been documented in behavioural tests for jumping spiders. The jumping spiders have three visual opsins that in phylogenetic analyses group with middle- and long wavelength sensitive and UV sensitive opsins of insects. We have recently shown that also the ctenid spider Cupiennius salei has three visual opsins expressed in its eyes. However, colour vision has never been documented behaviourally. If C. salei do not have colour vision we would have to seek for other explanations for the presence of multiple opsins, for instance in widening the spectral sensitivity range. In addition to having three main types of visual opsins C. salei also has multiple highly similar copies of these opsins. These copies typically differ in just a few amino acid residues. In other animals it has been shown that just a couple of amino acid differences can change the spectral sensitivity significantly. We want to take opportunity of the many copies of opsins and test their spectral sensitivities by introducing them into Drosophila melanogaster mutants that lack opsins of their own. The experiments with spider opsins expressed in D. melanogaster will show if the observed amino acid changes give rise to differing spectral sensitivities or differences in signal transduction efficiency. The phototransduction cascade is the first vital step by which the light stimuli are transferred into neuronal activity. Much is known about phototransduction in vertebrates and in the fly D. melanogaster but data from other animals is scarce and therefore we want to investigate this in the spider and onychophoran that represent other major branches of arthropods. Both spiders and onychophorans also have differing types of opsins than seen in D. melanogaster that we want to investigate concerning function and phototransduction pathways. Data from this investigation will teach us potentially new aspects of opsin function and signal transduction that can prove useful in understanding the evolution of vision in all animals that have eyes.

The process of vision start when light enters the retina of the eye and induces a biochemical reaction cascade (phototransduction chain) in the neuronal cells that are sensitive to light (photoreceptors). The phototransduction chain results in a bioelectrical pulse that propagates along the photoreceptive cells, following subsequent cells, and further to the brain into visual centres, where the signal is further processed. In these centres, an image of the visible world will be represented and subsequently may result in a behavioural response, e.g. fight or flight. The protein sensitive to light is called opsin and can be sensitive to light of different specific wavelengths, i.e. colours. A prerequisite to be able to discriminate different colours is the presence of at least two opsins with differing wavelength sensitivity. We made an analysis of all the expressed genes of the spider Cupiennius salei retinas as well as all expressed genes of developing velvet worm Euperipatoides kanangrensis embryos and were able to detect six different opsins in the spider and three different opsins in the velvet worm. Of the six different opsins of the spider four were expressed in the retinas. By comparing their proteins sequences with those of other arthropods we could conclude that the spider C. salei has three opsins with differing wavelength sensitivity expressed in the retina plus a peropsin. Peropsin has been shown in other animals to be involved in the recovery of opsins after their reaction with a photon. Based on the presence of three different visual opsins in the spider, the first prerequisite for colour vision in the spider is met. In order for an animal to be able to detect different colours other criteria have to be met. The different opsins have to be expressed in separate photoreceptors and furthermore the visual centres must be adapted for colour vision. In order to test this, we made antibodies that detect the opsins, and found that indeed photoreceptors did express only one opsin each, indicating that one more criteria for colour vision is met. However, so far any behavioural experiments with C. salei have failed to demonstrate that the spider can discriminate between different colours. Further studies of the visual centres of the brain and behavioural experiments could help to elucidate if C. salei really has the ability of colour vision or if the presence of the different opsins may have yet unknown functions. We also analysed the members of the phototransduction chain and found all the members that have been documented in the fruit fly Drosophila melanogaster also in the spider and velvet worm. However, in the spider retina we found duplicates of some of the phototransduction proteins that were differentially expressed in the different eye types indicating two subtypes of eyes with separate evolutionary history.