Evolution of vision in arthropods

The genetic and developmental mechanisms underlying vision in arthropods has been extensively investigated, mainly in Drosophila but some data are starting to be collected from other taxa such as beetles, bees and spiders. Some of the key molecules in vision are the photoreceptor proteins belonging to the opsin family. The spectral sensitivities of the opsins are in large determined by their amino acid composition. The presence of several visual opsins with different spectral sensitivity is a prerequisite for colour vision but the presence of multiple opsins may also help to distinguish different levels of grey. The spider Cupiennius salei is a nocturnal hunter that to a large extent relies on mechano sensors for prey capture. However, investigations have shown that its vision is well developed and its principal eyes detect shapes while the secondary eyes detect moving objects. Intracellular recordings of retinal cells have indicated the presence of three spectral sensitivities; blue, green and ultra violet. Behavioural tests have so far failed to indicate the ability to discriminate different colours. We propose to clone and sequence the opsin genes of C. salei in order to determine its receptor possibility for colour vision. By phylogenetic analysis of the opsin sequences we would gain information on the spectral sensitivity of the opsins as well as their phylogenetic position among opsins of other species, and hence, learn more about opsin evolution in spiders and arthropods. In order to facilitate the phylogenetic analyses of arthropod opsins we would clone and sequence the opsin genes of an onychophoran (Euperipatoides kanangrensis) and use it as an out-group representative. Characterization of the onychophoran opsin genes would also give insight into this group`s visual abilities, which is poorly understood. By doing expression analyses of the opsin genes we would get information on the localization of the different receptor types, within and between different eye types present in the spider, and this can tell us more about the visual i.e. spectral capabilities of this spider. In addition, the comparison of the spectral capabilities of the onychophoran with that of the spider would enable us to assess the evolution of photo receptors within arthropods.The genetic mechanisms behind the development of eyes have also been extensively studied in the fruit fly Drosophila and also the so-called retinal determination gene cascade (RDGC) were a number of genes are involved that have been indicated to be important in eye development also in other animals. However, recently it was shown that genes of the RDGC were not expressed in the eyes of the horseshoe crab (Limulus), a basal member of the chelicerates to which also the spiders belong. Considering the importance of these genes in other arthropods, and indeed in many other animals, it suggests that either the genetic machinery of eye development has diverged between the chelicerates and other arthropods, or that chelicerate eyes have evolved separately. We therefore intend to clone and characterize genes of the RDGC plus other genes that has proven to be vital for eye development in Drosophila or other animals in order to acquire the so-called gene expression fingerprint of the spider and onychophoran eyes. In order to do this we will carry out genetic and phylogenetic analysis of the orthologous genes found and perform expression analysis by in situ hybridisation, in order to localize their transcripts during development. Functional analysis of involved genes in the spider will be done by gene silencing with RNA interference experiments; this will also give information on regulatory interrelationships among genes and will help to map the gene regulatory network of spider eye development. In combination with the gene silencing experiments in the spider, we will also do behavioural experiments on individuals with non lethal effects since it is possible that the silencing of some genes might cause effects that are obvious in behaviour but not in morphology.