Molecular insights into the eye evolution of bivalvian molluscs

The intention of my PhD project was to gain more insights into eye evolution and to provide further evidence for the recently proposed idea that all eye-types found in eumetazoans derive from a common Pax6-dependent proto-type eye (Gehring and Ikeo, 1999). To do so, we decided to focus on eyes found in bivalves. Two main reasons prompted us to investigate the molecular basis of bivalvian eye formation. In the first place, all major eye-types, the compound eye, consisting of numerous ommatidia, the camera eye with a single lens and the mirror eye with a reflecting mirror in the back of the eye, are found in bivalves. Hence, the occurrence of different eye-types within the same phylogenetic class makes it very unlikely that these eyes arose as independent formations during evolution. A more elegant alternative is to assume that the compound-, camera-, and mirror eyes of clams evolved monophyletically from a common ancestral precursor. The second reason why we decided to investigate bivalvian eyes is their unusual anatomical position, the edge of the mantle. So far, molecular data and most prominently Pax6 expression were exclusively gathered from “cerebral eyes” of bilaterians, with the only exception of the non-cerebral Hesse eyecups of the lancelet, which by the way do not show any Pax6 expression (Glardon et al., 1998). In this study we focused on two bivalvian species, Arca noae and Pecten maximus, representing the compound eye-type and the mirror eye-type, respectively. We isolated two genes, Pax6 and Six1/2, known to be high up in the genetic regulatory cascade of eye development, from Arca and Pecten. Our expression studies of Pax6 and Six1/2 support the idea that these two genes are necessary for the formation of the olfactory system throughout the animal kingdom. In contrast, we could not assign Pax6 and Six1/2 expression to the visual system with absolute certainty. In a second project, we isolated three opsin genes, one from Arca and two opsin genes from Pecten. A Go-coupled opsin was isolated from Pecten which was shown to be exclusively expressed in the rhabdomeric photoreceptor cells of the proximal retina. The second opsin gene isolated from Pecten and the opsin gene from Arca were shown to be expressed in various tissues, suggesting a putative role in the photic regulation of peripheral circadian clocks. Moreover, phylogenetic analysis indicate that each of these two opsin genes may constitute a novel opsin subfamily.