FRIEDRICH, M.: Evolution of insect eye development: comparative and functional evidence for a conserved role of wingless in negatively controlling the spatial extent of retina differentiation

Molecular genetic studies in model organisms like mouse and Drosophila have identified a number of genes, which appear to be expressed and required in similar ways during eye development. Fundamental aspects of metazoan eye development are therefore likely to be evolutionarily conserved. Nonetheless, Drosophila exhibits a highly derived mode of insect eye development from imaginal discs, which may, to some degree, compromise its suitability for across phyla comparisons. It is also an important question, which developmental regulatory changes were involved in the transition from embryonic eye development in primitive insects to the advanced mode of postembryonic eye development in Drosophila. We are therefore studying eye development in the more primitive flour beetle Tribolium castaneum and the grasshopper Schistocerca americana. We have previously shown that the expression of the signaling factor wingless in front of the differentiating retina is highly conserved indicating a conserved function in negatively regulating retina differentiation in insects. Consistent with this, we find that exposure of in vitro cultured grasshopper retinas to lithium, which constitutively activates wingless signaling via inhibition of glycogen synthase kinase-3, specifically blocks progression of retina differentiation. A detailed analysis of wingless expression during Tribolium embryogenesis revealed that wingless is expressed in a manner consistent with a function in suppressing embryonic eye development thereby allowing only a small number of larval photoreceptors to differentiate and facilitating the delay of adult retina differentiation into postembryogenesis.