Functional analysis of eye development in the grasshopper, Schistocerca americana

To determine to which degree the underlying mechanism of eye differentiation during the postembryonic development of the Drosophila eye disc represents evolutionary novelty or heritage from embryonic eye development in primitive insects, both hormonal control and molecular genetic regulation of eye development have been studied during both the embryonic and postembryonic life cycle phases in the grasshopper Schistocerca americana.; Manipulating ecdysteroid signaling in cultured embryonic eye primordial revealed that early differentiation and terminal maturation of the embryonic retina are independent of ecdysteroid signaling. Nonetheless, supplementing cultures with 20-hydroxyecdysone accelerated differentiation and maturation, and enhanced cell proliferation. These data suggest that ecdysteroids are not an essential but possibly modulatory component of embryonic retina development in S. americana.; To study the molecular regulation of early embryonic eye development in S. americana, the expression pattern of the signaling gene wingless (wg) and genes projected to interact with wg were analyzed. It was found that the dynamic and complex expression of wg in the early grasshopper procephalon is compatible with a conserved function of wg during anteroposterior and dorsoventral axis formation in the grasshopper eye. Li induced Wg signaling activation experiments with cultured grasshopper embryo eye primordia suggest that these functions are mediated by repression of retinal differentiation and stimulation of tissue proliferation.; The expression of wg and the grasshopper orthologs of pannier, fringe, Delta, and Iroquois-Complex argue against induction of midline centered N signaling activity, which is essential for Drosophila retina development. Similarly substantial evolutionary divergence was found concerning the control of retina versus head epidermis specification by examining the expression of the transcription factor Extradenticle. These results suggest substantial modification of eye development during the evolution of the Drosophila mode of postembryonic eye development.; The fact that systemic RNAi in grasshopper nymphs leads to knockdown of related genes was also reported. Using the nymphal RNAi method, it was demonstrated that the coexpression of retina determination genes is still essential for the postembryonic development of the second half of the adult compound eye. The finding of reinitiation of retina differentiation after transient gene silencing of eye determination gene function suggests a mechanistic model for the evolution of postembryonic eye development in higher insects.