Visual system development: Invertebrates

In contrast to vertebrates, visual system development in most invertebrate groups commences with photoreceptor differentiation in an ectodermal epithelium. Photoreceptors, of both ciliary and rhabdomeric types in different invertebrates, differentiate under the control of the Pax6 gene, mostly independently of the nerve centers they innervate. Visual centers in the brain are highly developed in some forms, especially in arthropods and cephalopods. Most studies examine the compound eyes of the fruit fly. Drosophila melanogaster, to the unwarranted neglect of other deserving groups. The same is true for the development of neuronal connections within the visual centers in the brain, which generally occurs in a wave, with retinotopic projections often establishing chiasmata. Our understanding of many aspects of invertebrate visual system development must also encompass knowledge of eyes as diverse as, for example, the highly developed single-lens eyes of cephalopods, or ancestral eyecup eyes in forms such as planaria.