Evolution of eyes

Eye specification in Drosophila: perspectives and implications

The discovery that Drosophila eyeless is homologous to vertebrate Pax6 produced enormous interest in eye specification and a reappraisal of eye evolution. While the transcription factor Eyeless/Pax6 is necessary and in some circumstances sufficient to induce eye development the simple story of eye specification has become more epic than haiku. At least seven other nuclear proteins act with Eyeless/Pax6 to induce the eye and, furthermore, extrinsic developmental signals are required. Some striking similarities between later events of retinal patterning in vertebrates and insects have led to a deeper debate on the evolutionary path to these apparently quite different organs.

Signalling pathways in Drosophila and vertebrate retinal development

The near-catholic conservation of paired box gene 6 (Pax6) and its supporting cast of retinal determination genes throughout the animal kingdom has sparked a scientific war over the evolutionary origins of the eye. The battle pits those who support a polyphyletic history for the eye against those who argue for a common ancestor for all 'seeing' animals. Recent papers have shed light on how eyes in both vertebrates and invertebrates are patterned. New insights into the roles that signal-transduction cascades might have in determining the Drosophila melanogaster eye indicate that, like many developmental processes, eye specification is an inductive process.

Expression of evolutionarily conserved eye specification genes during Drosophila embryogenesis

Eye specification in Drosophila is thought be controlled by a set of seven nuclear factors that includes the Pax6 homolog, Eyeless. This group of genes is conserved throughout evolution and has been repeatedly recruited for eye specification. Several of these genes are expressed within the developing eyes of vertebrates and mutations in several mouse and human orthologs are the underlying causes of retinal disease syndromes. Ectopic expression in Drosophila of any one of these genes is capable of inducing retinal development, while loss-of-function mutations delete the developing eye. These nuclear factors comprise a complex regulatory network and it is thought that their combined activities are required for the formation of the eye. We examined the expression patterns of four eye specification genes, eyeless (ey), sine oculis (so), eyes absent (eya), and dachshund (dac) throughout all time points of embryogenesis and show that only eyeless is expressed within the embryonic eye anlagen. This is consistent with a recently proposed model in which the eye primordium acquires its competence to become retinal tissue over several time points of development. We also compare the expression of Ey with that of a putative antennal specifying gene Distal-less (Dll). The expression patterns described here are quite intriguing and raise the possibility that these genes have even earlier and wide ranging roles in establishing the head and visual field.