OAKLEY, T.H.; Univ. of California, Santa Barbara: The Tao of Eye Evolution and Development: A duplication model unifies apparently opposite resolutions of the paradox of eye evolution

The evolution of eyes presents biologists with a fascinating paradox because a highly conserved developmental program gives rise to a riotous display of morphologically distinct eye types. In general, proposals to resolve the paradox have taken two apparently opposing forms. First, eyes may have stemmed from a common ancestral prototype photoreceptor. Under this model, an ancestral receptor used the conserved developmental program and was elaborated differently during separate lines of evolution to arrive at the current diversity of eye types. Under a second model, eyes may have originated multiple times, each time recruiting or “co-opting” the same genes. Here I put forth a new model that unifies these seemingly opposing models. I suggest that eyes evolved by structural duplication, re-deploying the developmental program in new positions to produce serially homologous structures. Duplicate photoreceptors then could diverge, remain the same, or disappear during evolution. Under the new model, diverse eye types stem from an original prototype, yet also result from independent origins. Evidence from Drosophila and ostracod crustaceans is consistent with duplication of photoreceptors by redeployment of their developmental program. The unity of all opposites is a central theme in Taoist philosophy and the duplication model may serve as an example, unifying opposite hypotheses of single versus multiple origins of photoreceptors.