Eye Designs in Basal Metazoans
Vicki J. Martin
Professor
Ph.D., Wake Forest University
Postdoctoral, University of Alberta-Canada
Few topics have generated as much debate as the evolution of animal eyes. One camp argues that eyes originated numerous times independently in at least 40, but possibly up to 65 or more different animal lines, whereas, a second camp, proposes that the various animal eye-types evolved from a single common ancestral prototype eye. This hypothetical prototype eye had a photosensitive cell containing a light receptor protein (rhodopsin/opsin) associated with a pigment cell to form an organ. By divergent, parallel, and convergent evolution, all the various animal eye designs would be generated from the prototype. A common origin of animal eyes is supported by the observation that all multicellular animal eyes share the same visual pigments, rhodopsin/opsin. The phylum Cnidaria includes the first multicellular animals to form eyes; this group exhibits a diversity of eye designs ranging from a simple photosensitive sheet of cells to the complex image forming eyes of cubozoan jellyfish. Because of their basal position on the phylogenetic tree, cnidarians provide an excellent system in which to study the evolution of the first multicellular animal eyes and the evolution of photosensory mechanisms. The camera-type eyes of cubozoans represent the most highly evolved eyes in the Cnidaria. Further they contain the visual pigments involved in phototransduction: rhodopsin and opsins. These eyes resemble the proposed ancestral prototype eye. A comparative study of the structure, development, and physiology of representative eyes (simple to complex) from all classes of this phylum is being conducted in my lab to ascertain their overall complexity and their position in the scheme of animal eye evolution. Further, a search for key genes and gene products shown to be instrumental in eye development and eye function in higher animals is underway for the cnidarians. If we can demonstrate the presence of these genes/gene products in the first multicellular animals to form eyes, then evidence will exist to support a common origin of eyes in all animals, with the basic visual mechanisms dating back to the cnidarians, which are over 600 million years old.
We have examined the structure of the complex eyes of the cubomedusae Carybdea , Tripedalia , and Tamoya . Our work showed that these eyes are similar in design to vertebrate eyes, possessing a cornea, lens, and retina of ciliated photoreceptor cells. The jellyfish photoreceptors resemble vertebrate rod cells; further, different types of photoreceptors comprise these eyes, with some eyes containing more types of photoreceptors than others. Rhodopsins and opsin proteins are found in these photoreceptors, indicating that the jellyfish have the abilities to detect light and dark, as well as color. The photoreceptors contain neurochemicals that are used in the relaying of visual information from the photoreceptors to the main nerve ring (brain) of the jellyfish.
The Martin lab is presently comparing eyes from cubomedusae from around the world. We are using various techniques to: 1) to determine if all cubozoan eyes are built on a common architectural plan: cornea, lens, retina of ciliated photoreceptors, presence of rhodopsin/opsin proteins, presence of visual neurotransmitters, expression of known eye developmental control genes, 2) examine the architecture of cubozoan nervous systems and identify the neural connections linking the eyes to the main nerve rings of the animals, 3) reveal where neural processing of visual information is occurring, and 4) examine the development of the eyes. In addition, behavioral experiments are being done to examine the role of vision in feeding, navigation, and sexual reproduction of cubomedusae.
Structure of the complex eye of the box jellyfish Carybdea marsupialis.
Selected Publications
Martin, V. (1997) Cnidarians, the jellyfish and hydras. In: Embryology, Constructing the Organism. Gilbert, S., and Raunio, A. (eds.). pp. 57-86. Sinauer Associates, Inc.
Martin, V. (1999) Hydra. In: Encyclopedia of Reproduction. Knobil, E., and Neill, J. (eds.). pp. 707-718. Academic Press.
Martin, V., and Koss, R. (2001) Phylum Cnidaria. In: Atlas of Invertebrate Reproduction. Young, C., Rice, M., and Sewell, M. (eds.). Academic Press.
Martin, V. (2002) Photoreceptors of cnidarians. Canadian Journal of Zoology 80: 1703-1722.
Brumwell, G., and V. Martin. (2002) Immunocytochemically defined neuron populations progressively increase in size through embryogenesis of Hydra vulgaris (plus journal cover photo). Biol. Bull. 203: 70-79.
Martin, V. (2004) Eye designs in basal metazoans. Proceedings of the symposium Model Systems for the Basal Metazoa: Cnidarians, Ctenophores, and Placozoans, Society of Integrative and Comparative Biology, New Orleans, LA., January, p. 187.
Martin, V. (2004) Photoreceptors of cubozoan jellyfish. Hydrobiologia.
Martin, V. (2004) Photoreceptors of cubozoan jellyfish. In: Coelenterate Biology 2003: Trends in Research on Cnidaria and Ctenophora, D. Fautin, J. Westfall, P. Cartwright, M. Daly, C. Wyttenbach (eds.), Kluwer Press.
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