A GENOMIC AND ELECTROPHYSIOLOGICAL STUDY OF PHOTORECEPTION IN THE EYELESS CNIDARIAN HYDRA VULGARIS

Abstract

Hydra are tubular coelenterates with two germ layers, the endoderm and ectoderm. A ring of five to eight tentacles surround an oral pore, and the animal attaches to the substrate via adhesion of the peduncle at the opposite end. They possess a complete ectodermal nerve net, with nerve fibers running within the ectoderm throughout the body and along the length of the tentacles (Hufnagel 1976.) Hydra has two nerve rings. One nerve ring surrounds the mouth and is thought to coordinate mouth opening. A second ring, located at the base of the tentacles, coordinates movements of the body, tentacles, mouth, and nematocysts in response to chemical, photic, and tactile stimuli. The functional unit of the tentacle effector systems is the battery cell complex. The battery cell complex consists of a large epithelial cell called a battery cell and its associated complement of neurons and nematocytes. These battery cell complexes link to each other via interdigitating neuronal processes and myonemes. They have been shown to respond to diverse chemical and mechanical stimuli. When separated from the battery cell, the nematocyst is still able to respond to mechanical and photic stimuli, though not chemical cues. Hydra have been known to be photosensitive since the 1800’s, and have been shown to demonstrate a preference for some colors over others. They are sensitive to light at the base of the animal. Exposure to light causes contractions of the endodermal musculature and extensions of the body column. Light exposure also changes the frequency of both ectodermal contraction pulses and endodermal rhythmic pulses. Further, I have found in my current electrophysiological experiments that hydra’s ablated tentacles show some of the same differences in behavior across wavelengths. The Pax family of genes is found across taxa, from cnidarians all the way to humans. These highly conserved genes code for a transcription factor instrumental in the formation of the eye, to such a degree that it leads to the production of eyes where none should be. In fruit flies, exogenous expression of the Pax6 protein product causes the production of eyes on the legs or antennae. Box jellies like Cladonema, members of another cnidarian group, express a version of the Pax genes that also creates ectopic eyes in Drosophila. Nine mammalian Pax genes have been identified, in four subgroups; most of the Pax family is involved in the development of the nervous system, in particular those sections dealing