A genomic and electrophysiological study of photoreception in the eyeless cnidarian Hydra vulgaris

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 between the endoderm and the ectoderm along the length of the body and tentacles (Hufnagel 1976.) A nerve ring surrounds the mouth and coordinates movements of the body, tentacles, mouth, and nematocysts in response to chemical, photic, and tactile stimuli (Kass-Simon 1972 and Koizumi 1992). Hydra have been known to be photosensitive since the 1800\u27s, and have been shown to demonstrate a preference for some colors over others (Wilson, 1893). They are sensitive to light at the base of the animal. Exposure to light causes contractions of the endodermal musculature and body column extension (Passano and McCullough 1964). Light exposure also changes the frequency of both ectodermal contraction pulses and endodermal rhythmic pulses (Taddei-Ferretti et al. 2004). Further, I have found in my current electrophysiological experiments that they are also photosensitive in ablated tentacles showing some of the same differences 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 (Halder et al. 1995). Box jellies like Cladonema, members of another cnidarian group, express a version of the Pax genes that also create ectopic eyes in Drosophila (Suga et al. 2007). 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 with optical input. Connections between the Pax systems in more evolved organisms and those in more basal organisms have also been sought. A pair of Pax families, named PaxA and PaxB, has been found in both sea nettles and hydra (Sun et al. 1997). In addition, the protein products of the hydra PaxA family were found to bind to a site for Pax5/6 products, which means that the genes produce a very similar protein at all evolutionary levels. This similarity indicates that the Pax gene family has been involved in vision for a very long time. Because hydra are known to be basal to the eumetazoans, adding hydra to the list of Pax-expressing species, coupled with the hydra\u27s simple nerve net, allows us to examine the roots of vision and color sensitivity in its most primitive form. The study of PaxB in hydra indicated that it is expressed during head regeneration and development, in the cell types that will become neurons and nematocytes. Most particularly, the expression in the nematocytes is of interest, as it spatially couples the presence of PaxB to the response to light as demonstrated by Plachetzki and others (Plachetzki 2013). The electrophysiological results reported here add further support to this, with the greatest response to light found in the large tentacle contraction pulses seen in ablated tentacles. These tentacles are rich with nematocytes and battery cells, further spatially linking light reception to these cell types