Reaction patterns of dark- and light-adapted Hydra to light stimuli

Understanding the dermal light sense in the context of integrative photoreceptor cell biology

While the concept of a dermal light sense has existed for over a century, little progress has been made in our understanding of the mechanisms underlying dispersed photoreception and the evolutionary histories of dispersed photoreceptor cells. These cells historically have been difficult to locate and positively identify, but modern molecular techniques, integrated with existing behavioral, morphological, and physiological data, will make cell identification easier and allow us to address questions of mechanism and evolution. With this in mind, we propose a new classification scheme for all photoreceptor cell types based on two axes, cell distribution (aggregated vs. dispersed) and position within neural networks (first order vs. high order). All photoreceptor cells fall within one of four quadrants created by these axes: aggregated/high order, dispersed/high order, aggregated/first order, or dispersed/first order. This new method of organization will help researchers make objective comparisons between different photoreceptor cell types. Using integrative data from four major phyla (Mollusca, Cnidaria, Echinodermata, and Arthropoda), we also provide evidence for three hypotheses for dispersed photoreceptor cell function and evolution. First, aside from echinoderms, we find that animals often use dispersed photoreceptor cells for tasks that do not require spatial vision. Second, although there are both echinoderm and arthropod exceptions, we find that dispersed photoreceptor cells generally lack morphological specializations that either enhance light gathering or aid in the collection of directional information about light. Third, we find that dispersed photoreceptor cells have evolved a number of times in Metazoa and that most dispersed photoreceptor cells have likely evolved through the co-option of existing phototransduction cascades. Our new classification scheme, combined with modern investigative techniques, will help us address these hypotheses in great detail and generate new hypothesis regarding the function and evolution of dispersed photoreceptor cells.

The Sine oculis/Six class family of homeobox genes in jellyfish with and without eyes: development and eye regeneration

The development of visual organs is regulated in Bilateria by a network of genes where members of the Six and Pax gene families play a central role. To investigate the molecular aspects of eye evolution, we analyzed the structure and expression patterns of cognate members of the Six family genes in jellyfish (Cnidaria, Hydrozoa), representatives of a basal, non-bilaterian phylum where complex lens eyes with spherical lens, an epidermal cornea, and a retina appear for the first time in evolution. In the jellyfish Cladonema radiatum, a species with well-developed lens eyes in the tentacle bulbs, Six1/2-Cr and Six3/6-Cr, are expressed in the eye cup. Six4/5-Cr is mainly expressed in the manubrium, the feeding, and sex organ. All three Six genes are expressed in different subsets of epidermal nerve cells, possibly of the RFamide type which are part of a net connecting the different eyes with each other and the effector organs. Furthermore, expression is found in other tissues, notably in the striated muscle. During eye regeneration, expression of Six1/2-Cr and Six3/6-Cr is upregulated, but not of Six4/5-Cr. In Podocoryne carnea, a jellyfish without eyes, Six1/2-Pc and Six3/6-Pc are also expressed in the tentacle bulbs, Six1/2-Pc additionally in the manubrium and striated muscle, and Six3/6-Pc in the mechanosensory nematocytes of the tentacle. The conserved gene structure and expression patterns of all Cladonema Six genes suggest broad conservation of upstream regulatory mechanisms in eye development.

DNA-binding characteristics of cnidarian Pax-C and Pax-B proteins in vivo and in vitro: no simple relationship with the Pax-6 and Pax-2/5/8 classes

Cnidarians are the simplest animals in which distinct eyes are present. We have previously suggested that cnidarian Pax-Cam might represent a precursor of the Pax-6 class. Here we show that when expressed in Drosophila imaginal discs, Pax-Cam chimeric proteins containing the C-terminal region of EY were capable of eye induction and driving expression of a reporter gene under the control of a known EY target (the sine oculis gene). Whilst these results are consistent with a Pax-6-like function for Pax-Cam, in band shift experiments we were unable to distinguish the DNA-binding behaviour of the Pax-Cam Paired domain from that of a second Acropora Pax protein, Pax-Bam. The ability of a Pax-Bam/EY chimera to also induce eye formation in leg imaginal discs, together with the in vitro data, cast doubt on previously assumed direct relationships between cnidarian Pax genes and the Pax-6 and Pax-2/5/8 classes of bilateral animals.

Photoreceptors of cnidarians

Cnidarians are the most primitive present-day invertebrates to have multicellular light-detecting organs, called ocelli (eyes). These photodetectors include simple eyespots, pigment cups, complex pigment cups with lenses, and camera-type eyes with a cornea, lens, and retina. Ocelli are composed of sensory photoreceptor cells interspersed among nonsensory pigment cells. The photoreceptor cells are bipolar, the apical end forming a light-receptor process and the basal end forming an axon. These axons synapse with second-order neurons that may form ocular nerves. A cilium with a 9 + 2 arrangement of microtubules projects from the receptor-cell process. Depending on the species, the membrane covering the cilium shows several variations, including evaginating microvilli. In the cubomedusae stacks of membranes fill the apical regions of the photoreceptor cells. Pigment cells are rich in pigment granules, and in some animals the distal regions of these cells form tubular processes that project into the cavity of the ocellus. Microvilli may extend laterally from these tubular processes and interdigitate with the microvilli from the ciliary membranes of photoreceptor cells. Photoreceptor cells respond to changes in light intensity with graded potentials that are directly proportional to the range of the changes in light intensity. In the Hydrozoa these cells may be electrically coupled to each other through gap junctions. Light affects the behavioral activities of cnidarians, including diel vertical migration, responses to rapid changes in light intensity, and reproduction. Medusae with the most highly modified photoreceptors demonstrate the most complex photic behaviors. The sophisticated visual system of the cubomedusan jellyfish Carybdea marsupialis is described. Extraocular photosensitivity is widespread throughout the cnidarians, with neurons, epithelial cells, and muscle cells mediating light detection. Rhodopsin-like and opsin-like proteins are present in the photoreceptor cells of the complex eyes of some cubomedusae and in some neurons of hydras. Neurons expressing glutamate, serotonin, γ-aminobutyric acid, and RFamide (Arg-Phe-amide) are found in close proximity to the complex eyes of cubomedusae; these neurotransmitters may function in the photic system of the jellyfish. Pax genes are expressed in cnidarians; these genes may control many developmental pathways, including eye development. The photobiology of cnidarians is similar in many ways to that of higher multicellular animals.

Characterization and expression analysis of an ancestor-type Pax gene in the hydrozoan jellyfish Podocoryne carnea

We characterized a Pax gene from the hydrozoan Podocoryne carnea. It is most similar to cnidarian Pax-B genes and encodes a paired domain, a homeodomain and an octapeptide. Expression analysis demonstrates the presence of Pax-B transcripts in eggs, the ectoderm of the planula larva and in a few scattered cells in the apical polyp ectoderm. In developing and mature medusae, Pax-B is localized in particular endodermal cells, oriented toward the outside. Pax-B is not expressed in muscle cells. However, if isolated striated muscle tissue is activated for transdifferentiation, the gene is expressed within 1 h, before new cell types, such as smooth muscle and nerve cells, have formed. The expression data indicate that Pax-B is involved in nerve cell differentiation.

Photobehaviour of Hydra (Cnidaria, Hydrozoa) and correlated mechanisms: a case of extraocular photosensitivity

The morpho-functional organization correlated to photosensitivity in Cnidaria is that of ocelli and extraocular photoreception. Several examples of the second type of organization are reported. The photosensitivity of the cnidarian Hydra is of the extraocular (neural or dermal) type. The effects of photic stimulation (applied according to various experimental protocols: steady condition; step stimulus; single, twin, or repetitive pulses; different polarities and chromaticities of steady, step and pulse stimulation and different phases of pulse application) on the modulation of various bioelectric events linked to the periodic behaviour of the animal are reviewed. The mechanisms correlated with the photobehaviour of Hydra, as well as the problems still open on the molecular mechanisms of phototransduction, are discussed.

Electrical activities of a type of electroretinogram recorded from the ocellus of a jellyfish, Polyorchis penicillatus (Hydromedusae)

An electroretinogram (ERG), evoked by light stimuli, was recorded from ocelli of Polyorchis penicillatus (Hydromedusae). The ERG is a polyphasic response with a positive potential change at the onset of illumination followed by a slower biphasic pulse, and a positive deflection at the cessation of illumination which is followed by a series of high-frequency pulses. The most striking features of the initial pulse are its latency-log intensity relation and the gradation of pulse amplitude with respect to the intensity of the light stimulus and to different wavelengths. Maximum spectral sensitivity lies around 530 nm. Response patterns induced by shadowing and repeated stimulation of light- and dark-adapted ocelli are described. Morphological structures which could give rise to the mass response of the ocellus are discussed.