The anatomy of the eye and light organ of E. scolopes, 2 complex organs that interact with light. (A) The locations of the light organ and the eye in an adult animal. The specimen has been dissected ventrally to reveal the position of the light organ in the center of the mantle cavity. (Scale bar, 1 cm.) (B) A light micrograph of a cross section of the eye (black dotted line in A). The lens focuses light (dashed line, arrow) onto the retina, which contains the proteins of visual transduction; the pigmented layer (Pl) and reflector (Ref) collect stray light and also form the iris, which modifies amount of light entering the eye. (Scale bar, 2 mm.) (C) A light micrograph of a cross section of the light organ (white dotted line in A). The “lens” acts as a diffusion filter, diffusing the light produced by the symbionts in the epithelial central core into the environment (dashed line, arrow). The reflector (Ref) reflects the luminescence ventrally, and the ink (lost in histological preparation) in the ink sac creates a pigmented layer that absorbs stray luminescence. As in the eye, the pigmented layer and reflector act as an iris, contracting and expanding to allow more or less light emission, respectively. (Scale bar, 2 mm.)

Proteins of phototransduction encoded by genes expressed in the E. scolopes light organ. (A) Proteins predicted from derived amino acid sequences of the E. scolopes light organ cDNAs (red), their relatedness to the closest match in the NCBI database, and their clade, or phylogenetic relatedness to other sequences (confidence as aLRT values = approximate likelihood ratio test scores; see Fig. S1 and ref. 48). *, per cent identity to the light organ isoform. (B) Placement of the deduced light-organ proteins in diagrammatic representations of the phototransduction cascades typically associated with invertebrate rhabdomeric and vertebrate ciliary phototransduction in the visual system (red, components present in the light organ; blue, not detected). Biomolecule abbreviations: Arr-a, arrestin alpha; Arr-b, arrestin beta; cGMP, cyclic guanosine monophosphate; CNG, cyclic nucleotide-gated; C-opsin, ciliary opsin; G_i/ta, G protein alpha subunit i/t; G_o, G protein subunit o; G_q, G protein alpha subunit q; PDE, phosphodiesterase; PLC, phospholipase C; RALBP, retinal binding protein; RK rhodopsin kinase; R-opsin, rhabdomeric rhodopsin; TRP, transient receptor potential, photoreceptor-specific, Trp-C. Animal species abbreviations: Aa, Aedes aegypti; Hs, Homo sapiens; Lp, Loligo pealei; Lf, L. forbesi; Ov, Octopus vulgaris; Tp, Todarodes pacificus.

Representative electroretinograms from the eyes (Top) and light organs (Bottom) of E. scolopes. Diagrams indicate the position of the electrodes on organ tissues. Light stimulus (open triangles) to the eyes (n = 10 animals) resulted in a rapid depolarization characteristic of invertebrate rhabdomeric phototransduction. The membrane potential rapidly returned to the resting potential with the cessation of the light stimulus (closed triangles). In contrast, the light organs (n = 15 animals) exhibited hyperpolarization with a markedly slower onset and recovery than the eyes. The light organs of both hatchling (Bottom, Upper Panel) and juvenile animals (Bottom, Lower Panel) displayed an increase in levels of hyperpolarization with organ growth.

Localization of mRNA and proteins of phototransduction. (A) RT-PCR analysis for mRNA detection in an array of tissues. The gene encoding the ribosomal protein L21 (rpL21) was determined to be a suitable control (20 ng total RNA/lane). (B) Opsin message in the adult light organ. The RNA of the organ's central epithelial “core” (C; see Fig. 1C), where symbionts reside, was extracted and analyzed separately from the other light-organ tissues (O) (20 ng total RNA/lane). (C) Localization of proteins of phototransduction in juvenile light-organ tissues. (Upper image) Histological section for orientation, showing a pore (p), a duct (d), and portions of 3 crypts (cr, 1, 2, 3). (Lower 3 images) Confocal images of light-organ immunocytochemistry revealed that the crypts, as well as the pore and duct regions, were cross-reactive for opsin, arrestin, and rhodopsin kinase (green, fluorescein isothiocyanate-labeled secondary antibodies) in both symbiont-colonized and uncolonized light organs. Sections were counterstained with rhodamine phalloidin (red), which labels filamentous actin. In control experiments, the retinal tissues of the eye labeled with these antibodies, and labeling was below background in controls excluding the primary antibody (see Fig. 2S). RK, rhodopsin kinase. (Scale bar, 20 μm.)