Evidence for Multiple Phototransduction Pathways in a Reef-Building Coral

Abstract

Photosensitive behaviors and circadian rhythms are well documented in reef-building corals and their larvae, but the mechanisms responsible for photoreception have not been described in these organisms. Here we report the cloning, immunolocalization, and partial biochemical characterization of three opsins and four G proteins expressed in planulae of the Caribbean elkhorn coral, Acropora palmata. All three opsins (acropsins 1–3) possess conserved seven-pass transmembrane structure, and localize to distinct regions of coral planulae. Acropsin 1 was localized in the larval endoderm, while acropsin 2 was localized in solitary cells of the ectoderm. These rod-like cells displayed a remarkably polarized distribution, concentrated in the aboral end. We also cloned four A. palmata G protein alpha subunits. Three were homologs of vertebrate Gi, Go, and Gq. The fourth is presumably a novel G protein, which displays only 40% identity with the nearest known G protein, and we termed it Gc for “cnidarian”. We show that Gc and Gq can be activated by acropsins in a light-dependent manner in vitro. This indicates that at least acropsins 1 and 3 can form functional photoreceptors and potentially may play a role in color preference during settlement, vertical positioning and other light-guided behaviors observed in coral larvae.

Cubozoan genome illuminates functional diversification of opsins and photoreceptor evolution

“…Our data and data from other studies3960 show that Cnidarian intron-less opsins might have been derived from an ancient eumetazoan ciliary-like opsin containing introns by retro-transposition. Once anchored in the genome the ancient cnidopsin gene underwent several rounds of duplication, diversification and sensitivity tuning.…”

“…However, we were unable to obtain any light-mediated activation of signal transduction via this pathway for Tc-group 1 opsins. We propose that opsins that did not signal in our assay either use different G-protein pathways, as recently proposed in reef corals60, act as photoisomerases or for unknown reason do not signal in our cell-line based assay, but nonetheless use Gs signaling cascade under natural conditions. The later possibility is, however, in our opinion very unlikely, because we saw comparable expression of all Tcops on the cell membranes of our test cell system and moreover not even a repeated flash stimulation lead to any response.…”

“…Such a variety of larval opsins is astonishing considering that to date only three larval opsins have been reported from reef corals60. Planula larvae have an extremely simple organization with no nervous system at all.…”

The Comb Jelly Opsins and the Origins of Animal Phototransduction

“…In the case of Feuda et al (2012) data set, all the ctenophoran opsins identified by Schnitzler et al (2012) and the cnidarian opsins identified by Mason et al (2012) in the acroporan A . palmata were added to the alignment (generating FEAm1).…”

Impacts of temperature and lunar day on gene expression profiles during a monthly reproductive cycle in the brooding coral Pocillopora damicornis

“…Because spawning is cued by lunar and daily changes in light intensity and spectral quality, it has been suggested that cryptochromes are involved in this process (Hoadley, Szmant, & Pyott, ; Levy et al., ; Reitzel, Tarrant, & Levy, ). Similarly, red‐photosensitivity has been documented in coral larvae, and three coral opsins (acropsin 1–3) homologous to human rhodopsins have been characterized in the coral Acropora palmata, suggesting that light detection associated with multiple behaviours in corals may be the result of a rhodopsin‐based visual system (Mason & Cohen, ; Mason et al., ). In addition, calcium has been described as a mediator of light responsivity in corals, and empirical evidence has shown that changes in [Ca 2+ ], brought on by shifts from day to night, control the time of broadcast spawning (Hilton, Brady, Spaho, & Vize, ).…”