Molecular identification and developmental expression of UV and green opsin mRNAs in the pineal organ of the Atlantic halibut.

The pineal organ is the only differentiated photoreceptor organ present in embryos and early larvae of the Atlantic halibut (Hippoglossus hippoglossus). We investigated the molecular identity of opsins in the pineal organ, and their expression during different life stages. Using RT-PCR we identified two 681-bp gene sequences, named HPO1 and HPO4, in cDNA from adult pineal and whole embryos. The predicted amino acid sequences showed highest identity to the transmembrane regions of teleostean RH2 green cone opsins (HPO1, 72–91%) and SWS-1 UV cone opsins (HPO4, 71–83%). In situ hybridization revealed expression of HPO1 and HPO4 mRNA transcripts in photoreceptors in the pineal organ of embryos, larvae and adults. HPO1 and HPO4 mRNA transcripts were also expressed in the larval retina. Our study provides molecular evidence for short and middle wavelength light sensitive photoreceptors in the pineal organ of Atlantic halibut throughout life, and suggests that pineal photoreception may play an important role during embryonic and larval life stages, especially at the time when the retina does not possesses corresponding photoreceptor capacity.