Abstract
Eyes morphologies may differ but those differences are not reflected at the molecular level. Indeed, the ability to perceive light is thought to come from the same conserved gene families: opsins and cryptochromes. Even though cuttlefish (Cephalopoda) are known for their visually guided behaviors, there is a lack of data about the different opsins and cryptochromes orthologs represented in the genome and their expressions. Here we studied the evolutionary history of opsins, cryptochromes but also visual arrestins in molluscs with an emphasis on cephalopods. We identified 6 opsins, 2 cryptochromes and 1 visual arrestin in Sepia officinalis and we showed these families undergo several duplication events in Mollusca: one duplication in the arrestin family and two in the opsin family. In cuttlefish, we studied the temporal expression of these genes in the eyes of embryos from stage 23 to hatching and their expression in two extraocular tissues, skin and central nervous system (CNS = brain + optic lobes). We showed in embryos that some of these genes (Sof_CRY6, Sof_reti-1, Sof_reti-2, Sof_r-opsin1 and Sof_v-arr) are expressed in the eyes and not in the skin or CNS. By looking at a juvenile and an adult S. officinalis, it seems that some of these genes (Sof_r-opsin1 and Sof_reti1) are used for light detection in these extraocular tissues but that they set-up later in development than in the eyes. We also showed that their expression (except for Sof_CRY6) undergoes an increase in the eyes from stage 25 to 28 thus confirming their role in the ability of the cuttlefish embryos to perceive light through the egg capsule. This study raises the question of the role of Sof_CRY6 in the developing eyes in cuttlefish embryos and the role and localization of xenopsins and r-opsin2. Consequently, the diversity of molecular actors involved in light detection both in the eyes and extraocular tissues is higher than previously known. These results open the way for studying new molecules such as those of the signal transduction cascade.
Highlights
Eyes are specialized light-sensitive sensory structures, most of time involved in image forming vision
Opsin Family Besides the two opsins sequences already known in adult S. officinalis, we identified 4 new putative opsin sequences in transcripts from embryos
6 opsins receptors, 2 cryptochromes and 1 visual arrestin were identified in transcriptomes from Sepia officinalis embryos
Summary
Eyes are specialized light-sensitive sensory structures, most of time involved in image forming vision. The visual system of coleoid cephalopods is mainly composed of two large spherical eyes with a lens, a vitreous cavity and an iris, known as camera (or camerular) eyes They are linked to optic lobes through optic nerves. In Sepia officinalis, the two optic lobes represent about twice the size of the brain (Nixon and Young, 2003) and the eyes harbor rhabdomeric photoreceptor cells as in many protostomians. This visual system sets up during embryogenesis. Behavioural studies have shown that the embryo is able to answer a light stimulation as soon as the pigmentation starts to appear in the eyes (stage 25: Romagny et al, 2012). The transcription pathways of these molecules have just begun to be studied in cephalopods
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