Molecular analysis of the amphioxus frontal eye unravels the evolutionary origin of the retina and pigment cells of the vertebrate eye

Abstract

The origin of vertebrate eyes is still enigmatic. The "frontal eye" of amphioxus, our most primitive chordate relative, has long been recognized as a candidate precursor to the vertebrate eyes. However, the amphioxus frontal eye is composed of simple ciliated cells, unlike vertebrate rods and cones, which display more elaborate, surface-extended cilia. So far, the only evidence that the frontal eye indeed might be sensitive to light has been the presence of a ciliated putative sensory cell in the close vicinity of dark pigment cells. We set out to characterize the cell types of the amphioxus frontal eye molecularly, to test their possible relatedness to the cell types of vertebrate eyes. We show that the cells of the frontal eye specifically coexpress a combination of transcription factors and opsins typical of the vertebrate eye photoreceptors and an inhibitory Gi-type alpha subunit of the G protein, indicating an off-responding phototransductory cascade. Furthermore, the pigmented cells match the retinal pigmented epithelium in melanin content and regulatory signature. Finally, we reveal axonal projections of the frontal eye that resemble the basic photosensory-motor circuit of the vertebrate forebrain. These results support homology of the amphioxus frontal eye and the vertebrate eyes and yield insights into their evolutionary origin.evolution | vision | cephalochordate T he evolutionary origin of vertebrate eyes is enigmatic. Charles Darwin appreciated the conceptual difficulty in accepting that an organ as complex as the vertebrate eye could have evolved through natural selection (1). Part of the problem lies in the paucity of extant phyla with useful gradations that occurred during eye evolution, thus providing a scenario that led to the emergence of the vertebrate eye. For example, the eye of the adult lamprey (a jawless vertebrate) is remarkably similar to the eye of jawed vertebrates in the overall design, retina cell types, and multiple classes of opsins (2). Given these similarities, it is likely that the last common ancestor of jawless and jawed vertebrates already possessed an elaborate camera-type lens eye. To understand the seemingly sudden origin of the vertebrate eye, its evolutionary precursor must be identified within the nonvertebrate chordates lacking elaborated eye structures. Because of its basal phylogenetic position within the chordates (3), its slowly evolving genome (4), and its ancestral morphology, the cephalochordate amphioxus represents a traditional model organism for understanding the origin of vertebrate organs. Extensive electron microscopy studies of the cerebral vesicle of the basal chordate amphioxus revealed several putative photoreceptive organs-dorsal ocelli, Joseph cells, lamellar body, and the unpaired "frontal eye" (5). The pigmented dorsal ocelli and the Joseph cells are morphologically and molecularly related to invertebrate eye photoreceptors (6, 7), whereas the frontal eye and the lamellar body traditionally have been homologized to the vertebrate eyes and pineal g...

The opsin repertoire of the European lancelet: a window into light detection in a basal chordate

“…Studies on expression pattern and function of the Branchiostoma opsins are rather limited; the biochemical data support AmphiOp1 as a typical visual pigment and AmphiOp3 as a photoisomerase (Koyanagi et al, 2002), while the properties of the B. belcheri melanopsin homolog were found to resemble those of the visual opsins present in the intrinsically photosensitive rhabdomeric photoreceptor cells of vertebrates (Koyanagi et al, 2005). In agreement with their phylogenetic placement, antibody staining revealed specific expression of two ciliary-type opsins in the ciliary photoreceptor cells of the B. floridae frontal eye (Vopalensky et al, 2012). Here, we report for the first time the opsin repertoire of B. lanceolatum (Pallas, 1774) and provide information on the expression patterns of opsin genes across multiple tissues and developmental stages.…”

“…Function of the first two receptors is still not clear, nevertheless, rhabdomeric receptors similar to Joseph cells are present in the cerebral eyes of tunicates (salps). Amphioxus frontal eye is considered homologous to the paired eyes of vertebrates, since its photoreceptors and pigment cells co-express a combination of transcription factors and opsins typical of the vertebrate eye photoreceptors (Vopalensky et al, 2012). The lamellar body, on the other hand, is proposed as the amphioxus homolog of the pineal gland (Lacalli, 2004).…”

Amphioxus photoreceptors - insights into the evolution of vertebrate opsins, vision and circadian rhythmicity

“…More specifically Otx was shown to be expressed in Row1 photoreceptors and in pigment cells; Pax4/6 was detected in Row1 photoreceptors and Row3 and 4 cells, Rx is probably expressed in Row1 photoreceptors and later in developing Row3 and Row4 cells and Pax2/5/8 and Mitf expression was documented in pigment cells (Fig. 3) (Glardon et al, 1998;Kozmik et al, 1999;Vopalensky et al, 2012;Williams and Holland, 1996). Otx and Pax4/6 expression Vopalensky et al, (2012) (Lacalli, 1996).…”

“…Amphioxus orthologues of transcription factors important for development of vertebrate retinal neurons and retinal pigmented epithelium (RPE) were found to be expressed in the developing FE (Vopalensky et al, 2012). Molecular analysis uncovered distinct gene expression fingerprints for pigmented cells of the FE, for putative FE photoreceptors (Row1 cells) and for FE interneurons (Rows2-4).…”

The origin and evolution of chordate nervous systems

“…Additional evidence for a diencephalic homologue is the presence of the lamellar body, which has the same fine structure as the pineal in a larval lamprey [28,29]. In addition, consistent with the anterior tip of the amphioxus CNS being diencephalic, in both larvae and adults, the anterior-most part of the CNS includes a photoreceptor or frontal eye, which has been proposed to be homologous to the vertebrate paired eyes [22,29,30]. Evidence from gene expression indicates that amphioxus also has parts of the genetic mechanisms that specify three organizing centres in the vertebrate brain: the anterior neural ridge (ANR) zona limitans intrathalamica (ZLI) and the midbrain/ hindbrain boundary (isthmic organizer) (MHB/ISO).…”