title: Evolution and functional characterisation of melanopsins in a deep-sea chimaera (elephant shark, Callorhinchus milii).
abstract: Non-visual photoreception in mammals is primarily mediated by two splice variants that derive from a single melanopsin (OPN4M) gene, whose expression is restricted to a subset of retinal ganglion cells. Physiologically, this sensory system regulates the photoentrainment of many biological rhythms, such as sleep via the melatonin endocrine system and pupil constriction. By contrast, melanopsin exists as two distinct lineages in non-mammals, opn4m and opn4x, and is broadly expressed in a wide range of tissue types, including the eye, brain, pineal gland and skin. Despite these findings, the evolution and function of melanopsin in early vertebrates are largely unknown. We, therefore, investigated the complement of opn4 classes present in the genome of a model deep-sea cartilaginous species, the elephant shark (Callorhinchus milii), as a representative vertebrate that resides at the base of the gnathostome (jawed vertebrate) lineage. We reveal that three melanopsin genes, opn4m1, opn4m2 and opn4x, are expressed in multiple tissues of the elephant shark. The two opn4m genes are likely to have arisen as a result of a lineage-specific duplication, whereas "long" and "short" splice variants are generated from a single opn4x gene. By using a heterologous expression system, we suggest that these genes encode functional photopigments that exhibit both "invertebrate-like" bistable and classical "vertebrate-like" monostable biochemical characteristics. We discuss the evolution and function of these melanopsin pigments within the context of the diverse photic and ecological environments inhabited by this chimaerid holocephalan, as well as the origin of non-visual sensory systems in early vertebrates.
authors.affiliation: School of Animal Biology, University of Western Australia Oceans Institute and Lions Eye Institute, University of Western Australia, Perth, Western Australia, Australia.
authors.forename: Wayne I L
authors.initials: WI
authors.lastname: Davies
authors.forename: Boon-Hui
authors.initials: BH
authors.lastname: Tay
authors.forename: Lei
authors.initials: L
authors.lastname: Zheng
authors.forename: Janine A
authors.initials: JA
authors.lastname: Danks
authors.forename: Sydney
authors.initials: S
authors.lastname: Brenner
authors.forename: Russell G
authors.initials: RG
authors.lastname: Foster
authors.forename: Shaun P
authors.initials: SP
authors.lastname: Collin
authors.forename: Mark W
authors.initials: MW
authors.lastname: Hankins
authors.forename: Byrappa
authors.initials: B
authors.lastname: Venkatesh
authors.forename: David M
authors.initials: DM
authors.lastname: Hunt
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D005075:Biological Evolution; D002678:Chimera; D017931:DNA Primers; D008969:Molecular Sequence Data; D010802:Phylogeny; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017299:Rod Opsins / Q000737:chemistry / Q000235:genetics* / Q000502:physiology*; D017386:Sequence Homology, Amino Acid; D012754:Sharks / Q000235:genetics / Q000502:physiology*; D000099210:Melanopsin
chemical_list: D017931:DNA Primers; D017299:Rod Opsins; D000099210:Melanopsin