Most molecular studies on the visual system in fish have been
performed on freshwater teleosts such as goldfish and zebrafish
where cones and rods appear simultaneously. Many marine fishes
have long larval phase in the upper pelagic zone before
transformation into a juvenile and a benthic life style. The
retina at the larval stages consists of only single cone cells;
later during metamorphosis double cones and rods develop. The
flatfish Atlantic halibut (Hippoglossus hippoglossus)
is a typical example of a marine species with such a two-step
retina development. In this study, we have cloned five different
opsins from Atlantic halibut larvae and juvenile retinas. Sequence
comparisons with other opsins and phylogenetic analysis show
that the five genes belong to the opsins of long-wavelength
sensitive (L); middle-wavelength sensitive, MCone
and MRod; and short-wavelength sensitive,
SBlue and SUltraviolet, respectively.
In situ hybridization analysis reveals expression in
double cone (L and MCone), single cone (SBlue
and SUltraviolet), and rod (MRod) types
of photoreceptor cells in juvenile halibut retina. The visual
system in Atlantic halibut seems therefore to have all four
types of cone photoreceptors in addition to rod photoreceptors.
This work shows for the first time molecular isolation of a
complete set of retinal visual pigment genes from a marine teleost
and describes the first cloning of an ultraviolet-sensitive
opsin type from a marine teleost.