In Teleost fish examined to date the ocular rod opsin gene, rho, is intronless, 
unlike the rod opsin genes of other vertebrate classes which possess a five 
exon/four intron structure. We have examined in silico the structure of rho 
(which is expressed uniquely in the retina) and the closely related extraretinal 
rod-like opsin (exo-rhodopsin) gene, errlo (which is expressed uniquely in the 
pineal), in the puffer-fish, Fugu rubripes (Takifugu rubripes). Whilst the 
ocular rho is intronless in common with other Teleosts, the pineal errlo has the 
five exon/four intron structure common to the rod opsin gene of other 
vertebraes. A comparison of the sequence surrounding the errlo and rho loci 
indicates that the errlo locus is syntenic with RHO, the human rod opsin gene, 
rather than rho. We suggest that the intronless rho may have arisen through an 
ancient retrotransposition of a mature mRNA originating from errlo. This 
duplication event has occurred early in the evolution of the Actinopterygii 
(ray-finned fish) since the rho of the primitive Actinopterygians such as 
sturgeon, bowfin, and gar is also intronless. Since it appears that the intron 
containing errlo is the ancestral opsin gene that gave rise to the intronless 
rho in the Teleostei, errlo is therefore the true orthologue of the rod opsin 
gene in other vertebrate classes. We suggest that loss of expression of errlo in 
the retina could be related to the metabolic and physiological advantages, such 
as a reduction in splicing events during RNA processing, that may be conferred 
through possession of an additional, intronless rod opsin gene in the form of 
rho.