Correlated Expression of the Opsin Retrogene LWS-R and its Host Gene in Two Poeciliid Fishes.

The important role of retrogenes in genome evolution and species differentiation is becoming increasingly accepted. One synapomorphy among cyprinodontoid fish is a retrotransposed version of a long-wavelength sensitive (LWS) opsin gene, LWS-R, within an intron of the gephyrin (GPHN) gene. These two genes display opposing orientations. It had been speculated that LWS-R hijacks the cis-regulatory elements of GPHN for transcription, but whether their expression is correlated had remained unclear. Here, in silico predictions identified putative promoters upstream of the translation start site of LWS-R, indicating that its transcription is driven by its own promoter rather than by the GPHN promoter. However, consistent expression ratios of LWS-R:GPHN in the eyeball and brain of fishes indicate that the respective gene transcriptions are correlated. Co-expression is potentially modulated by histone exchange during GPHN transcription. Two isoforms were detected in this study, i.e., intron-free and intron-retaining. Intron-free LWS-R was only expressed in the eyeball of fishes, whereas intron-retaining LWS-R occurred in both eyeball and brain. Expression of vision-associated LWS-R beyond the eyeball supports that it is co-expressed with more ubiquitous GPHN.