Early opsin expression in Xenopus embryos precedes photoreceptor differentiation

The visual pigment which serves as the first step in the phototransduction cycle in vertebrate rod cells consists of a retinal chromophore which is linked to the transmembrane protein, opsin. Opsin genes have been isolated from a number of different organisms and studies have shown opsin to be developmentally regulated with both mRNA and protein expression associated with the morphological differentiation of photoreceptor cells. Due to its potential utility as a marker for rod photoreceptor determination in studies of retinal tissue interactions, and because no amphibian opsin genes have as yet been cloned, we isolated cDNA clones of the Xenopus laevis opsin gene. Sequence analysis shows that within the coding region Xenopus opsin shares a high degree of identity with other rod opsin genes, except at the C-terminal where it more closely resembles the mammalian color opsins. A developmental analysis, on the other hand, reveals that Xenopus opsin transcripts are detectable in a retina-specific fashion early in retinal development. Using in situ hybridization we find that Xenopus opsin mRNA is initially restricted to a few isolated cells in the presumptive photoreceptor layer which express the gene at relatively high levels. This suggests that rod photoreceptor determination occurs in single cells, and that the mechanisms controlling opsin expression in Xenopus are initiated well before any evidence of morphological differentiation.