Intron splice sites of Papilio glaucus PglRh3 corroborate insect opsin phylogeny

Full-length cDNA clones encoding the PglRh3 opsin from the tiger swallowtail butterfly Papilio glaucus were isolated from cDNA synthesized from adult head tissue total RNA. This cDNA consists of 1679 nucleotides and contains a single open reading frame predicted to be 379 amino acids in length. PCR amplification of genomic DNA with primers spanning the coding region yielded a single 2760bp fragment which was sequenced. The PglRh3 gene has nine exons and eight introns, four of which are in unique locations relative to the positions of introns in other known insect opsin sequences. Phylogenetic analyses of amino acid and nucleotide sequence data places PglRh3 within a clade of insect visual pigments thought to be sensitive to long wavelengths of light. The genomic structure of PglRh3 is the first characterized from a member of this opsin clade. Three PglRh3 intron positions are shared with Drosophila Rh1, and one of these is also shared with Drosophila Rh2. By contrast, none of the known intron locations in a clade of anciently diverged ultraviolet- and blue-sensitive visual pigments are shared by P. glaucus PglRh3, Drosophila Rh1 or Rh2. The placement of introns within opsin genes therefore independently supports the clustering of a putatively long-wavelength-sensitive clade with a clade of blue-green-sensitive visual pigments.