Iris Paralogs in the takashii/suzukii Species Groups
(A) An alignment of representative Iris-A and Iris-B proteins from the takahashii/suzukii species groups is shown. Iris-A and Iris-B proteins are highly similar to each other. Notable differences include pairs of cysteine residues that are conserved in the B lineage (indicated with “B”), but not in A. The B lineage also has a shorter cytoplasmic tail and is missing several residues (PLLEK amino acid residues) that are invariant in the A lineage. In addition, an internal segment of the Iris-A protein is lost from the Iris-B protein, by virtue of this genomic sequence becoming an intron (confirmed by RT-PCR).
(B and C) Hydropathy plots of representative Iris-A and Iris-B proteins show that the overall architecture of the two proteins is largely unaffected by the differences between the two lineages.
(D) A hypothetical model for the origin of the divergent Iris-B gene starts with the tandem gene duplication. A cryptic SA site is encountered by mutation, but this can be neutrally maintained. However, the simultaneous occurrence of an SD site activates the SA site and leads to a portion of the coding exon being spliced out from the mature message. If this is deleterious, the SD-SA combination is culled out by selection. However, in rare cases, like the Iris-B gene, this could lead to a novel functional gene that is favored by selection. Subsequently, the SD and SA sites are maintained by purifying selection.