Figure 12. Current model of the origins and evolution of the IRBP gene

The IRBP gene probably arose coincident with or shortly after the vertebrates diverged from the urochordates. This is the time period when the neural crest and the skull originated. No major changes are proposed in the steps resulting in the early internal quadruplication of the IRBP gene, but the time frame is now bounded, as the IRBP gene appears to be absent from the urochoradates. This model is based on Borst et al. [26] and Rajendran et al. [27]. The major change is the addition of a two-gene IRBP locus in the teleosts.