Figure 8. Distribution of photoreceptor opsins. Opsins have been at least partially sequenced for approximately 50 different cone photopigments. The diagram here is consistent with the topology derived from cladistic analyses of these genes (Hisatomi et al. 1996, Heath et al. 1997, Wilkie et al. 1998, Yokoyama et al. 1998), with question marks indicating relatively uncertain branch points. Note that the four reptilian cone opsins (represented here primarily by sequences from the domestic chicken, Gallus gallus, and the American anole, Anolis carolinensis) cluster together with four Teleost cone opsins (represented here primarily by the goldfish, Carassius auratus). This particular clustering (highlighted with asterisks on the chicken and goldfish opsins) indicates that the four opsins of reptiles are orthologous to four opsins of actinopterygian fish. Thus basal tetrapods bracketed by these two groups must also have had four opsins barring secondary losses. Also of note is the clustering of mammalian short-wave sensitive pigments with reptilian UV-sensitive pigments (the chicken SSWS cone has a peak sensitivity that is intermediate between the human SWS cone and the UV cones found in many other animals). The SWS/UV clustering suggests that despite the apparent paucity of UV-sensitivity in mammals, mammals were ancestrally sensitive to UV light.