Wright, Mathew William; (2000) Retinal photoreceptor complement of paleognathous birds. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Studies of the cone visual pigment complement from a wide range of representatives of the vertebrate classes have provided data on the origins and evolution of vertebrate colour vision. Tetrachromatic visual systems have been identified in many diurnal birds, as well as in some reptiles and fish. However, little data are available for the paleognathous birds, a group that represents the earliest offshoot of the main avian stem and may provide an important link between the visual system of reptiles and those of the neognathous birds. Microspectrophotometry (MSP) was used to determine the absorbance spectra of both rod and cone visual pigments and oil droplets from the retinae of the ostrich (Struthio camelus) and rhea (Rhea americana). Light and fluorescence microscopy of whole fresh tissue mounts was used to determine the relative numbers of oil droplets in the retinae. Both species possess rods, double cones and four classes of single cone identified by their oil droplets. The rods have [lambda]max at about 505-nm, whereas three cone pigments were recorded with [lambda]max at about 570, 505 and 445 nm. The 570-nm pigment is located in both members of the double cones and in a class of single cone containing a red (R-type) oil droplet with ?cut at about 560-nm. The 510-nm and 445-nm cone pigments were found in populations of single cones containing yellow (Y-type) and clear (C-type) oil droplets with ?cut at about 500 and 420-nm respectively. Double cones possess a pale (P-type) droplet in the principal member and a small droplet containing low concentrations of carotenoid (A-type) in the accessory member. The fourth class of single cone contains a transparent (T- type) droplet and in the ostrich a 405-nm pigment has been characterised. The complement of visual pigments and oil droplets, and the relative ratio of cone types in the ostrich and rhea are remarkably similar to that found in neognathous birds. Nucleotide sequences from opsin genes of both the ostrich and rhea have been identified. Comparison of these sequences with those from other avian species indicates that paleognathous opsins are highly homologous to the opsins found for neognathous avian species.

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