Heath, Lesley Annette; (1996) The molecular biology of avian visual pigments: Evolution and spectral tuning. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Avian vision is more complex than mammalian vision. The most complex mammalian systems, for example in old world primates, have three classes of cone, which mediate daylight vision and a single class of rod, which mediates low light vision Birds, however, often have four cone pigments alongside the rod system and sensitivity which extends into the ultraviolet. The makes avian vision very interesting, both to study the spectral tuning - looking at which amino acids cause variations in the spectral sensitivity of pigments - and the evolution of visual pigments. Previously it has been shown that the rod and green cone pigments in birds are very closely related - about 70% sequence identity - and it was suggested that rod pigments have evolved from green cone pigments (Okano et at. (1992)). I looked at the rod and green cone pigments from two othe avian species. Mallard duck (Anas platyrhynchos) and budgerigar (Mellopscittacus undulatus), to see if this pattern was repeated. The polymerase chain reaction was used to specifically amplify the central parts of the genes for the rod and green cone pigments from Mallard duck and budgerigar. These fragments were cloned and sequenced and the amino acid sequence deduced. The green cone and rod pigments are highly homologous in these avian species as well as chicken. The green pigemnt sequences were like the rod ones in almost all the major positions postulated to be important in rod function. The major difference between the rod and green cone pigments seemed to be the isoelectric point. The green cones share a basic isoelectric point with the other cone pigments, despite the overall similarity to rods, which have an acidic or neutral pl. This work extends and confirms earlier work on opsin evolution as it reinforces the theory that the rod opsins have evolved from cone opsins and adapted to work at relatively low light levels, possibly by changing their ioninc properties.

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