THE MOLECULAR-BASIS FOR UV VISION IN BIRDS - SPECTRAL CHARACTERISTICS, CDNA SEQUENCE AND RETINAL LOCALIZATION OF THE UV-SENSITIVE VISUAL PIGMENT OF THE BUDGERIGAR (MELOPSITTACUS-UNDULATUS)

Microspectrophotometric (msp) studies have shown that the colour-visio n system of many bird species is based on four pigments with absorptio n peaks in the red, green, blue and UV regions of the spectrum. The ex istence of a fourth pigment (UV) is the major difference between the t richromacy of humans and the tetrachromacy of such birds, and recent s tudies have shown that it may play a determining role in such diverse aspects of behaviour as mate selection and detection of food. Avian vi sual pigments are composed of an opsin protein covalently bound via a Schiff-base linkage to the chromophore 11-cis-retinal. Here we report the cDNA sequence of a UV opsin isolated from an avian species, Melops ittacus undulatus (budgerigar or small parakeet). This sequence has be en expressed using the recombinant baculovirus system; the pigment gen erated from the expressed protein on addition of 11-cis-retinal yielde d an absorption spectrum typical of a UV photopigment, with lambda(max ) 365 +/- 3 nm. This is the first UV opsin from an avian species to be sequenced and expressed in a heterologous system. In situ hybridizati on of this sequence to budgerigar retinas selectively labelled a sub-s et of UV cones, representing approx. 9 % of the total cone population, that are distributed in a semi-regular pattern across the entire reti na.