CHARACTERIZATION OF THE REFLECTIVE MATERIALS AND ORGANELLES IN THE BRIGHT IRIDES OF NORTH-AMERICAN BLACKBIRDS (ICTERINAE)

The reflective materials in the iris stroma of bright-irised American blackbirds (Icterinae, Emberizidae) and the red-eyed vireo (Vireo oliv aceus) (Vireonidae) were characterized using high-performance liquid c hromatography (HPLC) and diode-array detection. Two purines, guanine a nd hypoxanthine, and two pteridines, leucopterin and xanthopterin, wer e detected in large amounts in all bright irides. The brown iris of th e red-winged blackbird (Agelaius phoeniceus) by comparison contained o nly small amounts of these and additional unidentified compounds. The absolute and relative amounts of light-absorbing compounds in the iris varied somewhat among species of blackbirds with bright irides, and m arkedly within one species (brewer's blackbird, Euphagus cyanocephalus ) between sexes and age classes that vary in eye color. Differences in the types, numbers, and sizes of pigment organelles in the irides app eared to underlie the differences in amounts of light-absorbing compou nds. Guanine was the most abundant light-absorbing compound in all bri ght irides, accounting for about 90% of the total absorption at 250 nm . A wide range of concentrations of guanine, from 96 to 9 mu g per iri s, produced bright irides. The primary pigment organelles of pigment c ells in bright irides were reflecting platelets, which typically appea red as open spaces on electron micrographs. In the red-eyed vireo ther e were in addition red pterinosome-like pigment organelles in the pigm ent cells on the anterior surface of the iris stroma. Guanine was pres ent even in irides with no overt reflecting platelets.