EVOLUTIONARY ASPECTS OF THE DIVERSITY OF VISUAL PIGMENT CHROMOPHORES IN THE CLASS INSECTA

In the class Insecta, three retinal congeners are used as the chromoph ore of visual pigments: retinal, (3R)-3-hydroxyretinal and (3S)-3-hydr oxyretinal. The distribution of retinal and 3-hydroxyretinal superimpo sed on the phyletic tree of insects indicates that the original chromo phore of visual pigments was retinal, and that some insects arose arou nd the end of the Carboniferous period acquired the ability to use 3-h ydroxyretinal. Xanthophylls possessing 3-hydroxy-beta-ring have been c onsidered to be precursors of 3-hydroxyretinal, and the ''oxygen pulse '' in the late Palaeozoic era is discussed as a possible contributory factor in obtaining the ability to use 3-hydroxyretinal as the visual pigment chromophore. Xanthophylls possessing 3-hydroxy-beta-ring produ ced by plants and bacteria have only the (3R)-beta-ring, so the 3-hydr oxyretinal produced directly from such xanthophylls is expected to be (3R)-3-hydroxyretinal. On investigating the absolute structure of 3-hy droxyretinal in insect compound eyes, using a chiral column, the order s Odonata, Hemiptera, Neuroptera, Coleoptera, and Lepidoptera, and sub orders Nematocera and Brachycera of the Diptera were found to have onl y (3R)-3-hydroxyretinal. The members of the dipteran suborder Cyclorrh apha, however, were found to contain a mixture of both the (3R) and (3 S)-enantiomers of all-trans 3-hydroxyretinal and (3S)-11-cis 3-hydroxy retinal. The Cyclorrhapha, which arose in the Jurassic period, have ob tained the ability to produce (3S)-3-hydroxyretinal, but the metabolic pathway by which these ''higher flies'' form (3S)-3-hydroxyretinal ha s yet to be clarified. (C) 1998 Elsevier Science Inc.