Polyol metabolism in homopterans at high temperatures:: accumulation of mannitol in aphids (Aphididae: Homoptera) and sorbitol in whiteflies (Aleyrodidae: Homoptera)

Examination of four species of aphid and four of whitefly showed that mannitol was present in each species of aphid, while sorbitol was present in the whitefly species. In the cotton aphid, Aphis gossypii Glover, the total body content of mannitol was considerably higher at noon than during the early morning. A similar increase in the sorbitol content of the silverleaf whitefly, Bemisia argentifolii Bellows and Perring, was also demonstrated. In both species, polyol synthesis is stimulated by elevated temperatures. Enzyme assays were used to show that fructose is the substrate for mannitol synthesis in A. gossypii. The enzyme catalyzing this reaction, an NADP(H)-dependent ketose reductase/mannitol dehydrogenase, is analogous to the NADP(H)-dependent ketose reductase/sorbitol dehydrogenase that produces sorbitol in whiteflies. Western blot analysis verified that A. gossypii does not contain a protein that cross-reacts with antibodies against B. argentifolli NADP(H)-dependent ketose reductase/sorbitol dehydrogenase, whereas the greenhouse whitefly, Trialeurodes vaporariorum Westwood, does. Analysis of sugars in honeydew from aphids and whiteflies showed that the sugars in the excrement from these insects are very different from the sugars present in their bodies. Only small amounts of mannitol and sorbitol are excreted in the honeydew from these insects. Sorbitol accumulation provides a mechanism for thermo- and osmoprotection in whiteflies. Mannitol appears to function in a similar capacity in aphids. (C) 1998 Elsevier Science Inc. All rights reserved.