On the functional significance of symbiotic microorganisms in the Homoptera: a comparative study of Acyrthosiphon pisum and Nilaparvata lugens

All phloem-feeding Homoptera possess symbiotic microorganisms. Although the phylogenetic position and anatomical location of the micro- organisms diff er, the underlying theme of the symbiosis is the same; the microorganisms i mprove the nutritional quality of the diet through the provision of essenti al amino acids. The symbiosis has been well documented in aphids, but littl e information is available from other homopteran groups. The impact of the loss of bacterial symbionts in the pea aphid Acyrthosiphon pisum Harris and eukaryotic yeast-like symbionts in the Asian rice brown planthopper Nilapa rvata lugens Stal was examined in parallel. The weight and relative growth rate of aphids and planthoppers was significantly reduced by symbiont loss, and characteristic features of aposymbiotic pea aphids, so-called 'metabol ic signatures', were, for the first time, observed in aposymbiotic N. lugen s. For example, the amount of protein per unit fresh weight was reduced by 26 and 10%, and the free amino acid levels increased 1.8- and 1.4-fold, in aposymbiotic A. pisum and N. lugens, respectively. In addition, the concent ration of the amino acid glutamine was elevated in the tissues of aposymbio tic insects. The data are discussed in the context of our current understan ding of the nutritional role of the symbiosis and the mechanisms of nitroge n metabolism in the two insect species. It is concluded that the metabolic adjustments of the insects to symbiont loss are broadly equivalent.