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 differ, the underlying theme of the symbiosis is the same; the microorganisms improve the nutritional quality of the diet through the provision of essential amino acids. The symbiosis has been well documented in aphids, but little 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 Nilaparvata 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 'metabolic signatures', were, for the first time, observed in aposymbiotic N. lugens. 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 concentration of the amino acid glutamine was elevated in the tissues of aposymbiotic insects. The data are discussed in the context of our current understanding of the nutritional role of the symbiosis and the mechanisms of nitrogen metabolism in the two insect species. It is concluded that the metabolic adjustments of the insects to symbiont loss are broadly equivalent.