Most aphids (Hemiptera: Aphididae), like many homopteran

insects, feed on the phloem sap of plants (Pollard, 1973), which is a diet rich in sugars and usually dominated by sucrose (Ziegler, 1975). The main nitrogen sources of aphids are free amino acids of unbalanced composition, with only very low concentrations of most or all of the essential amino acids that insects and other animals cannot synthesize (Douglas, 1993; Girousse et al., 1991; Rahbé et al., 1990). The endosymbiotic bacteria of aphids, assigned to the genus Buchnera in the gamma-subdivision of Proteobacteria by Munson et al. (1991), have long been postulated to complement the aphid’s diet by supplying it with these essential amino acids (see Houk and Griffiths, 1980). The nutritional dependency of aphids upon Buchnera was first studied by maintaining aphids containing their bacteria, or experimentally deprived of their bacteria through the use of antibiotics (called symbiotic and aposymbiotic aphids, respectively), on defined artificial diets from which individual amino acids were omitted (Dadd and Krieger, 1968; Mittler, 1971). In recent years, molecular analyses of the genome of Buchnera have permitted the identification of almost 100 genes from these endosymbionts (Baumann et al., 1995, 1998). Among these, nearly 25 % were genes encoding enzymes in amino acid biosynthetic pathways, mostly essential amino acids. There are, however, very fe