THE TRYPTOPHAN BIOSYNTHETIC-PATHWAY OF APHID ENDOSYMBIONTS (BUCHNERA)- GENETICS AND EVOLUTION OF PLASMID-ASSOCIATED ANTHRANILATE SYNTHASE (TRPEG) WITHIN THE APHIDIDAE

The bacterial endosymbionts (Buchnera) from the aphids Rhopabosiphum p adi, R. maidis, Schizaphis graminum, and Acyrthosiphon pisum contain t he genes for anthranilate synthase (trpEG) on plasmids made up of one or more 3.6-kb units. Anthranilate synthase is the first as well as th e rate-limiting enzyme in the tryptophan biosynthetic pathway. The amp lification of trpEG on plasmids may result in an increase of enzyme pr otein and overproduction of this essential amino acid, which is requir ed by the aphid host. The nucleotide sequence of trpEG from endosymbio nts of different species of aphids is highly conserved, as is an appro ximately 500-bp upstream DNA segment which has the characteristics of an origin of replication. Phylogenetic analyses were performed using t rpE and trpG from the endosym bionts of these four aphids as well as f rom the endosymbiont of Schlechtendalia chinensis, in which trpEG occu rs on the chromosome. The resulting phylogeny was congruent with trees derived from sequences of two chromosome-located bacterial genes (par t of trpB and 16S ribosomal DNA). In turn, trees obtained from plas mi d-borne and bacterial chromosome-borne sequences were congruent with t he tree resulting from phylogenetic analysis of three aphid mitochondr ial regions (portions of the small and large ribosomal DNA subunits, a s well as cytochrome oxidase II). Congruence of trees based on genes f rom host mitochondria and from bacteria adds to previous support for e xclusively vertical transmission of the endosymbionts within aphid lin eages. Congruence with trees based on plasmid-borne genes supports the origin of the plasmid-borne trpEG from the chromosomal genes of the s ame lineage and the absence of subsequent plasmid exchange among endos ymbionts of different species of aphids.