Postsymbiotic plasmid acquisition and evolution of the repA1-replicon in Buchnera aphidicola

Buchnera aphidicola is an obligate, strictly vertically transmitted. bacter ial symbiont of aphids. It supplies its host with essential amino acids, nu trients required by aphids but deficient in their diet of plant phloem sap. Several lineages of Buchnera show adaptation to their nutritional role in the form of plasmid-mediated amplification of key-genes involved in the bio synthesis of tryptophan (trpEG) and leucine (leuABCD). Phylogenetic analyse s of these plasmid-encoded functions have thus far suggested the absence of horizontal plasmid exchange among lineages of Buchnera. Here, we describe three new Buchnera plasmids. obtained from species of the aphid host famili es Lachnidae and Pemphigidae. All three plasmids belong to the repA1 family of Buchnera plasmids, which is characterized by the presence of a repA1-re plicon responsible for replication initiation. A comprehensive analysis of this family of plasmids unexpectedly revealed significantly incongruent phy logenies for different plasmid and chromosomally encoded lad. We infer from these incongruencies a case of horizontal plasmid transfer in Buchnera. Th is process may have been mediated by secondary endosymbionts, which occasio nally undergo horizontal transmission in aphids.