Independent origins and horizontal transfer of bacterial symbionts of aphids

Many insect groups have obligate associations with primary endosymbionts: m utualistic bacteria that are maternally transmitted and derived from an anc ient infection. Often, the same insects are hosts to 'secondary' bacterial symbionts which are maternally transmitted but relatively labile within hos t lineages. To explore the dynamics of secondary symbiont associations in a phids, we characterized bacteria infecting 15 species of macrosiphine aphid s using DNA sequencing, diagnostic polymerase chain reaction (PCR), diagnos tic restriction digests, phylogenetic analyses, and electron microscopy to examine aphids from nature and from laboratory colonies. Three types of bac teria besides Buchnera were found repeatedly; all three fall within the Ent erobacteriaceae. The R-type has a 16S rDNA less than 0.1% different from th at of the secondary symbiont previously reported from Acyrthosiphon pisum a nd is related to Serratia species. The T-type includes a symbiont previousl y reported from a whitefly; the U-type comprises a new cluster near the T-t ype. The T-type was found in every one of 40 Uroleucon ambrosiae clones col lected throughout the United States. In contrast, A. pisum individuals were infected by any combination of the three symbiont types. Secondary symbion ts were maternally transmitted for 11 months within laboratory-reared A. pi sum clones and were present in sexually produced eggs. PCR screens for a ba cteriophage, APSE-1, indicated its presence in both A. pisum and LI. ambros iae containing secondary symbionts. Electron microscopy of R-type and T-typ e bacteria in A. pisum and in U; ambrosiae revealed rod-shaped organisms th at attain extremely high densities within a few bacteriocytes.