An aphid-borne bacterium allied to the secondary symbionts of whitefly

Bacterial 16S rDNA amplified by PCR from the pea aphid Acyrthosiphon pisum included a sequence with > 98% similarity to secondary symbionts in the whi tefly Bemisia tabaci. The 'pea aphid Bemisia-like bacterium' (PABS) and B. tabaci secondary symbionts are estimated to have diverged 17-34 million yea rs ago, a time considerably more recent than the common ancestor of aphids and whitefly and suggestive of horizontal transmission of this bacterial li neage. PABS was scored in both the gut and ovaries of aphids by PCR and ide ntified as a small rod by in situ hybridisation. PABS was not universal in pea aphids: 2/3 laboratory strains and 13/35 of field aphids were PABS-posi tive. Tt is suggested that the incidence of PABS in pea aphids is determine d by the balance between loss (processes may include occasional failure of vertical transmission and selection against PABS-positive aphids) and horiz ontal transfer between insects. (C) 2001 Federation of European Microbiolog ical Societies. Published by Elsevier Science B.V. All rights reserved.