Secondary endosymbionts of psyllids have been acquired multiple times

Previous studies have established that psyllids (Hemiptera, Psylloidea) con tain primary endosymbionts, designated as Carsonella ruddii, which cospecia te with the psyllid host. This association appears to be the consequence of a single infection of a psyllid ancestor with a bacterium. Some psyllids m ay have additional secondary (S-) endosymbionts. We have cloned and sequenc ed the 16S-23S ribosomal RNA genes of seven representative psyllid S-endosy mbionts. Comparison of the S-endosymbiont phylogenetic trees with those of C. ruddii indicates a lack of congruence, a finding consistent with multipl e infections of psyllids with different precursors of the S-endosymbionts a nd/or possible horizontal transmission. Additional comparisons indicate tha t the S-endosymbionts are related to members of the Enterobacteriaceae as w ell as to several other endosymbionts and insect-associated bacteria.