The secondary endosymbiotic bacterium of the pea aphid Acyrthosiphon pisum(Insecta : Homoptera)

The secondary intracellular symbiotic bacterium (S-symbiont) of the pea aph id Acryrthosiphon pisum was investigated to determine its prevalence among strains, its phylogenetic position, its localization in the host insect, it s ultrastructure, and the cytology of the endosymbiotic system. A total of 14 aphid strains were examined, and the S-symbiont was detected in 4 Japane se strains by diagnostic PCR, Two types of eubacterial 16S ribosomal DNA se quences were identified in disymbiotic strains; one of these types was obta ined from the primary symbiont Buchnera sp,, and the other was obtained fro m the S-symbiont, In situ hybridization and electron microscopy revealed th at the S-symbiont was localized not only in the sheath cells but also in a novel type of cells, the secondary mycetocytes (S-mycetocytes), which have not been found previously in A. pisum, The size and shape of the S-symbiont cells were different when we compared the symbionts in the sheath cells an d the symbionts in the S-mycetocytes, indicating that the S-symbiont is ple omorphic under different endosymbiotic conditions. Light microscopy, electr on microscopy, and diagnostic PCR revealed unequivocally that the hemocoel is also a normal location for the S-symbiont, Occasional disordered localiz ation of S-symbionts was also observed in adult aphids, suggesting that the re has been imperfect host-symbiont coadaptation over the short history of coevolution of these organisms.