OCCURRENCE OF CHAPERONIN-60 AND CHAPERONIN-10 IN PRIMARY AND SECONDARY BACTERIAL SYMBIONTS OF APHIDS - IMPLICATIONS FOR THE EVOLUTION OF ANENDOSYMBIOTIC SYSTEM IN APHIDS

All aphids harbor symbiotrophic prokaryotes (''primary symbionts'') in a specialized-abdominal cell, the bacteriocyte. Chaperonin 60 (Cpn60, symbionin) and chaperonin 10 (Cpn10), which are high and low molecula r weight heatshock proteins, were sought in tissues of more than 60 ap hid species. The endosymbionts were compared immunologically and histo logically. It was demonstrated that (1) there are two types of aphids in terms of the endosymbiotic system: some with only primary symbionts and others with, in addition, secondary symbionts; (2) the primary sy mbionts of various aphids are quite similar in morphology whereas the secondary symbionts vary; and (3) irrespective of the aphid species, C pn60 is abundant in both the primary and secondary symbionts, while Cp n10 is abundant in the secondary symbionts but present in small amount s in the primary ones. Based on these results, we suggest that the pri mary symbionts have been derived from a prokaryote that was acquired b y the common ancestor of aphids whereas the secondary symbionts have b een acquired by various aphids independently after divergence of the a phid species. In addition, we point out the possibility that the proka ryotes under intracellular conditions have been subject to some common evolutionary pressures. and as a result, have come to resemble cell o rganelles.