Abstract
In Japanese populations of the pea aphid Acyrthosiphon pisum, four types of facultative endosymbiotic bacteria, Serratia symbiotica, Regiella insecticola, Rickettsia sp. and Spiroplasma sp., have been identified in addition to the essential endosymbiont Buchnera aphidicola. On the vetch Vicia sativa and other leguminous plants, Aphis craccivora, Megoura crassicauda and A. pisum frequently co-occur and form mixed colonies. In this study, the endosymbiotic microbiotae in A. craccivora and M. crassicauda collected from different localities in Japan were investigated in comparison with the endosymbiotic microbiotae in sympatric samples of A. pisum. While the A. pisum samples frequently contained one or more of the four symbionts, the A. craccivora samples were infected with only Serratia and the M. crassicauda samples contained none of the facultative symbionts. When the four symbionts were artificially transferred from A. pisum to M. crassicauda by hemolymph injection, Regiella and Spiroplasma established heritable infections in the novel host while Serratia and Rickettsia did not. The Regiella infection in M. crassicauda was stably maintained with 100% fidelity for over 50 generations, whereas the Spiroplasma infection was unstable. On the basis of these results, we discussed the possible mechanisms whereby the different infection patterns with the facultative symbionts are maintained in the sympatric aphid communities.
