Diversity and geographic distribution of secondary endosymbiotic bacteria in natural populations of the pea aphid, Acyrthosiphon pisum

In addition to the essential intracellular symbiotic bacterium Buchnera , several facultative endosymbiotic bacteria called collectively secondary symbionts (S-symbionts) have been identified from the pea aphid Acyrthosiphon pisum . We conducted an extensive and systematic survey of S-symbionts in Japanese local populations of A. pisum using a specific PCR detection technique. Five S-symbionts of A. pisum , PASS, PAUS, PABS, Rickettsia and Spiroplasma , and two facultative endosymbionts universally found in various insects, Wolbachia and Arsenophonus , were targeted. Of 119 isofemale strains originating from 81 localities, 66.4% of the strains possessed either of four S-symbionts: PASS (38.7%); PAUS (16.0%); Rickettsia (8.4%); and Spiroplasma (3.4%), while 33.6% of the strains contained only Buchnera . PABS, Wolbachia and Arsenophonus were not detected from the Japanese strains of A. pisum . In order to understand intra- and interpopulational diversity of S-symbiont microbiota in detail, 858 insects collected from 43 localities were examined for infection with the four S-symbionts. It was demonstrated that different S-symbionts coexist commonly in the same local populations, but double infections with two S-symbionts were rarely detected. Notably, the S-symbionts exhibited characteristic geographical distribution patterns: PASS at high frequencies all over Japan; PAUS at high frequencies mainly in the northeastern part of Japan; and Rickettsia and Spiroplasma at low frequencies sporadically in the southwestern part of Japan. These results indicate that the geographical distribution and infection frequency of the S-symbionts, in particular PAUS, might be affected by environmental and/or historical factors. Statistical analyses suggested that the distribution of PAUS infection might be related to host plant species, temperature and precipitation.