Diversity of Secondary Endosymbionts among Different Geographical Populations of the Grain Aphid, Sitobion avenae (Fabricius) (Hemiptera:Aphididae) in China

Aphids are one of the most challenging agricultural pests because they feed on many economic crops, and transmit pathogens including some plant viruses. The intimate associations between aphid and secondary bacterial symbionts are increasingly being recognised by significantly influencing host biology. However, related evidences, e.g., the relationship between the infected frequencies of symbionts and natural environments, have been gathered only from model pea aphids. Here, we screened for six secondary symbionts, Regiella insecticola, Hamiltonella defensa, Serratia symbiotica, Spiroplasma, Rickettsia and Arsenophonus , by using polymerase chain reaction among natural populations of the grain aphid, Sitobion avenae . Of 510 individuals from 21 localities covering the main Chinese wheat farming region have demonstrated that: i) Rickettsia was detected in all individuals, ii) R. insecticola and H. defensa showed infection frequencies of 54.7% and 7.2%, respectively, iii) S. symbiotica, Arsenophonus and Spiroplasma were not detected at all. There were different secondary endosymbionts in the same aphid populations, and multiple infections with the secondary endosymbionts: 6.7% with triple infections, 48.4% with double infections and 44.9% with single infection. Notably, the secondary endosymbionts exhibited characteristic geographical distribution patterns: Rickettsia exhibited high frequencies in the main Chinese wheat farming regions, but R. insecticola and H. defensa exhibited a remarkable infection cline. Geographic localities exhibited differences in various environmental factors that can affect the infection frequency of secondary symbionts. In the study, we found there was a statistically significant positive correlation between R. insecticola infection and environmental factors (i.e. mean annual temperature and mean annual precipitation).