Effect of the Secondary Symbiont on Fitness and Relative Abundance of of Wheat Aphid, .

Bacterial symbionts associated with insects are often involved in host development and ecological fitness. In aphids, the role of these symbionts is variable and not fully understood across different host species. Here, we investigated the symbiont diversity of the grain aphid, (Takahashi), from 17 different geographical areas. Of these, two strains with the same symbiont profile, except for the presence of , were selected using PCR. The -infected strain, YX, was collected from a Yuxi wheat field in Yunnan Province, China. The -free strain, DZ, was collected from a Dezhou wheat field in Shandong Province, China. Using artificial infection with and antibiotic treatment, a -re-infected strain (DZ-H) and -significantly decreased strain (DZ-HT) were established and compared to the -free DZ strain in terms of ecological fitness. Infection with the DZ-H strain increased the fitness of , which led to increases in adult weight, percent of wingless individuals, and number of offspring. Meanwhile, decreased abundance of (DZ-HT strain) resulted in a lower adult weight and wingless aphid rate compared to the DZ-H strain. However, the indices of longevity in both the DZ-H and DZ-HT strains decreased slightly, but were not significantly different, compared to the DZ strain. Furthermore, quantitative PCR showed that the relative abundance of the primary symbiont in the DZ-H strain was significantly higher than in the DZ strain in all but the first developmental stage. These results indicate that may indirectly improve the fitness of by stimulating the proliferation of .