| The green peach aphid Myzus persicae(Sulzer)(Hemiptera: Aphididae: Myzus), is a worldwide significant pest of some Rosaceae fruit trees, many vegetables and certain herbal plants, causing great losses to agriculture. M. persicae is a classical polyphagous pest feeding on more than 400 plant species in over 50 plant families. Symbiotic microorganisms,especially bacteria, play a crucial role in host plant suitability and adaptation in M. persicae.In this study, I investigated how the dynamics of symbiont assemblages and densities affected M. persicae response of nutritional qualities in different host plants, and confirmed the potential benefits to M. persicae conferred by symbionts. This provided new insights for novel pest management strategies by targeting symbionts.I examined the diversity of symbionts in M. persicae with different morphs collected on several different host plants from many locations in China. The results showed that the partial sequences of 16 S rRNA gene of Buchnera aphidicola, the primary symbiont in all samples were almost the same. Secondary symbionts(S-symbionts) were closely related to geographical environments and reproductive modes. M. persicae collected from cabbages in Liaoning province was not infected any S-symbiont, and the parthenogenetic females collected in Guangzhou city from cabbages and tobaccos, in Beijing city from peach trees,and in Shanxi province from peach trees harbored one S-symbiont, Wolbachia, with low infection frequencies. The parthenogenetic females collected in Yangling, Shaanxi province from many host plants excepted peach trees and eggplant, were infected with one S-symbiont Serratia symbiotica with 100% infection frequencies. The parthenogenetic females collected from peach trees harbored no S-symbiont, while the samples from eggplants were co-infected with two S-symbionts S. symbiotica and Wolbachia and their infection frequencies were 100 and 80%, respectively. Different aphid morphs from peach trees were infected with various S-symbionts. The alate viviparous females, apterous viviparous females, males and oviparous females harbored no S-symbiont. Arsenophonus was detected in overwintering eggs;Wolbachia was found in fundatrigenia; and both S. symbiotica and Wolbachia were found in fundatrices. These results indicated that asexual reproduction can maintain a stable transmission of symbionts in aphids, but sexual reproduction can cause exchange ofS-symbionts or cut out one or more S-symbionts. The aphids could be infected with different S-symbionts or manage the density of S-symbionts in hosts under various selection pressures.Nitrogen quality of host plants was an important factor for aphids to infest and spread on new plant hosts. Concentration of total free amino acids(TAAs) and proportion of essential amino acids(EAAs) differed significantly among different host plants; amino acids in phloem sap were dominated by non-EAAs. Plant phloem sap was nutritionally poor and exceptionally unbalanced in amino acid composition profiles with different nitrogen nutrition qualities in different host plants. We transferred cabbage-reared M. persicae in the laboratory to three new host plants, i.e. eggplants, tobaccos, and spinaches, and the results showed that the aphids performed better on plants with higher nitrogen quality. The density of symbionts especially B.aphidicola in M. persicae was positively correlated with the nitrogen quality in phloem sap of host plants, suggesting that the population growth of B. aphidicola can be affected by the availability of nitrogen nutrients in their diets.Host plants fed by M. persicae for a week improved their nitrogen quality. Compared with those transferred to new clean host plants, M. persicae populations which were reared on four host plants decreased the amount of B. aphidicola but increased the abundance of S.symbiotica, which may be related with some benefits conferred by S. symbiotica for their host insects to adapt to the environment.The performance of M. persicae on artificial diets was closely correlated with nitrogen quality of the diets to some extent. M. persicae and Acyrthosiphon pisum with two populations of each species performed better on the diets with formulations C(normal formulation, 39 mol% of EAAs) and D(nutritionally balanced formulation, about 50% of EAAs), on which the aphids had higher accumulated molting rates, survival rates and body weights than on the diets with other formulations. M. persicae and A. pisum performed badly on the diets with formulations A and B(nutritionally poor), on which those aphids showed lower accumulated molting rates, survival rates and body weights. Dynamics of B. aphidicola in aphids on the diets with different proportions of EAAs were different in populations and aphid species, and no uniform pattern was found. We suspect that there may be distinct nutritional regulation mechanisms for symbionts in different aphid species or populations.Amino acid composition profiles in the honeydew of M. persicae reflected the composition profiles in diets. Proportions of individual amino acids in the honeydew of two aphid populations were positively correlated with those in the formulated diets. Sugars in the honeydew were predominately fructose, glucose, sucrose and trehalose, melizitose had the lowest abundance in honeydew.S. symbiontica brought benefits to host insects, i.e. increased daily fecundity. In contrast,they had an adverse impact on aphids, i.e. delayed the development of the aphids. S.symbiotica conferred different resistance levels in M. persicae to Aphelinus asychi and Aphidius gifuensis. S. symbiotica could reduce oviposition and emergence of A. asychi in M.persicae, but S. symbiotica did not seem to confer resistance to A. gifuensis. Heat shocked at37.5oC, aphids of 2-day-old infected with S. symbiotica had shorter developmental time and higher fecundity than those un-infected aphids.In conclusion, the primary symbionts, B. aphidicola in M. persicae transmitted with high fidelity and stability, and they changed the density to manipulate the nutritional adaptation of M. persicae to host plants. S-symbionts were greatly affected by environmental factors, i.e.geographical locations, host plants and reproductive modes. S. symbiotica provided some defense for host insects, including defense against A. asychi and against heat shock. These symbionts together increased the fitness of M. persicae on new host plants. |
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