GENETIC DIVERSITY AND FACULTATIVE SYMBIONTS INFECTION PATTERNS OF TWO RHOPALOSIPHUM APHIDS

Aphids, during the long-term evolution, kept their genetic traits for continuation of the species while some variations caused by selection, migration, mutation, and genetic drift occurred meanwhile to adapt to novel environments. The differentiation takes place not only among species but could also be observed within species that finally contributes to the population genetic diversity. However, the migrant behavior of aphid can enhances the gene flow among populations, as a result, decreasing the genetic variation. Additionally, insect such as aphids usually hold the mutualistic relationships with endosymbionts which may affect the adaptive characteristics of the host aphid. Both Corn leaf aphid Rhopalosiphum maidis (F.) (Hemiptera: Aphididae) and Bird cherry-oat aphid Rhopalosiphum padi (L.) (Hemiptera: Aphididae) are worldwidely severe pests that attack cereal crops such as maize, barley, wheat, sorghum and oats. They suck the phloem and can transmit viruses including Maize dwarf mosaic virus (MDMV) and Barley yellow dwarf virus (BYDV), the main virus of maize and barley, which may result in serious damage for the yield and quality of the host plant. Moreover, one aphid species usually exerts genetic differentiation among populations which is related to factors such as geographic conditions, host plants and life history. Hence, it is necessary to have a clear understanding of the genetic structure and variation among geographic populations to further explore the historical expanding routes and occurrence of these pests and design efficient control strategies. Targeting the genetic divergence among populations of two Rhopalosiphum aphids, 32 populations of R. padi and 38 populations of R. maidis were collected on maize from most regions of China as well as some countries in Europe. The sequencing results of two mitochondrial (COI and COII) and one nuclear (EF-1α) genes indicated that a high level of gene flow existed among the populations of R. maidis in terms of geographic locations, proving the migratory capacity of this aphid. With regard to R. padi, high genetic diversity were found between the populations from China and Europe based on combined COI-COII, however, the genetic diversification among Chinese populations and European populations were low, which illustrated that the length of geographic distance may play an important role in genetic diversity. The infection patterns of seven facultative symbionts (Serratia symbiotica, Hamiltonella defensa, Regiella insecticola, Rickettsia, Spiroplasma, Wolbachia and Arsenophonus) in R. padi and R. maidis were investigated as well. Both aphids were found to have wide symbiotic associations with variant symbionts. Unlike Acyrthosiphum pisim (H.) which is frequently infected with S. symbiotica, R. insecticola, Rickettsia and Spiroplasma were dominant facultative symbionts in these two Rhopalosiphum aphids. The results also indicated that the geographic conditions such as latitude of collecting locations may have an effect on the distribution of facultative symbionts, implying the adaptive function of the facultative symbionts to various environmental conditions on their host aphids. Considering the population diversity within species, we also tested the survival and reproduction rates of two R. maidis populations (i.e. Beijing and Mangshi) on barley and maize of small (10 cm) and high (50 cm) size. Direct observation method was used and none of the populations could survive on maize of small size whereas they were capable to develop on maize of high size, but with much lower growing speeds compared with barley on which both populations showed the best performances. In addition, Beijing population was found to perform significantly better than Mangshi population which may be related to the infection of facultative symbionts. We hypothesize that the lethality on small maize may result from the secondary metabolites synthesized to be more abundant in young maize seedlings