The Genomic Analysis Reveals The Symbiotic Relationship Between The Brown Planthopper And Its Endosymbionts

Insects have an intimate symbiotic relationship with microbes. Insect host can offer a stable inche for endosymbionts and endosymbionts can provide essentional nutritions for insect host or promote the capacity of insect host to adapt to environment conversely. These two live together and co-evolve. The brown planthopper (BPH)(Nilaparvata lugens) is one of the most serious insect pests of rice in China and Southeast Asia. Similar to most other sap-sucking insects belong to Hemiptera, N. lugens has diverse endosymbionts in its body. The previous studies have demonstrated that these endosymbionts were involved in the process of nutrional metabolism of the host and were regarded as a vital factor for the virulence shift and breakout of N. lugens. However this symbiotic mechanism has not been well explained at the molecular level. In this dissertation, supported by the genomic and proteomic data, we performed a research focusing on the molecular mechanism of symbiotic relationship between N. lugens and its endosymbionts and the evolution of endosymbionts themselves. The mainly results are as follows:(1) After being filtered and assembled from the BPH genome sequencing project based on the genome sequence of the closely-related species "Arsenophonus nasoniae", the resulting sequence of "Candidatus Arsenophonus nilaparvatae" totals 2.96 Mb and is annotated to contain 2,762 protein-coding genes. Through the analysis of this genome, we confirmed that this bacterium is a typical secondary endosymbiont and found evidence of genome reduction in its secretion, defence and immune systems along the evolution.(2) We successfully purified the yeast-like symbiont (YLS) from N. lugens and performed genome sequencing of the YLS using both 454 and Illumina approaches, generating a 26.8M draft genome which contains 7,155 protein-coding genes. Using the genomic data, we performed the phylogenomic analysis and confirmed the evolutional trace and divergence time of the YLS in N. lugens. Through the further analysis of this genome, we found genome redunction of infection related and sexual genes and explained the evolutional model of symbiotic fungi which is different from that of symbiotic bacteria. In addition, we denominated this YLS as "Entomomyces delphacidicola str. NLU".(3) Based on triple genomic annotations of N. lugens, symbiotic fungi and symbiotic bacteria, we found that the YLS contains all genes that needed in the biosynthesis pathway of ten essential amino acids and N. lugens can collaborate complementarily with YLS to accomplish the recycling of uric acid. Meanwhile, N. lugens also can utilize the intermediate of ergosterol biosynthesis pathway of YLS to produce cholesterol to meet its need for growth and development. Moreover, the symbiotic bacteria contain almost all genes needed for B vitamins biosynthesis, which demonstrated that the Arsenophonus bacteria might provide vitamins for N. lugens.(4) Combining the genomic data and mass spectrum, we successfully identified three putative cell wall proteins of YLS and cloned the corresponding gene sequences. However, we cannot obtain any interactive proteins of N. lugens using these three cell wall protein as baits adopting yeast-two-hybridization technology. Meanwhile, we also identified many chaperon proteins using mass spectrum and speculated that these proteins might play important roles in the symbiotic relationship.