Study Of Mitochondrial Genome And Rapid Adaptive Evolution Of Nilaparvata Lugens

Brown planthopper(Nilaparvata lugens Stal) (Hemiptera:Delphacidae) is economically important pest on rice that feed directly or serve as vectors of pathogenic microorganisms and viruses to crops, resulting in significant damage and yield losses for farmers. In recent decades, as the increased resistant rice varieties, N. lugens has become notorious due to the emergence of new biotypes that can overcome host resistance by adaptive evolution. The variations and adaptive evolution in virulent biotype populations of N. lugens and closely related insect species in Nilaparvata are revealed by studying in molecular evolution, which will be useful for developing more suitable method to control this pest.Animal mitochondrial genome is usually used for determining phylogenetic relationships and general evolutionary events because of its small genome size, stable gene content, uniparental inheritance, lack of extensive recombination and the accelerated rate of nucleotide substitution. In this work, complete mitochondrial (mt) genomes of five insect populations of N. lugens (four feeding on cultivated rice and one on the weed Leersia), and one population each of N. bakeri and N. muiri, were first sequenced and analyzed. We compared the mitochondrial genomes of Nilaparvata with other sequenced mitochondrial genomes in Hemiptera to explore the evolutionary traits and intra-and interspecific variation in mitochondrial genomes. In addition, the phylogenetic relationship of taxa with complete mitochondrial genomes from Hemiptera was reconstructed by concatenated DNA and amino acid sequences of mitochondrial protein-coding genes downloaded from GenBank.Mitochondrial genome of Nilaparvata contain a standard set of 13 protein coding genes (PCGs),22 transfer RNA genes,2 ribosomal RNA genes and a control region, which is different from the closely related Laodelphax striatellus by the gene rearrangement of trnHis. The nucleotide composition of Nilaparvata mitochondrial sequence is biased toward adenine and thymine, and the amino acid composition is affected to a similar degree by the bias to AT. The data above has shown that the GC-* AT evolutionary pressure is really strong. All protein coding genes of Nilaparvata mitochondrial genomes start with a typical ATN codon; and majority of the 13 protein coding genes have a complete termination codon (TAA or TAG), while several protein coding genes use incomplete termination codon T. There are obvious biases in both codon and amino acid usage in Nilaparvata mitochondrial protein coding genes. NNT and NNA are the most frequently used codons. Leu, Phe, Ile and Ser have the highest composition of all the amino acids.We compare the seven mitochondrial genomes and find that interspecific variation in gene length, base composition, nucleotide and amino acid sequence identity, and intergenic spacer and gene overlap is much larger than intraspecific variation. From the view of mitochondrial evolution, an endopathic for the generation of various insect biotypes/populations in one species is the synonymous nucleotide substitutions in protein coding genes with a very few adaptive mutations. The accumulation and increase of adaptive nucleotide mutations in mitochondrial genome with some other changes such as start codons varying, or intergenic spacers/gene overlap happening to change in small-scale, might make a contribution to the formation of a new species. In phylogenetic analyses, five populations of N. lugens group together, then N. muiri and N. bakeri; the species from the suborder Fulgoromorpha form a monophyletic group, as is the case for the others. Additionally, Fulgoromorpha is the sister group of Heteroptera and Sternorrhyncha.Furthermore, selected nuclear genes of the seven populations of Nilaparvata were sequenced and analyzed together with mitochondrial protein coding genes. The results showed that Nilaparvata which had shifted host plant to cultivated rice, when compared to wild natural Nilaparvata populations feeding on Leersia, accumulated more nonsynonymous nucleotide substitutions relative to synonymous substitutions in genes associated with oxidative phosphorylation, metabolism and detoxification. The positive selection acting on these genes is associated with the alteration of the Nilaparvata host plant to cultivated rice. Moreover, host-shifted N. lugens feeding on rice with resistance genes showed an increased rate in nucleotide substitutions within shorter years in comparison with wild N. lugens population, revealing rapid evolution of mitochondrial and nuclear genes in N. lugens in response to cultivated resistant rice. Our findings provide molecular evidence for plant-insect coevolution from the perspective of insect adaptive evolution, and facilitate insight into the mechanisms of host adaptation, species diversification and race formation in agricultural insects.