Susceptibility Variation Against Ectropis Obliqua Nucleopolyhedrovirus And Genetic Variation In Geographic Populations Of Tea Geometrid, Ectropis Obliqua Prout

Ectropis obliqua Prout (Lepidoptera: Geometridae) is one of the most important pest of tea plant, and is widely distriuted at the major tea producing areas in China. It severely damaged the yield and economic income of tea production, because of its large number of reproduction in Jiangsu, Zhejiang and Anhui Province. Ectropis obliqua Nucleopolyhedrovirus (EoNPV) is a kind of pathogenic natural enemy of E. obliqua. As an efficient biological insecticide, which has host specificity, no residue and non-toxic to non-host, EoNPV has been widely used in tea geometrid control. However, significant susceptibility to EoNPV was observed in some tea gardens recent years, and the mortality rate of E. obliqua larvae was lower than other tea producing areas. Therefore, studies on genetic variation and phylogeny of E. obliqua geographic populations were conducted to clarify the reason why this phenomnen exist, and it may provide scientific basis for integrated control of E. obliqua. In this study, laboratory bioassay of EoNPV virulence, phylogenetic analysis based on mitochondrial DNA-COâ… and COâ…¡gene sequences among 11 geographic populations, and two populations'hybridization and backcross test were conducted respectively to find out the relationship between susceptibility and geographic population genetic differentiation. The main resules were as follows:(1) There was a significant differentiation of susceptibility to EoNPV in 11 geographic populations of E. obliqua, and the largest rate of LC50 was 724.5. The results of laboratory bioassay experiment showed that there were 8 geographic populations which had shown a series of resistance degree to EoNPV. Compared with the susceptible population, EoNPV virulence ratio of the resistant population reached up to 724.5-fold, and the LT50 prolonged at least 5 days. However, susceptibility variation and geographic distribution of E. obliqua were not that related. Populations lived in the same province (e.g. HZ v.s QZ) may have huge differentiation in susceptibility, while some populations with long distance (e.g. YZ v.s WH) may share the same degree of resistance to EoNPV.(2) Based on the phylogenetic analysis of COâ… and COâ…¡gene sequences, genetic variations and divergence exist among 11 geographic populations of E. obliqua.The analysis of sequence contents indicated that average A+T content of COâ… and COâ…¡gene sequences in E. obliqua were 70.5% and 74.5%, showing a strong A+T bias. Among the nucleotides substitution of COâ… and COâ…¡gene, transition number was much more than transversion number, and substitutions mostly happened in the 3rd codon position, which made more synonymous substitutions and less significant amino acid variation. Some population show a high haplotype diversity and nucleotide diversity level. There were 31 haplotypes about COâ… gene and 11 haplotypes about COâ…¡gene among 86 specimens belonged to 11 geographic populations. Phylogenetic trees constructed by Neighbor-joining method and Maximum parsimony method were similar-topologies were alike with small differences of bootstrap value on tree branches. The analysis of phylogenetic trees indicated that all haplotypes clustered in 2 groups, with 5% sequence difference between them. Group 1 included all susceptible populations (HZ, YX and SZP), while other haplotypes belonged to 8 resistant populations clustered together.(3) There was an isolation between Hangzhou population and Quzhou population which had a largest differentiation of susceptibility to EoNPV.In this study, we used two populations (HZ and QZ) which have a large difference of susceptibility to EoNPV as parents. The statistic analysis of hybridization and backcross test suggested that the 1st hybrid offspring was weaker than parents at quantity and quality of oviposition, hatching rate, larvae duration, pupal weight and emerging rate, the viability was much lower than control. It also showed a distorted sex ratio as 1: 0.09 and 1: 52.5. What's more, the hybrids (F1) was sterility. An isolation had emerged between HZ and QZ populations yet, and this may be a beneficial evidence to classify and define the phylogeny of E. obliqua gographic population. We deduced that those two populations were probably two species, and the susceptibility differentiation may be caused by this specific divergence.