Population Diversity And Genetic Differentiation Of Heterodera Glycines

The soybean cyst nematode (SCN), Heterodera glycines, has been a major pathogen of soybean, which is hard to control and can cause serious yield reduction and great economic losses in China and worldwide. This pest is tiny, well-adapted, with a relatively broad host range and geographical distribution, and with obvious host specificity. Population diversity and genetic differentiation of SCN were generated by multiple factors including geographical environment, climate, host and human actions etc. during the natural selection and evolution progress. The study on physiological differentiation and genetic diversity of SCN will help to explain the mechanism of genetic differentiation and shed more light on understanding of its population evolution and provide theoretical basis and pratical significance for pathogen control.This study used physiology and biochemistry and various molecular biology methods to analyze population diversity and genetic variation of SCN’s geograpincal and host populations (virulence phenotypes), ascertain the phylogenetic relationships from genus-species to population by comparing morphological and molecular characters of other cyst nematode species in China. The results were as following:1. SCN distribution and population detection in China:According to National Soybean Industrial Technology System, a survey for SCN was undertaken in 150 demonstration counties of 30 comprehensive experimental stations in China during 2012-2014 and its population density was detected. The result shows SCN occurs in 20 provinces of China at present:Anhui, Gansu, Guangxi, Guizhou, Hebei, Henan, Heilongjiang, Jilin, Jiangsu, Jiangxi, Liaoning, Neimenggu, Ningxia, Shandong, Shanxi, Shananxi, Sichuan, Xinjiang, Yunnan and Zhejiang Province, which the total incidence of SCN was 69.4%.2. Comparing with SCN races and HG types:With the race determination test and HG typing method, ten SCN populations were identified by standard indicator lines. Total, eight HG types were first reported in China, which helps to understanding the relationship between races and HG types.3. Population diversity of cyst-forming nematodes in China:With morphological and molecular methods, seven cyst nematode species from two genera were identified, which referred to 2 new records,2 novel disease and new distributions of several cyst nematode species in China. An comparative analysis of rDNA among speices and populations of cyst nematodes on major crop was made by PCR-RFLP. Relative high genetic diveersity was found in SCN intra-species by digesting ITS- rDNA regions of SCN populations with Aval. Based on ITS and 28S D2D3 regions of rDNA, phylogenetic trees were constructed to analyze phylogenetic relationships among species and populations of cyst-forming nematodes.4. Genetic differentiation on mtDNA COI and CLE genes among SCN populations:A genetic polymorphism test on 318 individuals of 16 SCN populations with mtDNA COI gene was used to analyse population genetic structure and gene flows of SCN. The results reveals genetic differentiation among SCN populations whereas gene flows also occur; Genetic variability was mainly observed within populations, not from inter-populaion or inter-group; Geographical distance is not considered as a major reason for genetic differentiation. which is due to passive transport of cysts by human activities, water or wind. Genetic variation of SCN populations was analysed with parasitism gene CLE to reveal the correlation of gene polymorphism and pathogenicity (virulence). The results indicates Hg-cle-2 gene polymorphism occurrs witnin and among populations and SX population is different from others due to high nuleotide polymorphism and four clones as a separate clade in phylogenetic trees.5. Chorismate mutase (CM) gene and pathogenicity of SCN:Two H. glycines CM gene, Hg-cm-1 and Hg-cm-3, was found in this study. Hg-cm-3, with a shorer full-length and coding region sequence, is considered as a novel CM gene, which has homology to Hg-cm-1 but loss of 408 bp in coding sequence. The research shows Hg-cm-3 was only detected in SCN population parasiting tobacco but not in SCN populations on soybean, which provide a good evidence that a correlation occurs between CM gene variation and host adaptability or pathogenicity. Hg-cm-1 only existed in SCN populations which infected soybean, and three types (Hg-cm-1C、Hg-cm-1D and Hg-cm-1E) were found besides Hg-cm-1A and Hg-cm-1B that have been reported, which reveals virulence of SCN has high diversity and complexity.6. Comparing with parasite fitness between SCN race 3 and race 4:the results reveal race 3 with more eggs and hatched J2s per single cyst in average has a relatively higher parasite fitness on susceptible hosts than race 4 whereas difference in cyst numbers is non-significant. Therefore, a fitness cost may be associated with the virulence of SCN populations, whose reproductive ability would weaken with high virulence.