Population Genetic Structure Of Blumeria Graminis F.sp. Tritici From Major Epidemic Zones In China

Wheat powdery mildew, caused by obligate parasitic fungus Blumeria graminis f. sp. tritici （Bgt）,is a worldwide wheat disease. In China, this disease is recurrent with the largest area of incidence andthe heavy losses. The pathogen has high evolutionary potential as a result of a mixed reproductionsystem, a large population size, a wide adaptation and wind-aided dispersal mode. Analysis on the Bgtvirulence and molecular genetic structure from major epidemic zones of China, combining with airparcel trajectory to seek the evidence of pathogen long-distance dispersal, will be important to themonitoring and early forecasting of the virulence dynamics of the pathogen population and the ‘loss’ ofresistance in host, to reveal pathogen population genetic variation and the dissemination betweendifferent regions from the molecular level, and can be used to guide the disease management strategies.In this study, a collection of674single-colony wheat powdery mildew isolates were recoveredfrom samples collected in2010and2011, covering the regions of the southwest （Guizhou, Chongqing,Sichuan）, the northwest （Qinghai, Gansu, Shaanxi）, the central （Hubei, Anhui, Henan） and North China（Hebei, Beijing）. To gain more precise information about pathogen population structure, we pursued acombined pathological and genetic approach. Additionally, air parcel trajectory of the spore transportsof the pathogen were investigated by using Hysplit₄based on meteorological data in spring. The mainresults obtained are as follows:（1） Determined Bgt virulence structure from major epidemic zones of China in the two years.A total of418isolates were inoculated onto34differential wheat lines of powdery mildew. Theresults showed the virulence frequencies were nearly the same among different provinces in each yearexcept a few genes, and small changes existed between years. Over55%of the isolates were virulent toPm3a, Pm3b, Pm3c, Pm3e, Pm5a, Pm6, Pm7, Pm8, Pm17, Pm19, Pm25, and Pm34in both years. Thevirulence frequency was low to Pm12, Pm13, Pm16, and Pm21, indicating effective resistant resources.（2） Revealed a high genetic variation within populations but a lower genetic differentiation amongpopulations based on virulence factors.The estimation of virulence complexity showed each isolate was virulent to9-29Pm genes. Nei’sgenetic diversity index, Shannon’s information index, and polymorphism loci percentage were0.274,0.408,94.12%in2010, and0.268,0.399,82.35%in2011, respectively, revealing high virulencediversity. Genetic differentiation analysis from PopGen showed less than10%of the variation existedamong populations and more than90%of the variation within populations in both years. Analysis ofmolecular variance （AMOVA） and principal coordinate graph （PCA） revealed similar results, indicatinga high genetic variation within populations but a lower genetic differentiation among populations.（3） Revealed a high level of genetic diversity from the molecular level.A total of2316nucleotides from three house-keeping gene regions, Alternative oxidase （AOX）,Protein kinase A （PKA） and Protein phosphatase type2A （PPA）, for674isolates were sequenced.19variable characters were detected from multilocus sequences, comprised of7singleton variable sitesand12parsimony informative sites.45haplotypes （H1-H45） were inferred in total, with H1and H2 comprising over44.8%of the populations, and15rare haplotypes consisting of3.26%of thepopulations. The haplotype diversity for2010population was0.8592and0.9027for2011, indicating ahigh level of genetic diversity.（4） Explored molecular genetic structure of the pathogen population.Neighbor-joining phylogenetic tree and minimum spanning network based on haplotypes did notshow obvious geographical subdivision. Regression analysis indicated there was a certain degree ofgeographic differentiation among populations. AMOVA analysis revealed92.44%of the total variationwithin populations in2010and73.01%of that in2011, suggesting distance isolation was not the mainfactor causing Bgt population genetic differentiation and there was difference between years. Mismatchdistributions of multilocus sequences presented a bimodal fashion, and Tajima’s D value （-0.84732） andFu’s Fs value （-40.768） were not significant （P>0.05）, implying that there might not be a populationexpansion in recent time and mildew populations was at demographic equilibrium.（5） Confirmed genetic recombination and pathogen long-distance migration.5minimum recombination events were detected from variable sites, proving the existence ofsexual recombination. Most of pairwise Nm values between populations were higher than4, suggestinga frequent gene exchange among populations. The findings of shared dominant haplotypes confirmedthis. The distributions of rare haplotypes revealed local evolution. Trajectory analysis in two yearssuggested that the inoculum sources in Southwestern China mainly affect the disease epidemics inSouthwestern, Northwestern, Central, and Northern China in spring. The sources in Northwestern Chinaaffected mainly on Northwestern, Central, Northern China, and Sichuan. The sources in Central Chinamainly cover Central and Northern China. The pathogen from Northern China could be dispersed withinthe region and to Northeastern China. All of these evidences support the existence of the frequentlong-distance migration of Bgt populations among different regions in China and independent evolutionwithin the regions.（6） Clarified the center of Bgt genetic diversity.The overall evaluation of virulence and genetic data showed a higher diversity in Southwestern andNorthwestern China than that in Central and Northern China, especially in Chongqing, Sichuan, andGansu populations. Trajectory analysis of spring spore transports indicated that the inoculum sources inSouthwestern and Northwestern China can impact the disease epidemics in other regions in Chinadirectly or indirectly. In view of this, we infer that Southwestern China and Gansu are the center of Bgtgenetic diversity, and speculate they may be the origin of Bgt in China.