| Stripe rust (or yellow rust), caused by Puccinia striiformis Westend. f. sp. tritici Eriks., is one of the most important diseases of wheat (Triticum aestivum L.) worldwide. In China, stripe rust has been considered the most important disease of wheat since the first major epidemic in 1950. Gansu province, located in northwestern China, is one of the largest and the most important over-summering areas in China. The region provides inoculum to the major wheat-growing regions to the east. Almost all Chinese races of P. striiformis f. sp. tritici were first detected in the Longnan regions of Gansu, which serve as sources of new races and inoculum for rust development in the other regions of China because of their unique geographic topography and ideal environmental and cropping conditions. The objectives of this study were to (1) characterize genetic diversity of P. striiformis f. sp. tritici , (2)determine the levels of migration among different regions , (3) evaluate stability of genetic structure of P. striiformis f. sp. tritici in different seasons and years, (4) infer the pathway of pathogen spread in northwest China, used SSR marker. To determine population structures of P. striiformis f. sp. tritici , a total of 1447 isolates were studied using 11 microsatellite markers, including 270 isolates from Qinghai, 206 isolates from Linxia, 50 isolates from Pingliang and 971 isolates from Longnan, in which two provinces and 20 countries. The main results are summarized as following:(1) To determine the relativity between virulence and DNA polymorphism, a total of 90 isolates of Puccinia striiformis f. sp. tritici were studied. 14 pathotypes and 75 genotypes were found, suggesting DNA polymorphism was much more diversity than virulence. Moreover, there were the same or near genotype in different races, and the same race may be had different genotype. Therefore, there was no relativity between virulence and DNA polymorphism.(2) To determine the number of isolates for a population or subpopulation, we analyzed the effect of number of isolates to population genetic structure of P. striiformis f. sp. tritici. The result showed that population genetic structure was untrue when isolates less than 10. Which isolates more than 20, population genetic structure were be close to true, when isolates more than 30, population genetic structure was actually ture. Therefore a population should have more than 30 isolates.(3) To determine population structures of P. striiformis f. sp. tritici in Gansu, a total of 850 isolates were studied using 11 microsatellite markers. The genetic diversities in the Tianshui and Longnan populations were much higher than those of the Linxia and Pingliang populations. The Nei's gene diversity and Shannon information index of the Tianshui (H = 0.28; I = 0.41) and Longnan (H = 0.26; I = 0.42) populations were higher than those of the Linxia (H = 0.22; I = 0.36) and Pingliang (H = 0.18; I = 0.28) populations. For the Gansu population, the genetic differentiation was low (Gst = 0.15). Thus, the results suggest there are extensive gene exchanges or short-distance migrations of pathogen among regions in Gansu. The most (83.05%) of the total variation presented within subpopulations; 15.57% was among subpopulations; and only 1.38% was among the four regions. These data supported considerable gene flow and insignificant separation among the regional populations. Our molecular data support the hypothesis that the Longnan regions are the center of wheat stripe rust in Gansu .(4) To determine population structures of P. striiformis f. sp. tritici in this region, a total of 330 isolates from 11 collections were studied using SSR markers. The populations of P. striiformis f. sp. tritici possessed relatively high levels of genetic diversity in high mountain area but a low gene flow. Analysis of molecular variation showed that only 1.43% of the total variation presented among areas, 24.21% among collections within areas, and the remaining 74.36% within collections. There is extensive gene exchange within areas and short-distance migration of pathogen among areas in different agro-ecological area in southeast of Gansu, China. The most significant finding of this study is that recombination of P. striiformis f. sp. tritici was detected in the southeast of Gansu. Our molecular data confirmed the hypothesis that the stripe rust pathogen is able to over-winter in the lowland areas and over-summer in high mountain areas, which finish its life cycle in southeast of Gansu .(5) To determine the change of population genetic diversity and evaluate stability of genetic structure of P. striiformis f. sp. tritici in different seasons, a total of 685 isolates were studied using SSR markers. There was no significant difference of genetic diversity in different seasons. While genetic differentiation was significant difference in different seasons. Although there was a little change, the population genetic structure of P. striiformis f. sp. tritici was stability in different seasons in Longnan.(6) To determine the change of population genetic diversity and evaluate stability of genetic structure of P. striiformis f. sp. tritici in different years, a total of 728 isolates were studied using SSR markers. There was no significant difference of genetic diversity and genetic differentiation in different years. Although there was a little change, the population genetic structure of P. striiformis f. sp. tritici was stability in different seasons in Longnan.(7) Interregional long-distance spread of wheat stripe rust and the pathway in the Northwest China were inferred by disease surveys and molecular markers. Thirteen genotypes in Linxia and 12 genotypes in Qinghai were also found in Longnan, where 17 genotypes were identified. The genetic diversity in the Longnan population was much higher than those in the Linxia and Qinghai populations. The low genetic differentiation (Gst = 0.15) and the extensive gene flow (Nm=1.37) were found among these regions. The most important molecular finding was that collections from different regions were clustered together and isolates from different regions shared the same DNA fingerprinting. Therefore, the most important conclusion of this study is that the stripe rust inoculum in Qinghai can from both Longnan and Linxia, but mainly directly from Longan in the spring.(8) The somatic genetic recombination of P. striiformis f. sp. tritici in Longnan of Gansu Province was detected by 4 of 11 SSR marker. Recombinant frequency of P. striiformis f. sp. tritici was different. Recombinant frequency were 20.0%, 18.5%, 12.8% and 15.0% detected by RJ CPS15, CPS34, RJ20 and RJ18. The average recombinant frequency of P. striiformis f. sp. tritici was 16.6% in Longnan. This study revealed genetic recombination by molecular markers and thus strongly suggests that genetic recombination is another way of producing a new race of P. striiformis f. sp. tritici and lead to virulence variation. |
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