Molecular Population Genetic Structure Of Puccinia Striiformis F.sp. Tritici In China

Puccinia striiformis f.sp. tritici (PST) is an obligate biotrophic pathogen, causing wheat yellow rust (stripe rust), one of the most economical important diseases of wheat (Triticum aestivum). Spores of fungal plant pathogens may be spread by wind over long distances, thereby enhancing disease epidemics across large areas. It is very essential for the development of more effective disease control to understand the population genetic structure in the regions wheat stripe rust occurs frequently. A great deal of valuable theory was obtained through continual study on population biology of pathogens since 1950s in China, which play an important role to prevent and control diseases epidemic. However, the inferring results about the epidemiological study of pathogens over a large area were mainly based on traditional methods of virulence identification and observation for disease degree, and some of relation between the regions is still unknown. So those need to be approved and complemented using the molecular population genetic methods. Our goal of the present research is to establish a system of SSR molecular marker and then to analyze the genetic diversity and population genetic structure and to infer the dispersal relations of pathogens according to the information of gene flow and shared fingerprints among the regions. The main results are summarized as following:1. The frequency and characteristics of SSR derived from ESTs by sequencing complementary DNA (cDNA) library of PST were analyzed. Of the 1307 nonredundant EST sequences, 170 SSR loci distribute in 135 ESTs (10.33%). The average length of EST-SSR searched is 16.48 bp and the average distance of distribution is 1/3.69 kb. The trinucleotide repeat is the dominant types with repeat motif GTT/CAA being the most common. These results lay the foundation for the future development and application of EST-SSR markers in PST.2. The SSRs markers derived from PST cDNA library and three others Puccinia species reported were tested for molecular polymorphism by isolates sampled from different geographical regions. The fluorescent labeling system of TP-M13-SSR (simple sequence repeat with tailed primer M13) was established based on polymorphic markers screened and the fluorescent labeling technique of TP-M13. The results showed that the TP-M13-SSR approach is of high polymorphism, sensitivity and good steady. The new system molecular marker provides technological support for large-scale and high-throughput population genetic analysis.3. The 17 physiological races and pathotypes were tested for diversity using the TP-M13-SSR system. The results showed that there is much diversity revealed by SSR markers. However, there is no significant correspondence between two measures of labeling. These molecular markers would become the useful labeling resources for population genetic research of PST.4. The 980 isolates grouped into 20 different geographical populations sampled from nine provinces in China were analyzed for the population genetic structure of PST using the TP-M13-SSR system. The results as followed:(1) Results showed that the genetic diversity level of PST in China is relatively high and is different from each other among 20 populations. The highest one was found in Tianshui population which means the central population of diversity and the lowest in Xiangfan population.(2) The results of genetic differentiation among 20 natural populations showed that it could be found within and among populations (Gst=0.2335, Fst=0.2215). The majority of genetic variation occurred within populations. But the level of differentiation was different from each other between two populations and parts of it were higher. The mantle test showed significant correlation between genetic and geographic distances among natural populations(P <0.001). Gene flow among natural populations was relatively weak (Nm=0.8787), which might have caused the genetic differentiation among the populations.(3) The 20 natural populations were clustered into five groups by UPGMA method according to their genetic distances. The largest group included 12(60%) populations which were mainly sampled from the oversummer zone of PST in northwest China and northwest Sichuan province and three neighboring basins except for Zhongtong population collected from Yunnan province. Eight others populations were clusterd into four groups, which were mostly from south of Sichuan basin and eastern China.(4) The 20 natural populations were divided into six zones according to their geographic distribution. These were the oversummer zone of PST in northwest China and northwest Sichuan province, GuanZhong basin, Southern Shaanxi province basin, Sichuan basin, south of Sichuan basin and eastern China zones, respectively. On the one hand, the genetic diversity of oversummer zone is highest among six zones, which showed the central zone of genetic diversity in China, and the lowest in eastern China zone. On the other hand, the maximal genetic differentiation was found among populations within eastern China zone and the minimum one within Southern Shaanxi province basin zone.(5) The evidence of gene flow between the two oversummer zones in northwest China and northwest Sichuan province was obtained, which showed that genetic relationship among populations located in two regions was not correlative with their geographical attribution. The genetic relationship of PST among the populations within the oversummer zone of PST in northwest China and northwest Sichuan province and the three neighboring basins was also clarified. The closest genetic relationships were found between GuanZhong basin and Pingliang, Sichuan basin and Aba, Southern Shaanxi province basin and Longnan populations, respectively.(6) The direct molecular evidence for long-distance dispersal in PST was obtained according to the finding of some identical fingerprints between and among different geographical populations. Parts of results about migration direction in PST inferred using traditional methods of virulence identification were proved. For instance, the relations of pathogens dispersal occurred between Pingliang and GuanZhong basin, also between Hanzhong and Ankang regions. The strong possibility of pathogens dispersal was found for the first time over very long distances like between the oversummer zone in northwest China and south of Sichuan basin regions.