GENETIC DIVERSITY IN PUCCINIA-STRIIFORMIS WESTEND. F-SP TRITICI REVEALED BY PATHOGEN GENOME-SPECIFIC REPETITIVE SEQUENCE

DNA fingerprinting was used to examine genetic variation in population s of Puccinia striiformis Westend. f. sp. tritici, an obligate fungus that causes wheat stripe rust, using as a probe a moderately repetitiv e DNA sequence PSR331 that shows species specificity in the genome of this pathogen. One hundred and sixty isolates sampled from six provinc es throughout China were examined for genetic variation over 26 putati ve genetic loci defined by PSR331 and the restriction enzyme Bg/II. Be cause of the dikaryotic nature of this fungus, DNA fingerprints can no t differentiate heterozygotes from homozygotes. We refer to the PSR DN A fingerprints as phenotypes rather than genotypes. Phenotypic diversi ty analysis revealed a high level of genetic variation. A total of 97 phenotypes was detected among 160 isolates. Phenotypic diversity varie d among regions, ranging from 0.3742 in Shaanxi to 0.9380 in Gansu, as calculated with the normalized Shannon's index. Genetic subdivision a nalysis revealed a low level of genetic differentiation (G(ST) = 0.008 4) among regions (Gansu, Henan, Shaanxi, Sichuan, and Yunnan provinces ) as well as within regions (Gansu and Sichuan provinces). This, toget her with the detection of the same phenotypes among regions, provided the molecular evidence for gene flow in P. striiformis f. sp. tritici. The results support conclusions from virulence surveys that Tianshui of southern Gansu is probably the most important ''hotspot'' area with respect to the potential to generate and maintain virulence variation . DNA polymorphism analysis also detected potential hotspot areas in a ddition to southern Gansu. This may result in more difficulties in man agement of genetic variation and thus the potential virulence variatio n in P. striiformis f. sp. tritici as well as providing opportunities for searching disease resistance factors.