GENETIC STRUCTURE OF SEPTORIA TRITICI POPULATIONS IN GERMANY ASSESSED BY AFLP ANALYSIS
F SCHNIEDER, F SCHNIEDER, G KOCH, C JUNG and J-A VERREET
Institute of Phytopathology, University of Kiel, Kiel, Germany
Background and objectives
Materials and methodsIn 1996 we collected 380 single spore isolates of S. tritici from five different locations and two different susceptible cultivars in Germany. The preamplification of the single spore isolate DNA was carried out with two primers having each a single selective nucleotide. The labelling reaction was performed with 33P-ATP. In the following amplification step we used two primers each having two selective nucleotides. After amplification each sample was loaded on a 4 % denaturing (sequencing) polyacrylamide gel.
The fragments from each technique were scored for presence or absence of bands showing the same mobility in the gel, regardless of their optical density. The fragment data were coded as a binary matrix where 1 designated presence and 0 absence of a particular band. Based on the binary data matrix, genetic similarities were calculated for all pair-wise isolate-comparisons. The genetic similarity between two isolates x and y was calculated according to the formula of DICE (1945), first applied to molecular data by Nei and Li (1979): Sxy=2nxy/(nx+ny), where nxy is the number of fragments in common between isolates x and y, and nx and ny are the total numbers of fragments in isolates x and y, respectively. Based on similarity values, associations among isolates were revealed by UPGMA cluster analysis specifying the DICE coefficient.
Results and conclusions
The populations showed similar levels of genetic diversity. The correlation between genetic distance and geographical distance depends largely on the dispersal ability of an organism. Organisms with the potential for widespread, long-distance dispersal will display greater genetic uniformity across local populations than organisms with very limited dispersal ability. The sexual state of S. tritici is now thought to be important for long-distance dispersal of the pathogen. The sexual cycle may also be an important source of genetic variation in S. tritici populations. Our data suggest that the 'geographical populations' we used are not evolving independently and therefore may be considered part of the same genetic population.