Electrophoretic and cytological karyotyping of the foliar wheat pathogen Mycosphaerella graminicola reveals many chromosomes with a large size range

The karyotypes of three isolates of Mycosphaerella graminicola, the septoria tritici blotch pathogen of wheat, were analyzed with both pulsed field gel electrophoresis (PFGE) and the cytological technique called germ tube burst method (GTBM). These analyses revealed a chromosome length polymorphism among these isolates. The estimated genome size was 31¿40 Mb depending on the isolates, indicating 17¿22% redundancy in the genome of the standard strain IPO323 because such differences do not affect development, pathogenicity and sexual reproduction of the other isolates. The chromosome numbers in the three isolates were 18¿20 and the chromosome size was 0.3¿6 Mb. These data show that M. graminicola has the highest chromosome number and the smallest autosomes (A chromosomes) in filamentous ascomycetes. Our data also confirmed a large ( 6 Mb) chromosome that was assembled recently in the IPO323 genome sequence. GTBM analyses revealed the mitotic metaphase chromosomes, enabling chromosome quantification, which was fully congruent with the PFGE analyses. These data will be instrumental in the final assembly of the M. graminicola genome.