Evolution of Mycosphaerella graminicola at the wheat leaf level and at the field level

Evolution of M. graminicola wheat field populations from a given French county (Morbihan, 56) between years 2005 and 2006 was investigated for thirty seven strains using molecular fingerprinting by microsatellite markers (ST1A4, ST1E3, ST1E7, and ST1D7), and SSCP analysis study of partial actin and beta-tubulin encoding sequences. In addition, twenty nine strains collected from 3 distinct lesions on a same wheat leaf in 2006 in another French county (Nord, 59) were also investigated for genetic diversity. At the field level, we observed similar gene diversity in the 2005 and in the 2006 populations, with no common clones between the two years. This indicates frequent sexual recombination undergone by the fungus. When considering each marker independently and comparing genetic variability of the two populations, we noticed a decrease in genetic variability of the 2006 strains for three of them (ST1A4, ST1D7 and the partial sequence of actin) and an increase for ST1E3, ST1E7 and the partial sequence of beta-tubulin, revealing the importance of the chosen markers. At the lesion level, 69% of the studied strains were haplotypes with 31% of the clonal population found in 2 lesions out of 3. This suggests that at least parts of the lesions were formed after asexual reproduction and dissemination of pycnidiospores by splashing. We also confirmed the exploitative competition that exists between the strains at the lesion level.