Evolution of the CYP51 gene in Mycosphaerella graminicola:: evidence for intragenic recombination and selective replacement

Recent findings are consistent with a slow but constant shift towards reduced sensitivity of Mycosphaerella graminicola to azole fungicides, which target the CYP51 gene. The goal of this study was to elucidate the evolutionary mechanisms through which CYP51-based mutations associated with altered sensitivity have evolved in M. graminicola over space and time. To accomplish this, we sequenced and compared a portion of the CYP51 gene encompassing the main mutations associated with altered sensitivity towards demethylation inhibitor fungicides. The CYP51 gene showed an extraordinary dynamic shift consistent with a selective haplotype replacement both in space and in time. No mutations associated with increased resistance to azoles were found in non-European populations. These mutations were also absent in the oldest collections from Europe, whereas they dominated in the recent European populations. Intragenic recombination was identified as an important evolutionary process in populations affected by high fungicide selection, suggesting the creation of novel alleles among existing mutations as a potential source of novel resistance alleles. We propose that CYP51 mutations giving resistance in M. graminicola arose only locally (perhaps in Denmark or the UK) and were then spread eastward across Europe through wind-dispersed ascospores. We conclude that recurring cycles of recombination coupled with selection due to the widespread use of azole fungicides will increase the frequency of novel mutants or recombinants with higher resistance. Long-distance gene flow due to wind dispersal of ascospores will move the resulting new alleles to new areas following the prevailing wind directions. A selective replacement favouring haplotypes with various coding mutations at the target site for azole fungicides during the last 5-10 years is the most likely cause of the decrease in sensitivity reported for many azole fungicides in the same period.