Relations intra et interspecifiques chez Mycosphaerella populorum (anamorphe Septoria musiva) agent pathogene des peupliers (French and English text)

To understand the evolutionary potential of Mycosphaerella populorum (anamorph Septoria musiva), a fungal pathogen of poplars, we have investigated its genetic variability using neutral molecular markers. An appropriate sampling of the S. musiva populations was first required to examine the relationships of this fungus with the related poplar pathogens S. populicola and S. populi. Firstly, a polymerase chain reaction (PCR) assay was developed for an accurate identification of the three fungi based on interspecific polymorphisms in the internal transcribed spacer (ITS) of nuclear ribosomal RNA gene (rDNA) repeats. The specificity of the three primer pairs developed was successfully tested on a large fungal collection, providing evidence that the procedure can be applied for an accurate identification of the targeted Septoria species. Secondly, in order to improve our understanding of phylogenetic relationships among S. musiva, S. populicola and S. populi and within the Mycosphaerella fungal genera, three molecular data sets of nuclear and mitochondrial genes were evaluated. Separate gene analyses, congruence tests among data partitions and combined analyses were performed using maximum parsimony and bayesian frameworks. Specifically, the results pointed out the occurrence of distinct but closely related species sampled on identical host species or from identical botanical families. This tendency was particularly illustrated within the monophyletic clade which includes poplar pathogenic Septoria species. Based on morphological traits and previously defined molecular characteristics of S. musiva , we then sampled seven North American populations of this fungus to characterize their genetic variation using 21 polymorphic markers revealed with the PCR-RAPD (Random Amplified Polymorphic DNA) technique. A high and significant correlation between genetic and geographic distances among populations was found; it was mirrored (1) by no significant differentiation between the subpopulations sampled on Populus deltoides and on hybrid trees and (2) by a moderate to high genetic differentiation between intra- and inter-regional populations. Moreover, due to these differentiation levels, the high haplotypic diversity, and the gametic equilibrium observed at the RAPID loci sampled, S. musiva appears to be differentiated into sub-populations in which both asexual and sexual recombination contribute to the local level of genetic structure.