Résumé
Understanding how geographical and environmental features affect genetic variation at
both the population and individual levels is crucial in biology, especially in the case of
pathogens. However, distinguishing between these factors and the effects of historical
range expansion on spatial genetic structure remains challenging. In the present study, we
investigated the case of Mycosphaerella fijiensis—a plant pathogenic fungus
that has recently colonized an agricultural landscape characterized by the presence of
potential barriers to gene flow, including several commercial plantations in which disease
control practises such as the use of fungicides are applied frequently, and low host
density areas. We first genotyped 300 isolates sampled at a global scale on untreated
plants in two dimensions over a 50 × 80-km area. Using two different clustering
algorithms, no genetic structure was detected in the studied area, suggesting expansion of
large populations and/or no influence of potential barriers. Second, we investigated
the potential effect of disease control practises on M. fijiensis diversity by
comparing populations sampled in commercial vs food-crop plantations. At this local scale,
we detected significantly higher allelic richness inside commercial plantations compared
with the surrounding food-crop plantation populations. Analysis of molecular variance
indicated that 99% of the total genetic variance occurred within populations. We
discuss the suggestion that high population size and/or high migration rate between
populations might be responsible for the absence of any effect of disease control
practises on genetic diversity and differentiation.