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 2
dimensions over a 50x80 Km area. Using two different clustering
algorithms, no genetic structure was detected in the studied area,
suggesting expansion of large populations and/or no influ ence 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 to the surrounding food-crop plantation populations.
Analysis of molecular variance (AMOVA) 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.