Overall stability in the genetic structure of a Zymoseptoria tritici population from epidemic to interepidemic stages at a small spatial scale

Subpopulations of the wheat pathogen Zymoseptoria tritici (26 sample groups composed of 794 isolates) were collected in two nearby wheat fields in the Paris basin, during both epidemic and inter-epidemic periods of three successive years (2009-2013). In addition to the type of inoculum (ascospores vs. pycnidiospores), the alternative presence of wheat debris allowed taking into account its putative origin (local vs. distant). We used a molecular epidemiology approach, based on population genetic indices derived from SSR marker analysis, to describe putative changes in the structure and genotypic diversity of these subpopulations over 3years, at a spatiotemporal scale consistent with epidemiological observations. Genetic structure was broadly stable over time (within and between years) and between fields, however with weak population differentiation over time. All subpopulations displayed very high diversity and the occurrence of regular sexual reproduction was confirmed in the two fields. A significant increase of the MAT1-1/MAT1-2 ratio was observed over the course of the epidemics. This original finding suggests a competitive advantage of MAT1-1 strains consistently with their greater pathogenicity reported in the literature and may reveal undescribed adaptation. Finally, we found that the period, the type of inoculum and its putative origin had little effect on the short term evolution of the local population of Z. tritici. Fungal population size and diversity are apparently large enough to prevent genetic drift at this fine spatiotemporal scale, and more likely short distance migration contributes strongly to the stabilization of genetic diversity among and within plots.