Résumé
In 2007, Rete Semi Rurali (RSR), the Italian farmers' seed network, facilitated the emergence of a broad movement of seeds re-appropriation and on-farm diversity management by farmers by creating a bread wheat Community Seed System (RSR-CSS). RSR-CSS is now facing changes in farmers' seed demand in the context of the current seed regulation for heterogenous material in Europe (2014/150 & 2018/848 EU). One major issue for the RSR-CSS is to organize seed circulation among farmers in order to foster local adaptation of wheat populations, while maintaining a sufficient level of genetic diversity over generations. To address this issue, we started an iterative participatory simulation-based approach with RSR-CSS, consisting in: i) the description of the CSS' functioning; ii) the co-construction of scenarios; iii) the scenario simulation using a population genetics model of crop metapopulations and the output analyses. Discussions and meetings with RSR farmers and facilitators, helped to describe the CSS' functioning and to co-design scenarios. These included different characteristics of the seed circulation network based on longitudinal data and three ways of representing the local selective pressures due to the pedo-climatic conditions and farming practices: 1) a single environment; 2) an environmental gradient; and, 3) two contrasting environments. The functioning of the model and the results were shown and discussed with RSR-CSS actors, allowing us to iteratively co-construct: a) more accurate scenarios to represent the current functioning of the CSS; and, b) new scenarios to explore alternative management practices and organisation modes. We found that the way environmental selective pressures were represented in relation to the seed circulation network strongly affected genetic diversity and genetic differentiation, whilst the gradient scenario maintains more genetic diversity over generations. These results will feed into the RSR-CSS discussion on the best practices for seed distribution/ciruclation among an increasing number of farmers.