& Key message A moderate genetic diversity, the absence of a significant genetic differentiation between wild and cultivated stands and a highly admixed genetic structure of sweet chestnut with two main clusters were observed in France using two different data sets with 10 and 18 microsatellites. & Context Renewed interest in European chestnut in France is focused on finding locally adapted populations partially resistant to ink disease and identifying local landraces. & Aims We genotyped trees to assess (i) the genetic diversity of wild and cultivated chestnut across most of its range in France, (ii) their genetic structure, notably in relation with the sampled regions, and (iii) to a lesser extent the relations between French chestnuts with 10 cultivated chestnuts from the Northwest of Spain that were previously classified in the Iberian or Italian groups. & Methods A total of 693 trees in 16 sampling regions in France were genotyped at 24 SSRs and 1401 trees in 17 sampling regions at 13 SSRs. & Results Genetic diversity was moderate in most sampling regions, with redundancy between them. No significant differentiation was found between wild and cultivated chestnut. A genetic structure analysis with no a priori information found a low yet Timothée Flutre and Luc Harvengt contributed equally to this work.

Key messageThis paper presents the results of the first assessment of genetic diversity and structure of wild and cultivated sweet 25 chestnut in France. It reveals high diversity, a low but significant structure, and strongly suggests that the French gene pool is at the intersection between the Italian and Spanish gene pools. Abstract Context 30Renewed interest in European chestnut in France is focussed on finding locally adapted populations partially resistant to ink disease and identifying local landraces. AimsWe genotyped trees to assess (i) the genetic diversity of wild and cultivated chestnut across most of its range in France, (ii) their genetic structure, notably in relation with the sampled regions, and (iii) relations with its neighbors in Spain 35 and Italy. MethodsA total of 1,401 trees in 17 sampling regions in France were genotyped at 13 SSRs, and a subset of 693 trees at 24 SSRs. ResultsGenetic diversity was high in most sampling regions, with redundancy between them. No significant differentiation was 40 found between wild and cultivated chestnut. A genetic structure analysis with no a priori information found a low, yet significant structure, and identified three clusters. Two clusters of sampling regions, south east France and Corsica, were 2/17 less admixed than the others. A substructure was detected in the admixed cluster suggesting differentiation in wild chestnut trees in Finistère and Aveyron sampling regions. Conclusion 45The genetic structure within and between our sampling regions is likely the result of natural events (recolonization after the last glaciation) and human activities (migration and exchanges). Notably, we provide evidence for a common origin of most French and Iberian chestnut trees, except those from, south east France that were associated with the Italian gene pool. This advance in our knowledge of chestnut genetic diversity and structure will benefit conservation and help our local partners' valorization efforts. 50Keywords Castanea sativa Mill.; chestnut; genetic diversity; genetic structure; microsatellite markers; landraces 55 3/17